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Automatically generate the tables of contents for livepatch documentation files that have tables of contents rather than open coding them so things are a little easier to maintain. Signed-off-by: Mark Brown <broonie@kernel.org> Acked-by: Josh Poimboeuf <jpoimboe@redhat.com> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
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448 lines
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=========
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Livepatch
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=========
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This document outlines basic information about kernel livepatching.
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.. Table of Contents:
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.. contents:: :local:
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1. Motivation
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=============
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There are many situations where users are reluctant to reboot a system. It may
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be because their system is performing complex scientific computations or under
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heavy load during peak usage. In addition to keeping systems up and running,
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users want to also have a stable and secure system. Livepatching gives users
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both by allowing for function calls to be redirected; thus, fixing critical
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functions without a system reboot.
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2. Kprobes, Ftrace, Livepatching
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================================
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There are multiple mechanisms in the Linux kernel that are directly related
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to redirection of code execution; namely: kernel probes, function tracing,
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and livepatching:
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- The kernel probes are the most generic. The code can be redirected by
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putting a breakpoint instruction instead of any instruction.
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- The function tracer calls the code from a predefined location that is
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close to the function entry point. This location is generated by the
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compiler using the '-pg' gcc option.
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- Livepatching typically needs to redirect the code at the very beginning
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of the function entry before the function parameters or the stack
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are in any way modified.
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All three approaches need to modify the existing code at runtime. Therefore
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they need to be aware of each other and not step over each other's toes.
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Most of these problems are solved by using the dynamic ftrace framework as
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a base. A Kprobe is registered as a ftrace handler when the function entry
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is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
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a live patch is called with the help of a custom ftrace handler. But there are
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some limitations, see below.
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3. Consistency model
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====================
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Functions are there for a reason. They take some input parameters, get or
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release locks, read, process, and even write some data in a defined way,
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have return values. In other words, each function has a defined semantic.
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Many fixes do not change the semantic of the modified functions. For
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example, they add a NULL pointer or a boundary check, fix a race by adding
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a missing memory barrier, or add some locking around a critical section.
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Most of these changes are self contained and the function presents itself
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the same way to the rest of the system. In this case, the functions might
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be updated independently one by one.
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But there are more complex fixes. For example, a patch might change
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ordering of locking in multiple functions at the same time. Or a patch
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might exchange meaning of some temporary structures and update
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all the relevant functions. In this case, the affected unit
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(thread, whole kernel) need to start using all new versions of
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the functions at the same time. Also the switch must happen only
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when it is safe to do so, e.g. when the affected locks are released
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or no data are stored in the modified structures at the moment.
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The theory about how to apply functions a safe way is rather complex.
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The aim is to define a so-called consistency model. It attempts to define
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conditions when the new implementation could be used so that the system
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stays consistent.
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Livepatch has a consistency model which is a hybrid of kGraft and
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kpatch: it uses kGraft's per-task consistency and syscall barrier
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switching combined with kpatch's stack trace switching. There are also
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a number of fallback options which make it quite flexible.
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Patches are applied on a per-task basis, when the task is deemed safe to
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switch over. When a patch is enabled, livepatch enters into a
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transition state where tasks are converging to the patched state.
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Usually this transition state can complete in a few seconds. The same
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sequence occurs when a patch is disabled, except the tasks converge from
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the patched state to the unpatched state.
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An interrupt handler inherits the patched state of the task it
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interrupts. The same is true for forked tasks: the child inherits the
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patched state of the parent.
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Livepatch uses several complementary approaches to determine when it's
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safe to patch tasks:
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1. The first and most effective approach is stack checking of sleeping
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tasks. If no affected functions are on the stack of a given task,
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the task is patched. In most cases this will patch most or all of
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the tasks on the first try. Otherwise it'll keep trying
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periodically. This option is only available if the architecture has
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reliable stacks (HAVE_RELIABLE_STACKTRACE).
