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[...] > If an external reference holds mm->mm_users > 1 across that mmput(), mm > outlives tsk. A new task allocated at the same slab address as tsk can > then match current == READ_ONCE(mm->rss_owner) in __add_mm_counter() and > perform unsynchronized non-atomic updates via > percpu_counter_tree_add_owner(), losing counter updates or triggering a > Bad rss-counter state report in check_mm(). > > Clearing mm->rss_owner alongside mm->owner on both error paths before > mmput() would avoid leaving the dangling pointer. Yes, both error paths leave rss_owner pointing to the child. This was found by reading the code, I did not reproduce it. No update would be lost, though. rss_owner is only compared with current, never dereferenced. At most one live task has that address at a time, so the plain counter still has a single writer. The other tasks update the atomic counter. The counters would still work. What breaks is the identity of the owner: a task which does not run on the mm would update the plain counter as its owner. The owner should stay the task running on the mm. Planned fix for the next round: clear mm->rss_owner next to mm->owner on both error paths. In copy_process() this is done only when rss_owner is the child, because with CLONE_VM the mm is the parent's. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com