Messages in this thread Patch in this message | | | Date | Thu, 29 May 2014 19:00:24 +0200 | From | Peter Zijlstra <> | Subject | Re: perf: use after free in perf_remove_from_context |
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On Thu, May 29, 2014 at 06:50:57PM +0200, Peter Zijlstra wrote: > On Thu, May 29, 2014 at 12:44:23PM -0400, Sasha Levin wrote: > > On 05/29/2014 11:07 AM, Peter Zijlstra wrote: > > > On Thu, May 29, 2014 at 10:47:09AM -0400, Sasha Levin wrote: > > >> It doesn't work out well because we later lock a mutex in sync_child_event(). > > >> > > > > > > Urgh, right you are. I'll go stare at it more. It shouldn't have > > > mattered, because the mutex we take just before should ensure existence, > > > but.. you know.. :-) > > > > > > > So the only caller to sync_child_event() is that loop. According to what you said > > it should be safe to remove that mutex lock, but doing that triggers a list > > corruption: > > > > [ 1204.341887] WARNING: CPU: 20 PID: 12839 at lib/list_debug.c:62 __list_del_entry+0xa1/0xe0() > > [ 1204.347597] list_del corruption. next->prev should be ffff8806ca68b108, but was ffff88051a67c398 > > [...] > > > > I don't see how that would happen :/ > > No, what I said is that the mutex in perf_event_exit_task() should be > sufficient to guard the list iteration calling __perf_event_exit_task(). > > Ading the RCU was a bit of paranoia..
Hmm, so can you try this..
While that mutex should guard the elements, it doesn't guard against the use-after-free that's from list_for_each_entry_rcu(). __perf_event_exit_task() can actually free the event.
And because list addition/deletion is guarded by both ctx->mutex and ctx->lock, holding ctx->mutex is sufficient for reading the list, so we don't actually need the rcu list iteration.
--- kernel/events/core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c index 689237a0c5e8..2bb45d483325 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -7432,7 +7432,7 @@ __perf_event_exit_task(struct perf_event *child_event, static void perf_event_exit_task_context(struct task_struct *child, int ctxn) { - struct perf_event *child_event; + struct perf_event *child_event, *next; struct perf_event_context *child_ctx; unsigned long flags; @@ -7486,7 +7486,7 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn) */ mutex_lock(&child_ctx->mutex); - list_for_each_entry_rcu(child_event, &child_ctx->event_list, event_entry) + list_for_each_entry_safe(child_event, next, &child_ctx->event_list, event_entry) __perf_event_exit_task(child_event, child_ctx, child); mutex_unlock(&child_ctx->mutex);
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