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2. The second approach, if needed, is kernel exit switching. A
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task is switched when it returns to user space from a system call, a
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user space IRQ, or a signal. It's useful in the following cases:
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a) Patching I/O-bound user tasks which are sleeping on an affected
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function. In this case you have to send SIGSTOP and SIGCONT to
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force it to exit the kernel and be patched.
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b) Patching CPU-bound user tasks. If the task is highly CPU-bound
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then it will get patched the next time it gets interrupted by an
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IRQ.
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3. For idle "swapper" tasks, since they don't ever exit the kernel, they
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instead have a klp_update_patch_state() call in the idle loop which
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allows them to be patched before the CPU enters the idle state.
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(Note there's not yet such an approach for kthreads.)
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Architectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on
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the second approach. It's highly likely that some tasks may still be
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running with an old version of the function, until that function
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returns. In this case you would have to signal the tasks. This
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especially applies to kthreads. They may not be woken up and would need
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to be forced. See below for more information.
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Unless we can come up with another way to patch kthreads, architectures
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without HAVE_RELIABLE_STACKTRACE are not considered fully supported by
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the kernel livepatching.
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The /sys/kernel/livepatch/<patch>/transition file shows whether a patch
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is in transition. Only a single patch can be in transition at a given
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time. A patch can remain in transition indefinitely, if any of the tasks
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are stuck in the initial patch state.
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A transition can be reversed and effectively canceled by writing the
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opposite value to the /sys/kernel/livepatch/<patch>/enabled file while
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the transition is in progress. Then all the tasks will attempt to
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converge back to the original patch state.
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There's also a /proc/<pid>/patch_state file which can be used to
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determine which tasks are blocking completion of a patching operation.
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If a patch is in transition, this file shows 0 to indicate the task is
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unpatched and 1 to indicate it's patched. Otherwise, if no patch is in
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transition, it shows -1. Any tasks which are blocking the transition
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can be signaled with SIGSTOP and SIGCONT to force them to change their
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patched state. This may be harmful to the system though. Sending a fake signal
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to all remaining blocking tasks is a better alternative. No proper signal is
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actually delivered (there is no data in signal pending structures). Tasks are
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interrupted or woken up, and forced to change their patched state. The fake
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signal is automatically sent every 15 seconds.
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Administrator can also affect a transition through
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/sys/kernel/livepatch/<patch>/force attribute. Writing 1 there clears
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TIF_PATCH_PENDING flag of all tasks and thus forces the tasks to the patched
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state. Important note! The force attribute is intended for cases when the
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transition gets stuck for a long time because of a blocking task. Administrator
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is expected to collect all necessary data (namely stack traces of such blocking
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tasks) and request a clearance from a patch distributor to force the transition.
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Unauthorized usage may cause harm to the system. It depends on the nature of the
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patch, which functions are (un)patched, and which functions the blocking tasks
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are sleeping in (/proc/<pid>/stack may help here). Removal (rmmod) of patch
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modules is permanently disabled when the force feature is used. It cannot be
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guaranteed there is no task sleeping in such module. It implies unbounded
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reference count if a patch module is disabled and enabled in a loop.
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Moreover, the usage of force may also affect future applications of live
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patches and cause even more harm to the system. Administrator should first
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consider to simply cancel a transition (see above). If force is used, reboot
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should be planned and no more live patches applied.
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3.1 Adding consistency model support to new architectures
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---------------------------------------------------------
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For adding consistency model support to new architectures, there are a
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few options:
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1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and
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for non-DWARF unwinders, also making sure there's a way for the stack
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tracing code to detect interrupts on the stack.
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2) Alternatively, ensure that every kthread has a call to
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klp_update_patch_state() in a safe location. Kthreads are typically
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in an infinite loop which does some action repeatedly. The safe
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location to switch the kthread's patch state would be at a designated
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point in the loop where there are no locks taken and all data
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structures are in a well-defined state.
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The location is clear when using workqueues or the kthread worker
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API. These kthreads process independent actions in a generic loop.
