On Wed, Oct 09, 2019 at 01:48:27PM +0200, Marco Elver wrote:
On Wed, 9 Oct 2019 at 13:31, Christian Brauner christian.brauner@ubuntu.com wrote:
When assiging and testing taskstats in taskstats_exit() there's a race when writing and reading sig->stats when a thread-group with more than one thread exits:
cpu0: thread catches fatal signal and whole thread-group gets taken down do_exit() do_group_exit() taskstats_exit() taskstats_tgid_alloc() The tasks reads sig->stats without holding sighand lock seeing garbage.
cpu1: task calls exit_group() do_exit() do_group_exit() taskstats_exit() taskstats_tgid_alloc() The task takes sighand lock and assigns new stats to sig->stats.
Fix this by using smp_load_acquire() and smp_store_release().
Reported-by: syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation") Cc: stable@vger.kernel.org Signed-off-by: Christian Brauner christian.brauner@ubuntu.com Reviewed-by: Dmitry Vyukov dvyukov@google.com
/* v1 */ Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.co...
/* v2 */ Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@ubuntu.com
- Dmitry Vyukov dvyukov@google.com, Marco Elver elver@google.com:
- fix the original double-checked locking using memory barriers
/* v3 */ Link: https://lore.kernel.org/r/20191007110117.1096-1-christian.brauner@ubuntu.com...
- Andrea Parri parri.andrea@gmail.com:
- document memory barriers to make checkpatch happy
/* v4 */
- Andrea Parri parri.andrea@gmail.com:
- use smp_load_acquire(), not READ_ONCE()
- update commit message
Acked-by: Marco Elver elver@google.com
Note that this now looks almost like what I suggested, except the
Right, I think we all just needed to get our heads clear about what exactly is happening here. This codepath is not a very prominent one. :)
return at the end of the function is accessing sig->stats again. In this case, it seems it's fine assuming sig->stats cannot be written elsewhere. Just wanted to point it out to make sure it's considered.
Yes, I considered that but thanks for mentioning it.
Note that this patch has a bug. It should be smp_load_acquire(&sig->stats) and not smp_load_acquire(sig->stats). I accidently didn't automatically recompile the patchset after the last change I made. Andrea thankfully caught this.
Thanks! Christian