Messages in this thread Patch in this message | | | From | Yang Jihong <> | Subject | [PATCH v2] perf/core: Fix data race between perf_event_set_output and perf_mmap_close | Date | Mon, 4 Jul 2022 20:00:06 +0800 |
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Data race exists between perf_event_set_output and perf_mmap_close. The scenario is as follows:
CPU1 CPU2 perf_mmap_close(event2) if (atomic_dec_and_test(&event2->rb->mmap_count) // mmap_count 1 -> 0 detach_rest = true; ioctl(event1, PERF_EVENT_IOC_SET_OUTPUT, event2) perf_event_set_output(event1, event2) if (!detach_rest) goto out_put; list_for_each_entry_rcu(event, &event2->rb->event_list, rb_entry) ring_buffer_attach(event, NULL) // because event1 has not been added to event2->rb->event_list, // event1->rb is not set to NULL in these loops
ring_buffer_attach(event1, event2->rb) list_add_rcu(&event1->rb_entry, &event2->rb->event_list)
The above data race causes a problem, that is, event1->rb is not NULL, but event1->rb->mmap_count is 0. If the perf_mmap interface is invoked for the fd of event1, the kernel keeps in the perf_mmap infinite loop:
again: mutex_lock(&event->mmap_mutex); if (event->rb) { <SNIP> 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. */ mutex_unlock(&event->mmap_mutex); goto again; } <SNIP>
This problem has occurred on the Syzkaller, causing the softlockup. The call stack is as follows: [ 3666.984385][ C2] watchdog: BUG: soft lockup - CPU#2 stuck for 23s! [syz-executor.2:32404] [ 3666.986137][ C2] Modules linked in: [ 3666.989581][ C2] CPU: 2 PID: 32404 Comm: syz-executor.2 Not tainted 5.10.0+ #4 [ 3666.990697][ C2] Hardware name: linux,dummy-virt (DT) [ 3666.992270][ C2] pstate: 80000005 (Nzcv daif -PAN -UAO -TCO BTYPE=--) [ 3666.993787][ C2] pc : __kasan_check_write+0x0/0x40 [ 3666.994841][ C2] lr : perf_mmap+0x3c8/0xf80 [ 3666.995661][ C2] sp : ffff00001011f8f0 [ 3666.996598][ C2] x29: ffff00001011f8f0 x28: ffff0000cf644868 [ 3666.998488][ C2] x27: ffff000012cad2c0 x26: 0000000000000000 [ 3666.999888][ C2] x25: 0000000000000001 x24: ffff000012cad298 [ 3667.003511][ C2] x23: 0000000000000000 x22: ffff000012cad000 [ 3667.005504][ C2] x21: ffff0000cf644818 x20: ffff0000cf6d2400 [ 3667.006891][ C2] x19: ffff0000cf6d24c0 x18: 0000000000000000 [ 3667.008295][ C2] x17: 0000000000000000 x16: 0000000000000000 [ 3667.009528][ C2] x15: 0000000000000000 x14: 0000000000000000 [ 3667.010658][ C2] x13: 0000000000000000 x12: ffff800002023f17 [ 3667.012169][ C2] x11: 1fffe00002023f16 x10: ffff800002023f16 [ 3667.013780][ C2] x9 : dfffa00000000000 x8 : ffff00001011f8b7 [ 3667.015265][ C2] x7 : 0000000000000001 x6 : ffff800002023f16 [ 3667.016683][ C2] x5 : ffff0000c0f36400 x4 : 0000000000000000 [ 3667.018078][ C2] x3 : ffffa00010000000 x2 : ffffa000119a0000 [ 3667.019343][ C2] x1 : 0000000000000004 x0 : ffff0000cf6d24c0 [ 3667.021276][ C2] Call trace: [ 3667.022598][ C2] __kasan_check_write+0x0/0x40 [ 3667.023666][ C2] __mmap_region+0x7a4/0xc90 [ 3667.024679][ C2] __do_mmap_mm+0x600/0xa20 [ 3667.025700][ C2] do_mmap+0x114/0x384 [ 3667.026583][ C2] vm_mmap_pgoff+0x138/0x230 [ 3667.027532][ C2] ksys_mmap_pgoff+0x1d8/0x570 [ 3667.028537][ C2] __arm64_sys_mmap+0xa4/0xd0 [ 3667.029597][ C2] el0_svc_common.constprop.0+0xf4/0x414 [ 3667.030682][ C2] do_el0_svc+0x50/0x11c [ 3667.031545][ C2] el0_svc+0x20/0x30 [ 3667.032368][ C2] el0_sync_handler+0xe4/0x1e0 [ 3667.033305][ C2] el0_sync+0x148/0x180
perf_mmap_close and ring_buffer_attach involve complicated lock recursion. The solution is to modify ring_buffer_attach function, that is, after new event is added to rb->event_list, check whether rb->mmap_count is 0, if the value is 0, another core performs perf_mmap_close operation on ring buffer during this period. In this case, we set event->rb to NULL.
Signed-off-by: Yang Jihong <yangjihong1@huawei.com> ---
Changes since v1: - Modify commit message description.
kernel/events/core.c | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c index 80782cddb1da..c67c070f7b39 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -5900,6 +5900,7 @@ static void ring_buffer_attach(struct perf_event *event, struct perf_buffer *rb) { struct perf_buffer *old_rb = NULL; + struct perf_buffer *new_rb = rb; unsigned long flags; WARN_ON_ONCE(event->parent); @@ -5928,6 +5929,20 @@ static void ring_buffer_attach(struct perf_event *event, spin_lock_irqsave(&rb->event_lock, flags); list_add_rcu(&event->rb_entry, &rb->event_list); + + /* + * When perf_mmap_close traverses rb->event_list during + * detach all other events, new event may not be added to + * rb->event_list, let's check again, if rb->mmap_count is 0, + * it indicates that perf_mmap_close is executed. + * Manually delete event from rb->event_list and + * set event->rb to null. + */ + if (!atomic_read(&rb->mmap_count)) { + list_del_rcu(&event->rb_entry); + new_rb = NULL; + } + spin_unlock_irqrestore(&rb->event_lock, flags); } @@ -5944,7 +5959,7 @@ static void ring_buffer_attach(struct perf_event *event, if (has_aux(event)) perf_event_stop(event, 0); - rcu_assign_pointer(event->rb, rb); + rcu_assign_pointer(event->rb, new_rb); if (old_rb) { ring_buffer_put(old_rb); @@ -11883,6 +11898,13 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event) goto unlock; } + /* + * If rb->mmap_count is 0, perf_mmap_close is being executed, + * the ring buffer is about to be unmapped and cannot be attached. + */ + if (rb && !atomic_read(&rb->mmap_count)) + goto unlock; + ring_buffer_attach(event, rb); ret = 0; -- 2.30.GIT
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