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It's much more complicated with kthreads which have a custom loop.
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There the safe location must be carefully selected on a case-by-case
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basis.
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In that case, arches without HAVE_RELIABLE_STACKTRACE would still be
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able to use the non-stack-checking parts of the consistency model:
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a) patching user tasks when they cross the kernel/user space
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boundary; and
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b) patching kthreads and idle tasks at their designated patch points.
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This option isn't as good as option 1 because it requires signaling
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user tasks and waking kthreads to patch them. But it could still be
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a good backup option for those architectures which don't have
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reliable stack traces yet.
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4. Livepatch module
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===================
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Livepatches are distributed using kernel modules, see
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samples/livepatch/livepatch-sample.c.
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The module includes a new implementation of functions that we want
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to replace. In addition, it defines some structures describing the
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relation between the original and the new implementation. Then there
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is code that makes the kernel start using the new code when the livepatch
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module is loaded. Also there is code that cleans up before the
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livepatch module is removed. All this is explained in more details in
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the next sections.
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4.1. New functions
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------------------
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New versions of functions are typically just copied from the original
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sources. A good practice is to add a prefix to the names so that they
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can be distinguished from the original ones, e.g. in a backtrace. Also
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they can be declared as static because they are not called directly
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and do not need the global visibility.
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The patch contains only functions that are really modified. But they
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might want to access functions or data from the original source file
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that may only be locally accessible. This can be solved by a special
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relocation section in the generated livepatch module, see
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Documentation/livepatch/module-elf-format.rst for more details.
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4.2. Metadata
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-------------
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The patch is described by several structures that split the information
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into three levels:
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- struct klp_func is defined for each patched function. It describes
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the relation between the original and the new implementation of a
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particular function.
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The structure includes the name, as a string, of the original function.
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The function address is found via kallsyms at runtime.
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Then it includes the address of the new function. It is defined
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directly by assigning the function pointer. Note that the new
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function is typically defined in the same source file.
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As an optional parameter, the symbol position in the kallsyms database can
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be used to disambiguate functions of the same name. This is not the
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absolute position in the database, but rather the order it has been found
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only for a particular object ( vmlinux or a kernel module ). Note that
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kallsyms allows for searching symbols according to the object name.
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- struct klp_object defines an array of patched functions (struct
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klp_func) in the same object. Where the object is either vmlinux
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(NULL) or a module name.
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The structure helps to group and handle functions for each object
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together. Note that patched modules might be loaded later than
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the patch itself and the relevant functions might be patched
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only when they are available.
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- struct klp_patch defines an array of patched objects (struct
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klp_object).
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This structure handles all patched functions consistently and eventually,
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synchronously. The whole patch is applied only when all patched
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symbols are found. The only exception are symbols from objects
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(kernel modules) that have not been loaded yet.
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For more details on how the patch is applied on a per-task basis,
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see the "Consistency model" section.
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5. Livepatch life-cycle
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=======================
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Livepatching can be described by five basic operations:
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loading, enabling, replacing, disabling, removing.
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Where the replacing and the disabling operations are mutually
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exclusive. They have the same result for the given patch but
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not for the system.
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5.1. Loading
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------------
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The only reasonable way is to enable the patch when the livepatch kernel
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module is being loaded. For this, klp_enable_patch() has to be called
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in the module_init() callback. There are two main reasons:
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First, only the module has an easy access to the related struct klp_patch.
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Second, the error code might be used to refuse loading the module when
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the patch cannot get enabled.
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5.2. Enabling
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-------------
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The livepatch gets enabled by calling klp_enable_patch() from
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the module_init() callback. The system will start using the new
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implementation of the patched functions at this stage.
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First, the addresses of the patched functions are found according to their
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names. The special relocations, mentioned in the section "New functions",
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are applied. The relevant entries are created under
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/sys/kernel/livepatch/<name>. The patch is rejected when any above
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operation fails.
