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Message-Id: <20250414132610.677644-6-sashal@kernel.org>
Date: Mon, 14 Apr 2025 09:25:42 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Gabriel Shahrouzi <gshahrouzi@...il.com>,
	syzbot+ff3aa851d46ab82953a3@...kaller.appspotmail.com,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ravi Bangoria <ravi.bangoria@....com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Sasha Levin <sashal@...nel.org>,
	mingo@...hat.com,
	acme@...nel.org,
	namhyung@...nel.org,
	linux-perf-users@...r.kernel.org
Subject: [PATCH AUTOSEL 6.14 06/34] perf/core: Fix WARN_ON(!ctx) in __free_event() for partial init

From: Gabriel Shahrouzi <gshahrouzi@...il.com>

[ Upstream commit 0ba3a4ab76fd3367b9cb680cad70182c896c795c ]

Move the get_ctx(child_ctx) call and the child_event->ctx assignment to
occur immediately after the child event is allocated. Ensure that
child_event->ctx is non-NULL before any subsequent error path within
inherit_event calls free_event(), satisfying the assumptions of the
cleanup code.

Details:

There's no clear Fixes tag, because this bug is a side-effect of
multiple interacting commits over time (up to 15 years old), not
a single regression.

The code initially incremented refcount then assigned context
immediately after the child_event was created. Later, an early
validity check for child_event was added before the
refcount/assignment. Even later, a WARN_ON_ONCE() cleanup check was
added, assuming event->ctx is valid if the pmu_ctx is valid.
The problem is that the WARN_ON_ONCE() could trigger after the initial
check passed but before child_event->ctx was assigned, violating its
precondition. The solution is to assign child_event->ctx right after
its initial validation. This ensures the context exists for any
subsequent checks or cleanup routines, resolving the WARN_ON_ONCE().

To resolve it, defer the refcount update and child_event->ctx assignment
directly after child_event->pmu_ctx is set but before checking if the
parent event is orphaned. The cleanup routine depends on
event->pmu_ctx being non-NULL before it verifies event->ctx is
non-NULL. This also maintains the author's original intent of passing
in child_ctx to find_get_pmu_context before its refcount/assignment.

[ mingo: Expanded the changelog from another email by Gabriel Shahrouzi. ]

Reported-by: syzbot+ff3aa851d46ab82953a3@...kaller.appspotmail.com
Signed-off-by: Gabriel Shahrouzi <gshahrouzi@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ravi Bangoria <ravi.bangoria@....com>
Cc: Kan Liang <kan.liang@...ux.intel.com>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Link: https://lore.kernel.org/r/20250405203036.582721-1-gshahrouzi@gmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ff3aa851d46ab82953a3
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 kernel/events/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index f6cf17929bb98..6503ff28a782c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -13729,6 +13729,9 @@ inherit_event(struct perf_event *parent_event,
 	if (IS_ERR(child_event))
 		return child_event;
 
+	get_ctx(child_ctx);
+	child_event->ctx = child_ctx;
+
 	pmu_ctx = find_get_pmu_context(child_event->pmu, child_ctx, child_event);
 	if (IS_ERR(pmu_ctx)) {
 		free_event(child_event);
@@ -13751,8 +13754,6 @@ inherit_event(struct perf_event *parent_event,
 		return NULL;
 	}
 
-	get_ctx(child_ctx);
-
 	/*
 	 * Make the child state follow the state of the parent event,
 	 * not its attr.disabled bit.  We hold the parent's mutex,
@@ -13773,7 +13774,6 @@ inherit_event(struct perf_event *parent_event,
 		local64_set(&hwc->period_left, sample_period);
 	}
 
-	child_event->ctx = child_ctx;
 	child_event->overflow_handler = parent_event->overflow_handler;
 	child_event->overflow_handler_context
 		= parent_event->overflow_handler_context;
-- 
2.39.5


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