On 2019-10-14, Tejun Heo <t...@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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