linux-kernel - Re: [PATCH] cgroup: pids: use {READ,WRITE}

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Message-ID: <20191014155931.jl7idjebhqxb3ck3@yavin.dot.cyphar.com>
Date:   Tue, 15 Oct 2019 02:59:31 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] cgroup: pids: use {READ,WRITE}_ONCE for pids->limit
 operations

On 2019-10-14, Tejun Heo <tj@...nel.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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