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(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c index 88dd1398ae88..ba66ea3ca705 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -5719,10 +5719,10 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)

if (!atomic_inc_not_zero(&event->rb->mmap_count)) { /* - * Raced against perf_mmap_close() through - * perf_event_set_output(). Try again, hope for better - * luck. + * Raced against perf_mmap_close(); remove the + * event and try again. */ + ring_buffer_attach(event, NULL); mutex_unlock(&event->mmap_mutex); goto again; } @@ -10396,14 +10396,25 @@ static int perf_copy_attr(struct perf_event_attr __user *uattr, goto out; }

+static void mutex_lock_double(struct mutex *a, struct mutex *b) +{ + if (b < a) + swap(a, b); + + mutex_lock(a); + mutex_lock_nested(b, SINGLE_DEPTH_NESTING); +} + static int perf_event_set_output(struct perf_event *event, struct perf_event *output_event) { struct ring_buffer *rb = NULL; int ret = -EINVAL;

- if (!output_event) + if (!output_event) { + mutex_lock(&event->mmap_mutex); goto set; + }

/* don't allow circular references */ if (event == output_event) @@ -10441,8 +10452,15 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event) event->pmu != output_event->pmu) goto out;

+ /* + * Hold both mmap_mutex to serialize against perf_mmap_close(). Since + * output_event is already on rb->event_list, and the list iteration + * restarts after every removal, it is guaranteed this new event is + * observed *OR* if output_event is already removed, it's guaranteed we + * observe !rb->mmap_count. + */ + mutex_lock_double(&event->mmap_mutex, &output_event->mmap_mutex); set: - mutex_lock(&event->mmap_mutex); /* Can't redirect output if we've got an active mmap() */ if (atomic_read(&event->mmap_count)) goto unlock; @@ -10452,6 +10470,12 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event) rb = ring_buffer_get(output_event); if (!rb) goto unlock; + + /* did we race against perf_mmap_close() */ + if (!atomic_read(&rb->mmap_count)) { + ring_buffer_put(rb); + goto unlock; + } }

ring_buffer_attach(event, rb); @@ -10459,20 +10483,13 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event) ret = 0; unlock: mutex_unlock(&event->mmap_mutex); + if (output_event) + mutex_unlock(&output_event->mmap_mutex);

out: return ret; }

-static void mutex_lock_double(struct mutex *a, struct mutex *b) -{ - if (b < a) - swap(a, b); - - mutex_lock(a); - mutex_lock_nested(b, SINGLE_DEPTH_NESTING); -} - static int perf_event_set_clock(struct perf_event *event, clockid_t clk_id) { bool nmi_safe = false;