Messages in this thread | | | Date | Mon, 28 Jun 2021 07:30:03 +0200 | From | Ingo Molnar <> | Subject | Re: [GIT PULL] sigqueue cache fix |
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* Ingo Molnar <mingo@kernel.org> wrote:
> > * Ingo Molnar <mingo@kernel.org> wrote: > > > - Producer <-> consumer: this is the most interesting race, and I think > > it's unsafe in theory, because the producer doesn't make sure that any > > previous writes to the actual queue entry (struct sigqueue *q) have > > reached storage before the new 'free' entry is advertised to consumers. > > > > So in principle CPU#0 could see a new sigqueue entry and use it, before > > it's fully freed. > > > > In *practice* it's probably safe by accident (or by undocumented > > intent), because there's an atomic op we have shortly before putting the > > queue entry into the sigqueue_cache, in __sigqueue_free(): > > > > if (atomic_dec_and_test(&q->user->sigpending)) > > free_uid(q->user); > > > > And atomic_dec_and_test() implies a full barrier - although I haven't > > found the place where we document it and > > Documentation/memory-ordering.txt is silent on it. We should probably > > fix that too. > > > > At minimum the patch adding the ->sigqueue_cache should include a > > well-documented race analysis firmly documenting the implicit barrier after > > the atomic_dec_and_test(). > > I just realized that even with that implicit full barrier it's not safe: > the producer uses q->user after the atomic_dec_and_test(). That access is > not serialized with the later write to ->sigqueue_cache - and another CPU > might see that entry and use the ->sigqueue_cache and corrupt q->user ... > > So I think this code might have a real race on LL/SC platforms and we'll > need an smp_mb() in sigqueue_cache_or_free()?
pps. free_uid() happens to have an implicit barrier via refcount_dec_and_lock_irqsave():
void free_uid(struct user_struct *up) { unsigned long flags;
if (!up) return;
if (refcount_dec_and_lock_irqsave(&up->__count, &uidhash_lock, &flags)) free_user(up, flags);
So the q->user read in __sigqueue_free() appears to be implicitly serialized by free_uid() with the later write of 'q' to ->sigqueue_cache.
This needs to be robustly documented though.
Thanks,
Ingo
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