Re: [PATCH] cgroup: pids: use {READ,WRITE}_ONCE for pids->limit operations

[Tejun Heo]

Hello, Aleksa.

On Tue, Oct 15, 2019 at 02:59:31AM +1100, Aleksa Sarai wrote:
> On 2019-10-14, Tejun Heo <tj@xxxxxxxxxx> 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