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Second, livepatch enters into a transition state where tasks are converging
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to the patched state. If an original function is patched for the first
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time, a function specific struct klp_ops is created and an universal
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ftrace handler is registered\ [#]_. This stage is indicated by a value of '1'
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in /sys/kernel/livepatch/<name>/transition. For more information about
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this process, see the "Consistency model" section.
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Finally, once all tasks have been patched, the 'transition' value changes
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to '0'.
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.. [#]
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Note that functions might be patched multiple times. The ftrace handler
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is registered only once for a given function. Further patches just add
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an entry to the list (see field `func_stack`) of the struct klp_ops.
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The right implementation is selected by the ftrace handler, see
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the "Consistency model" section.
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That said, it is highly recommended to use cumulative livepatches
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because they help keeping the consistency of all changes. In this case,
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functions might be patched two times only during the transition period.
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5.3. Replacing
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--------------
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All enabled patches might get replaced by a cumulative patch that
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has the .replace flag set.
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Once the new patch is enabled and the 'transition' finishes then
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all the functions (struct klp_func) associated with the replaced
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patches are removed from the corresponding struct klp_ops. Also
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the ftrace handler is unregistered and the struct klp_ops is
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freed when the related function is not modified by the new patch
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and func_stack list becomes empty.
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See Documentation/livepatch/cumulative-patches.rst for more details.
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5.4. Disabling
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--------------
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Enabled patches might get disabled by writing '0' to
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/sys/kernel/livepatch/<name>/enabled.
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First, livepatch enters into a transition state where tasks are converging
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to the unpatched state. The system starts using either the code from
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the previously enabled patch or even the original one. This stage is
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indicated by a value of '1' in /sys/kernel/livepatch/<name>/transition.
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For more information about this process, see the "Consistency model"
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section.
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Second, once all tasks have been unpatched, the 'transition' value changes
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to '0'. All the functions (struct klp_func) associated with the to-be-disabled
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patch are removed from the corresponding struct klp_ops. The ftrace handler
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is unregistered and the struct klp_ops is freed when the func_stack list
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becomes empty.
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Third, the sysfs interface is destroyed.
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5.5. Removing
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-------------
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Module removal is only safe when there are no users of functions provided
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by the module. This is the reason why the force feature permanently
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disables the removal. Only when the system is successfully transitioned
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to a new patch state (patched/unpatched) without being forced it is
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guaranteed that no task sleeps or runs in the old code.
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6. Sysfs
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========
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Information about the registered patches can be found under
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/sys/kernel/livepatch. The patches could be enabled and disabled
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by writing there.
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/sys/kernel/livepatch/<patch>/force attributes allow administrator to affect a
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patching operation.
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See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
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7. Limitations
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==============
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The current Livepatch implementation has several limitations:
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- Only functions that can be traced could be patched.
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Livepatch is based on the dynamic ftrace. In particular, functions
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implementing ftrace or the livepatch ftrace handler could not be
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patched. Otherwise, the code would end up in an infinite loop. A
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potential mistake is prevented by marking the problematic functions
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by "notrace".
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- Livepatch works reliably only when the dynamic ftrace is located at
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the very beginning of the function.
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The function need to be redirected before the stack or the function
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parameters are modified in any way. For example, livepatch requires
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using -fentry gcc compiler option on x86_64.
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One exception is the PPC port. It uses relative addressing and TOC.
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Each function has to handle TOC and save LR before it could call
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the ftrace handler. This operation has to be reverted on return.
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Fortunately, the generic ftrace code has the same problem and all
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this is handled on the ftrace level.
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- Kretprobes using the ftrace framework conflict with the patched
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functions.
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Both kretprobes and livepatches use a ftrace handler that modifies
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the return address. The first user wins. Either the probe or the patch
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is rejected when the handler is already in use by the other.
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- Kprobes in the original function are ignored when the code is
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redirected to the new implementation.
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There is a work in progress to add warnings about this situation.
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