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

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Message-ID: <20191016083218.ttsaqnxpjh5i5bgv@yavin.dot.cyphar.com>
Date:   Wed, 16 Oct 2019 19:32:19 +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:
> Hello, Aleksa.
> 
> On Tue, Oct 15, 2019 at 02:59:31AM +1100, Aleksa Sarai wrote:
> > 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.
> 
> 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.

Maybe I'm misunderstanding Documentation/atomic_t.txt, but it looks to
me like it's explicitly saying that I shouldn't use atomic64_t if I'm
just using it for fetching and assignment.

> The non-RMW ops are (typically) regular LOADs and STOREs and are
> canonically implemented using READ_ONCE(), WRITE_ONCE(),
> smp_load_acquire() and smp_store_release() respectively. Therefore, if
> you find yourself only using the Non-RMW operations of atomic_t, you
> do not in fact need atomic_t at all and are doing it wrong.

As for 64-bit on 32-bit machines -- that is a separate issue, but from
[1] it seems to me like there are more problems that *_ONCE() fixes than
just split reads and writes.

[1]: https://lwn.net/Articles/793253/

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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