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From e085354dde254bc6c83ee604ea66c2b36f9f9067 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 20 May 2022 20:38:06 +0200
Subject: [PATCH] perf: Fix sys_perf_event_open() race against self
commit 3ac6487e584a1eb54071dbe1212e05b884136704 upstream.
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: f63a8daa5812 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/events/core.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 7c891a8eb32341..565910de92e9b6 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -12283,6 +12283,9 @@ 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;
@@ -12348,6 +12351,7 @@ SYSCALL_DEFINE5(perf_event_open,
} else {
perf_event_ctx_unlock(group_leader, gctx);
move_group = 0;
+ goto not_move_group;
}
}
@@ -12364,7 +12368,17 @@ 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;
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