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From: Tejun Heo <tj@kernel.org>
To: Aleksa Sarai <cyphar@cyphar.com>
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: Mon, 14 Oct 2019 09:33:07 -0700	[thread overview]
Message-ID: <20191014163307.GG18794@devbig004.ftw2.facebook.com> (raw)
In-Reply-To: <20191014155931.jl7idjebhqxb3ck3@yavin.dot.cyphar.com>

Hello, Aleksa.

On Tue, Oct 15, 2019 at 02:59:31AM +1100, Aleksa Sarai wrote:
> 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.

READ/WRITE_ONCE provides protection against compiler generating
multiple accesses for a single operation.  It won't prevent split
writes / reads of 64bit variables on 32bit machines.  For that, you'd
have to switch them to atomic64_t's.

Thanks.

-- 
tejun

  reply	other threads:[~2019-10-14 16:33 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
2019-10-14 16:33     ` Tejun Heo [this message]
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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