Patch "perf: Fix perf_event_validate_size() lockdep splat" has been added to the 4.19-stable tree

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This is a note to let you know that I've just added the patch titled

    perf: Fix perf_event_validate_size() lockdep splat

to the 4.19-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-perf_event_validate_size-lockdep-splat.patch
and it can be found in the queue-4.19 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From 7e2c1e4b34f07d9aa8937fab88359d4a0fce468e Mon Sep 17 00:00:00 2001
From: Mark Rutland <mark.rutland@xxxxxxx>
Date: Fri, 15 Dec 2023 11:24:50 +0000
Subject: perf: Fix perf_event_validate_size() lockdep splat

From: Mark Rutland <mark.rutland@xxxxxxx>

commit 7e2c1e4b34f07d9aa8937fab88359d4a0fce468e upstream.

When lockdep is enabled, the for_each_sibling_event(sibling, event)
macro checks that event->ctx->mutex is held. When creating a new group
leader event, we call perf_event_validate_size() on a partially
initialized event where event->ctx is NULL, and so when
for_each_sibling_event() attempts to check event->ctx->mutex, we get a
splat, as reported by Lucas De Marchi:

  WARNING: CPU: 8 PID: 1471 at kernel/events/core.c:1950 __do_sys_perf_event_open+0xf37/0x1080

This only happens for a new event which is its own group_leader, and in
this case there cannot be any sibling events. Thus it's safe to skip the
check for siblings, which avoids having to make invasive and ugly
changes to for_each_sibling_event().

Avoid the splat by bailing out early when the new event is its own
group_leader.

Fixes: 382c27f4ed28f803 ("perf: Fix perf_event_validate_size()")
Closes: https://lore.kernel.org/lkml/20231214000620.3081018-1-lucas.demarchi@xxxxxxxxx/
Closes: https://lore.kernel.org/lkml/ZXpm6gQ%2Fd59jGsuW@xxxxxxxxxxxxxxxx/
Reported-by: Lucas De Marchi <lucas.demarchi@xxxxxxxxx>
Reported-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20231215112450.3972309-1-mark.rutland@xxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/events/core.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1834,6 +1834,16 @@ static bool perf_event_validate_size(str
 				   group_leader->nr_siblings + 1) > 16*1024)
 		return false;
 
+	/*
+	 * When creating a new group leader, group_leader->ctx is initialized
+	 * after the size has been validated, but we cannot safely use
+	 * for_each_sibling_event() until group_leader->ctx is set. A new group
+	 * leader cannot have any siblings yet, so we can safely skip checking
+	 * the non-existent siblings.
+	 */
+	if (event == group_leader)
+		return true;
+
 	for_each_sibling_event(sibling, group_leader) {
 		if (__perf_event_read_size(sibling->attr.read_format,
 					   group_leader->nr_siblings + 1) > 16*1024)


Patches currently in stable-queue which might be from mark.rutland@xxxxxxx are

queue-4.19/perf-fix-perf_event_validate_size-lockdep-splat.patch




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