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Date: Mon, 05 Apr 2021 12:03:05 -0500
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Alexey Gladkov <gladkov.alexey@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linux Containers <containers@...ts.linux-foundation.org>, linux-mm@...ck.org, Alexey Gladkov <legion@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Christian Brauner <christian.brauner@...ntu.com>, Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>, Kees Cook <keescook@...omium.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v9 0/8] Count rlimits in each user namespace
Alexey Gladkov <gladkov.alexey@...il.com> writes:
> 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
> ---------
> 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 (8):
> 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
>
Overall this looks good, and it is a very good sign that the automatic
testing bots have not found anything. I found a few little nits.
But thing are looking very good.
Eric
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