This is a note to let you know that I've just added the patch titled perf: Fix sys_perf_event_open() race against self to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: perf-fix-sys_perf_event_open-race-against-self.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 3ac6487e584a1eb54071dbe1212e05b884136704 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Date: Fri, 20 May 2022 20:38:06 +0200 Subject: perf: Fix sys_perf_event_open() race against self From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/events/core.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) --- 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; Patches currently in stable-queue which might be from peterz@xxxxxxxxxxxxx are queue-5.15/crypto-x86-chacha20-avoid-spurious-jumps-to-other-fu.patch queue-5.15/perf-fix-sys_perf_event_open-race-against-self.patch