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perf: Fix sys_perf_event_open() race against self

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Norbert reported that it's possible to race sys_perf_event_open() such
that the looser ends up in another context from the group leader,
triggering many WARNs.

The move_group case checks for races against itself, but the
!move_group case doesn't, seemingly relying on the previous
group_leader->ctx == ctx check. However, that check is racy due to not
holding any locks at that time.

Therefore, re-check the result after acquiring locks and bailing
if they no longer match.

Additionally, clarify the not_move_group case from the
move_group-vs-move_group race.

Fixes: f63a8daa58 ("perf: Fix event->ctx locking")
Reported-by: Norbert Slusarek <nslusarek@gmx.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Peter Zijlstra 2022-05-20 20:38:06 +02:00 committed by Linus Torvalds
parent 3b5e1590a2
commit 3ac6487e58
1 changed files with 14 additions and 0 deletions
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14
kernel/events/core.c
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@ -12217,6 +12217,9 @@ SYSCALL_DEFINE5(perf_event_open,
* Do not allow to attach to a group in a different task
* or CPU context. If we're moving SW events, we'll fix
* this up later, so allow that.
*
* Racy, not holding group_leader->ctx->mutex, see comment with
* perf_event_ctx_lock().
*/
if (!move_group && group_leader->ctx != ctx)
goto err_context;
@ -12282,6 +12285,7 @@ SYSCALL_DEFINE5(perf_event_open,
} else {
perf_event_ctx_unlock(group_leader, gctx);
move_group = 0;
goto not_move_group;
}
}
@ -12298,7 +12302,17 @@ SYSCALL_DEFINE5(perf_event_open,
}
} else {
mutex_lock(&ctx->mutex);
/*
* Now that we hold ctx->lock, (re)validate group_leader->ctx == ctx,
* see the group_leader && !move_group test earlier.
*/
if (group_leader && group_leader->ctx != ctx) {
err = -EINVAL;
goto err_locked;
}
}
not_move_group:
if (ctx->task == TASK_TOMBSTONE) {
err = -ESRCH;