On Sat, Mar 16, 2019 at 05:38:54PM +0800, zhong jiang wrote: > On 2019/3/16 5:39, Andrea Arcangeli wrote: > > On Fri, Mar 08, 2019 at 03:10:08PM +0800, zhong jiang wrote: > >> I can reproduce the issue in arm64 qemu machine. The issue will leave after applying the > >> patch. > >> > >> Tested-by: zhong jiang <zhongjiang@xxxxxxxxxx> > > Thanks a lot for the quick testing! > > > >> Meanwhile, I just has a little doubt whether it is necessary to use RCU to free the task struct or not. > >> I think that mm->owner alway be NULL after failing to create to process. Because we call mm_clear_owner. > > I wish it was enough, but the problem is that the other CPU may be in > > the middle of get_mem_cgroup_from_mm() while this runs, and it would > > dereference mm->owner while it is been freed without the call_rcu > > affter we clear mm->owner. What prevents this race is the > As you had said, It would dereference mm->owner after we clear mm->owner. > > But after we clear mm->owner, mm->owner should be NULL. Is it right? > > And mem_cgroup_from_task will check the parameter. > you mean that it is possible after checking the parameter to clear the owner . > and the NULL pointer will trigger. :-( Dereference mm->owner didn't mean reading the value of the mm->owner pointer, it really means to dereference the value of the pointer. It's like below: get_mem_cgroup_from_mm() failing fork() ---- --- task = mm->owner mm->owner = NULL; free(mm->owner) *task /* use after free */ We didn't set mm->owner to NULL before, so the window for the race was larger, but setting mm->owner to NULL only hides the problem and it can still happen (albeit with a smaller window). If get_mem_cgroup_from_mm() can see at any time mm->owner not NULL, then the free of the task struct must be delayed until after rcu_read_unlock has returned in get_mem_cgroup_from_mm(). This is the standard RCU model, the freeing must be delayed until after the next quiescent point. BTW, both mm_update_next_owner() and mm_clear_owner() should have used WRITE_ONCE when they write to mm->owner, I can update that too but it's just to not to make assumptions that gcc does the right thing (and we still rely on gcc to do the right thing in other places) so that is just an orthogonal cleanup. Thanks, Andrea
