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.



commit e31ebccbc85ea67281fe2cbde5afa452044c17b4
Author: Mark Rutland <mark.rutland@xxxxxxx>
Date:   Fri Dec 15 11:24:50 2023 +0000

    perf: Fix perf_event_validate_size() lockdep splat
    
    [ Upstream commit 7e2c1e4b34f07d9aa8937fab88359d4a0fce468e ]
    
    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: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4182e265176dc..1f215aa8f30d6 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1834,6 +1834,16 @@ static bool perf_event_validate_size(struct perf_event *event)
 				   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)




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