perf: Reject exited events as group leaders

[ Upstream commit fa091f4 ]

perf_event_remove_on_exec() sets remove-on-exec events to the EXIT state
and detaches their group relationships.  The event's file descriptor can
remain open, however, and perf_event_open() currently accepts that event
as a group leader because its early validation rejects only REVOKED and
DEAD events.

A new sibling can consequently be linked to the detached leader.  When
the leader is closed, perf_group_detach() observes that its
PERF_ATTACH_GROUP bit is already clear and skips the new sibling.  The
sibling then retains a group_leader pointer to the freed event.

Reject group leaders in the EXIT state.  Perform the check while holding
the shared context mutex so that an exec in the target task cannot detach
the leader between validation and group attachment.

[peterz: make the earlier test fully consistent]
Fixes: 037a3c4 ("perf/core: Detach event groups during remove_on_exec")
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260806205655.75722-1-kylebot@openai.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: kylebot-oai

kernel/events/core.c

@@ -12939,6 +12939,10 @@ SYSCALL_DEFINE5(perf_event_open,
 		if (err)
 			goto err_fd;
 		group_leader = fd_file(group)->private_data;
+		if (group_leader->state <= PERF_EVENT_STATE_EXIT) {
+			err = -ENODEV;
+			goto err_group_fd;
+		}
 		if (flags & PERF_FLAG_FD_OUTPUT)
 			output_event = group_leader;
 		if (flags & PERF_FLAG_FD_NO_GROUP)
@@ -13066,6 +13070,12 @@ SYSCALL_DEFINE5(perf_event_open,
 		if (group_leader->ctx != ctx)
 			goto err_locked;
 
+		/* Recheck under ctx::mutex to serialize against remove-on-exec. */
+		if (group_leader->state <= PERF_EVENT_STATE_EXIT) {
+			err = -ENODEV;
+			goto err_locked;
+		}
+
 		/*
 		 * Only a group leader can be exclusive or pinned
 		 */