[PATCH] perf: Fix the aux_output group inheritance fix

[Alexander Shishkin]

Commit

  f733c6b508bc ("perf/core: Fix inheritance of aux_output groups")

adds a null pointer dereference in case inherit_group() races with
perf_release(), which causes the below.

> BUG: kernel NULL pointer dereference, address: 000000000000010b
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 3b203b067 P4D 3b203b067 PUD 3b2040067 PMD 0
> Oops: 0000 [#1] SMP KASAN
> CPU: 0 PID: 315 Comm: exclusive-group Tainted: G B 5.4.0-rc3-00181-g72e1839403cb-dirty #878
> RIP: 0010:perf_get_aux_event+0x86/0x270
> Call Trace:
>  ? __perf_read_group_add+0x3b0/0x3b0
>  ? __kasan_check_write+0x14/0x20
>  ? __perf_event_init_context+0x154/0x170
>  inherit_task_group.isra.0.part.0+0x14b/0x170
>  perf_event_init_task+0x296/0x4b0

Fix this by skipping over events that are getting closed, in the
inheritance path.

Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
---
 kernel/events/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index b1aa5237052b..8ff1218e91b1 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -12129,7 +12129,7 @@ static int inherit_group(struct perf_event *parent_event,
 		if (IS_ERR(child_ctr))
 			return PTR_ERR(child_ctr);
 
-		if (sub->aux_event == parent_event &&
+		if (sub->aux_event == parent_event && child_ctr &&
 		    !perf_get_aux_event(child_ctr, leader))
 			return -EINVAL;
 	}
-- 
2.24.0.rc1