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Date:   Fri, 20 May 2022 20:38:06 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     x86@kernel.org, linux-kernel@vger.kernel.org
Cc:     nslusarek@gmx.net, Linus Torvalds <torvalds@linux-foundation.org>
Subject: [PATCH] perf: Fix sys_perf_event_open() race against self
Message-ID: <20220520183806.GV2578@worktop.programming.kicks-ass.net>
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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: f63a8daa5812 ("perf: Fix event->ctx locking")
Reported-by: Norbert Slusarek <nslusarek@gmx.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/core.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -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;