linux-kernel - [tip: perf/urgent] perf/core: Fix WARN_ON(!ctx) in __free

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Message-ID: <174396494203.31282.1919457781258540750.tip-bot2@tip-bot2>
Date: Sun, 06 Apr 2025 18:42:22 -0000
From: "tip-bot2 for Gabriel Shahrouzi" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: syzbot+ff3aa851d46ab82953a3@...kaller.appspotmail.com,
 Gabriel Shahrouzi <gshahrouzi@...il.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>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: perf/urgent] perf/core: Fix WARN_ON(!ctx) in __free_event() for
 partial init

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     0ba3a4ab76fd3367b9cb680cad70182c896c795c
Gitweb:        https://git.kernel.org/tip/0ba3a4ab76fd3367b9cb680cad70182c896c795c
Author:        Gabriel Shahrouzi <gshahrouzi@...il.com>
AuthorDate:    Sat, 05 Apr 2025 16:30:36 -04:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Sun, 06 Apr 2025 20:30:28 +02:00

perf/core: Fix WARN_ON(!ctx) in __free_event() for partial init

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
---
 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 128db74..9af9726 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -14016,6 +14016,9 @@ 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);
@@ -14037,8 +14040,6 @@ 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,
@@ -14059,7 +14060,6 @@ 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;