From: Aleksa Sarai <cyphar@cyphar.com>
To: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>,
Johannes Weiner <hannes@cmpxchg.org>,
cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org
Subject: Re: [PATCH] cgroup: pids: use {READ,WRITE}_ONCE for pids->limit operations
Date: Tue, 15 Oct 2019 02:59:31 +1100 [thread overview]
Message-ID: <20191014155931.jl7idjebhqxb3ck3@yavin.dot.cyphar.com> (raw)
In-Reply-To: <20191014154136.GF18794@devbig004.ftw2.facebook.com>
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On 2019-10-14, Tejun Heo <tj@kernel.org> wrote:
> On Sat, Oct 12, 2019 at 12:05:39PM +1100, Aleksa Sarai wrote:
> > Because pids->limit can be changed concurrently (but we don't want to
> > take a lock because it would be needlessly expensive), use the
> > appropriate memory barriers.
>
> I can't quite tell what problem it's fixing. Can you elaborate a
> scenario where the current code would break that your patch fixes?
As far as I can tell, not using *_ONCE() here means that if you had a
process changing pids->limit from A to B, a process might be able to
temporarily exceed pids->limit -- because pids->limit accesses are not
protected by mutexes and the C compiler can produce confusing
intermediate values for pids->limit[1].
But this is more of a correctness fix than one fixing an actually
exploitable bug -- given the kernel memory model work, it seems like a
good idea to just use READ_ONCE() and WRITE_ONCE() for shared memory
access.
[1]: https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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next prev parent reply other threads:[~2019-10-14 15:59 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-12 1:05 [PATCH] cgroup: pids: use {READ,WRITE}_ONCE for pids->limit operations Aleksa Sarai
2019-10-14 15:41 ` Tejun Heo
2019-10-14 15:59 ` Aleksa Sarai [this message]
2019-10-14 16:33 ` Tejun Heo
2019-10-16 8:32 ` Aleksa Sarai
2019-10-16 14:27 ` Tejun Heo
2019-10-16 15:29 ` Aleksa Sarai
2019-10-16 15:32 ` Tejun Heo
2019-10-16 15:35 ` Aleksa Sarai
2019-10-16 15:54 ` Tejun Heo
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