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Preface ------- These patches are for binding the rlimit counters to a user in user namespace. This patch set can be applied on top of: git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git v5.12-rc4 Problem ------- The RLIMIT_NPROC, RLIMIT_MEMLOCK, RLIMIT_SIGPENDING, RLIMIT_MSGQUEUE rlimits implementation places the counters in user_struct [1]. These limits are global between processes and persists for the lifetime of the process, even if processes are in different user namespaces. To illustrate the impact of rlimits, let's say there is a program that does not fork. Some service-A wants to run this program as user X in multiple containers. Since the program never fork the service wants to set RLIMIT_NPROC=1. service-A \- program (uid=1000, container1, rlimit_nproc=1) \- program (uid=1000, container2, rlimit_nproc=1) The service-A sets RLIMIT_NPROC=1 and runs the program in container1. When the service-A tries to run a program with RLIMIT_NPROC=1 in container2 it fails since user X already has one running process. The problem is not that the limit from container1 affects container2. The problem is that limit is verified against the global counter that reflects the number of processes in all containers. This problem can be worked around by using different users for each container but in this case we face a different problem of uid mapping when transferring files from one container to another. Eric W. Biederman mentioned this issue [2][3]. Introduced changes ------------------ To address the problem, we bind rlimit counters to user namespace. Each counter reflects the number of processes in a given uid in a given user namespace. The result is a tree of rlimit counters with the biggest value at the root (aka init_user_ns). The limit is considered exceeded if it's exceeded up in the tree. [1]: https://lore.kernel.org/containers/87imd2incs.fsf@x220.int.ebiederm.org/ [2]: https://lists.linuxfoundation.org/pipermail/containers/2020-August/042096.html [3]: https://lists.linuxfoundation.org/pipermail/containers/2020-October/042524.html Changelog --------- v11: * Revert most of changes in signal.c to fix performance issues and remove unnecessary memory allocations. * Fixed issue found by lkp robot (again). v10: * Fixed memory leak in __sigqueue_alloc. * Handled an unlikely situation when all consumers will return ucounts at once. * Addressed other review comments from Eric W. Biederman. v9: * Used a negative value to check that the ucounts->count is close to overflow. * Rebased onto v5.12-rc4. v8: * Used atomic_t for ucounts reference counting. Also added counter overflow check (thanks to Linus Torvalds for the idea). * Fixed other issues found by lkp-tests project in the patch that Reimplements RLIMIT_MEMLOCK on top of ucounts. v7: * Fixed issues found by lkp-tests project in the patch that Reimplements RLIMIT_MEMLOCK on top of ucounts. v6: * Fixed issues found by lkp-tests project. * Rebased onto v5.11. v5: * Split the first commit into two commits: change ucounts.count type to atomic_long_t and add ucounts to cred. These commits were merged by mistake during the rebase. * The __get_ucounts() renamed to alloc_ucounts(). * The cred.ucounts update has been moved from commit_creds() as it did not allow to handle errors. * Added error handling of set_cred_ucounts(). v4: * Reverted the type change of ucounts.count to refcount_t. * Fixed typo in the kernel/cred.c v3: * Added get_ucounts() function to increase the reference count. The existing get_counts() function renamed to __get_ucounts(). * The type of ucounts.count changed from atomic_t to refcount_t. * Dropped 'const' from set_cred_ucounts() arguments. * Fixed a bug with freeing the cred structure after calling cred_alloc_blank(). * Commit messages have been updated. * Added selftest. v2: * RLIMIT_MEMLOCK, RLIMIT_SIGPENDING and RLIMIT_MSGQUEUE are migrated to ucounts. * Added ucounts for pair uid and user namespace into cred. * Added the ability to increase ucount by more than 1. v1: * After discussion with Eric W. Biederman, I increased the size of ucounts to atomic_long_t. * Added ucount_max to avoid the fork bomb. -- Alexey Gladkov (9): Increase size of ucounts to atomic_long_t Add a reference to ucounts for each cred Use atomic_t for ucounts reference counting Reimplement RLIMIT_NPROC on top of ucounts Reimplement RLIMIT_MSGQUEUE on top of ucounts Reimplement RLIMIT_SIGPENDING on top of ucounts Reimplement RLIMIT_MEMLOCK on top of ucounts kselftests: Add test to check for rlimit changes in different user namespaces ucounts: Set ucount_max to the largest positive value the type can hold fs/exec.c | 6 +- fs/hugetlbfs/inode.c | 16 +- fs/proc/array.c | 2 +- include/linux/cred.h | 4 + include/linux/hugetlb.h | 4 +- include/linux/mm.h | 4 +- include/linux/sched/user.h | 7 - include/linux/shmem_fs.h | 2 +- include/linux/signal_types.h | 4 +- include/linux/user_namespace.h | 31 +++- ipc/mqueue.c | 40 ++--- ipc/shm.c | 26 +-- kernel/cred.c | 50 +++++- kernel/exit.c | 2 +- kernel/fork.c | 18 +- kernel/signal.c | 25 +-- kernel/sys.c | 14 +- kernel/ucount.c | 116 ++++++++++--- kernel/user.c | 3 - kernel/user_namespace.c | 9 +- mm/memfd.c | 4 +- mm/mlock.c | 22 ++- mm/mmap.c | 4 +- mm/shmem.c | 10 +- tools/testing/selftests/Makefile | 1 + tools/testing/selftests/rlimits/.gitignore | 2 + tools/testing/selftests/rlimits/Makefile | 6 + tools/testing/selftests/rlimits/config | 1 + .../selftests/rlimits/rlimits-per-userns.c | 161 ++++++++++++++++++ 29 files changed, 467 insertions(+), 127 deletions(-) create mode 100644 tools/testing/selftests/rlimits/.gitignore create mode 100644 tools/testing/selftests/rlimits/Makefile create mode 100644 tools/testing/selftests/rlimits/config create mode 100644 tools/testing/selftests/rlimits/rlimits-per-userns.c Link: https://lkml.kernel.org/r/cover.1619094428.git.legion@kernel.org |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.