2018-04-29 13:20:11 +00:00
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===================
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Speculation Control
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===================
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2018-05-08 13:43:45 +00:00
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Quite some CPUs have speculation-related misfeatures which are in
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fact vulnerabilities causing data leaks in various forms even across
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privilege domains.
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2018-04-29 13:20:11 +00:00
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The kernel provides mitigation for such vulnerabilities in various
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2018-05-08 13:43:45 +00:00
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forms. Some of these mitigations are compile-time configurable and some
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can be supplied on the kernel command line.
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2018-04-29 13:20:11 +00:00
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There is also a class of mitigations which are very expensive, but they can
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be restricted to a certain set of processes or tasks in controlled
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environments. The mechanism to control these mitigations is via
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:manpage:`prctl(2)`.
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There are two prctl options which are related to this:
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* PR_GET_SPECULATION_CTRL
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* PR_SET_SPECULATION_CTRL
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PR_GET_SPECULATION_CTRL
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-----------------------
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PR_GET_SPECULATION_CTRL returns the state of the speculation misfeature
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2018-05-03 20:09:15 +00:00
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which is selected with arg2 of prctl(2). The return value uses bits 0-3 with
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the following meaning:
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2019-01-16 22:01:36 +00:00
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==== ====================== ==================================================
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Bit Define Description
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==== ====================== ==================================================
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0 PR_SPEC_PRCTL Mitigation can be controlled per task by
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PR_SET_SPECULATION_CTRL.
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1 PR_SPEC_ENABLE The speculation feature is enabled, mitigation is
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disabled.
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2 PR_SPEC_DISABLE The speculation feature is disabled, mitigation is
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enabled.
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3 PR_SPEC_FORCE_DISABLE Same as PR_SPEC_DISABLE, but cannot be undone. A
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subsequent prctl(..., PR_SPEC_ENABLE) will fail.
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4 PR_SPEC_DISABLE_NOEXEC Same as PR_SPEC_DISABLE, but the state will be
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cleared on :manpage:`execve(2)`.
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==== ====================== ==================================================
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If all bits are 0 the CPU is not affected by the speculation misfeature.
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2018-05-08 13:43:45 +00:00
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If PR_SPEC_PRCTL is set, then the per-task control of the mitigation is
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2018-04-29 13:20:11 +00:00
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available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation
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misfeature will fail.
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2019-06-20 23:10:50 +00:00
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.. _set_spec_ctrl:
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2018-04-29 13:20:11 +00:00
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PR_SET_SPECULATION_CTRL
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-----------------------
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2018-04-29 13:20:11 +00:00
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PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, which
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is selected by arg2 of :manpage:`prctl(2)` per task. arg3 is used to hand
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in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or
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PR_SPEC_FORCE_DISABLE.
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Common error codes
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------------------
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======= =================================================================
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Value Meaning
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======= =================================================================
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EINVAL The prctl is not implemented by the architecture or unused
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prctl(2) arguments are not 0.
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2018-05-08 13:43:45 +00:00
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ENODEV arg2 is selecting a not supported speculation misfeature.
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======= =================================================================
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PR_SET_SPECULATION_CTRL error codes
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-----------------------------------
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======= =================================================================
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Value Meaning
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======= =================================================================
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0 Success
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ERANGE arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor
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PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE.
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ENXIO Control of the selected speculation misfeature is not possible.
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See PR_GET_SPECULATION_CTRL.
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EPERM Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller
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tried to enable it again.
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======= =================================================================
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Speculation misfeature controls
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-------------------------------
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- PR_SPEC_STORE_BYPASS: Speculative Store Bypass
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Invocations:
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* prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, 0, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE_NOEXEC, 0, 0);
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x86/speculation: Add prctl() control for indirect branch speculation
Add the PR_SPEC_INDIRECT_BRANCH option for the PR_GET_SPECULATION_CTRL and
PR_SET_SPECULATION_CTRL prctls to allow fine grained per task control of
indirect branch speculation via STIBP and IBPB.
Invocations:
Check indirect branch speculation status with
- prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, 0, 0, 0);
Enable indirect branch speculation with
- prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_ENABLE, 0, 0);
Disable indirect branch speculation with
- prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, 0, 0);
Force disable indirect branch speculation with
- prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);
See Documentation/userspace-api/spec_ctrl.rst.
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Casey Schaufler <casey.schaufler@intel.com>
Cc: Asit Mallick <asit.k.mallick@intel.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Jon Masters <jcm@redhat.com>
Cc: Waiman Long <longman9394@gmail.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Dave Stewart <david.c.stewart@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20181125185005.866780996@linutronix.de
2018-11-25 18:33:53 +00:00
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- PR_SPEC_INDIR_BRANCH: Indirect Branch Speculation in User Processes
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(Mitigate Spectre V2 style attacks against user processes)
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Invocations:
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* prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, 0, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_ENABLE, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, 0, 0);
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* prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);
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