On Tue, Jul 26, 2022 at 09:33:56AM +0800, Yang Jihong wrote:
commit 68e3c69803dada336893640110cb87221bb01dcf upstream.
Yang Jihing reported a race between perf_event_set_output() and perf_mmap_close():
CPU1 CPU2 perf_mmap_close(e2) if (atomic_dec_and_test(&e2->rb->mmap_count)) // 1 - > 0 detach_rest = true ioctl(e1, IOC_SET_OUTPUT, e2) perf_event_set_output(e1, e2) ... list_for_each_entry_rcu(e, &e2->rb->event_list, rb_entry) ring_buffer_attach(e, NULL); // e1 isn't yet added and // therefore not detached ring_buffer_attach(e1, e2->rb) list_add_rcu(&e1->rb_entry, &e2->rb->event_list)After this; e1 is attached to an unmapped rb and a subsequent perf_mmap() will loop forever more:
again: mutex_lock(&e->mmap_mutex); if (event->rb) { ... if (!atomic_inc_not_zero(&e->rb->mmap_count)) { ... mutex_unlock(&e->mmap_mutex); goto again; } }The loop in perf_mmap_close() holds e2->mmap_mutex, while the attach in perf_event_set_output() holds e1->mmap_mutex. As such there is no serialization to avoid this race.
Change perf_event_set_output() to take both e1->mmap_mutex and e2->mmap_mutex to alleviate that problem. Additionally, have the loop in perf_mmap() detach the rb directly, this avoids having to wait for the concurrent perf_mmap_close() to get around to doing it to make progress.
Fixes: 9bb5d40cd93c ("perf: Fix mmap() accounting hole") Reported-by: Yang Jihong yangjihong1@huawei.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Tested-by: Yang Jihong yangjihong1@huawei.com Link: https://lkml.kernel.org/r/YsQ3jm2GR38SW7uD@worktop.programming.kicks-ass.net [YJH: backport to 4.19: adjusted context] Signed-off-by: Yang Jihong yangjihong1@huawei.com
kernel/events/core.c | 45 ++++++++++++++++++++++++++++++-------------- 1 file changed, 31 insertions(+), 14 deletions(-)
Sasha already queued this up, thanks.
greg k-h