linux-kernel - [tip: perf/core] perf: Fix failing inherit_event() doing extra refcount decrement on parent

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <174670046994.406.11347888958227901456.tip-bot2@tip-bot2>
Date: Thu, 08 May 2025 10:34:29 -0000
From: "tip-bot2 for Frederic Weisbecker" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Frederic Weisbecker <frederic@...nel.org>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: perf/core] perf: Fix failing inherit_event() doing extra
 refcount decrement on parent

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

Commit-ID:     f162bd64d95f42f42d1efdba555d898dd14af758
Gitweb:        https://git.kernel.org/tip/f162bd64d95f42f42d1efdba555d898dd14af758
Author:        Frederic Weisbecker <frederic@...nel.org>
AuthorDate:    Thu, 24 Apr 2025 18:11:25 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 02 May 2025 12:40:40 +02:00

perf: Fix failing inherit_event() doing extra refcount decrement on parent

When inherit_event() fails after the child allocation but before the
parent refcount has been incremented, calling put_event() wrongly
decrements the reference to the parent, risking to free it too early.

Also pmu_get_event() can't be holding a reference to the child
concurrently at this point since it is under pmus_srcu critical section.

Fix it with restoring the deleted free_event() function and call it on
the failing child in order to free it directly under the verified
assumption that its refcount is only 1. The refcount to the parent is
then voluntarily omitted.

Fixes: da916e96e2de ("perf: Make perf_pmu_unregister() useable")
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20250424161128.29176-2-frederic@kernel.org
---
 kernel/events/core.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 05136e8..882db7b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5628,6 +5628,22 @@ static void _free_event(struct perf_event *event)
 }
 
 /*
+ * Used to free events which have a known refcount of 1, such as in error paths
+ * of inherited events.
+ */
+static void free_event(struct perf_event *event)
+{
+	if (WARN(atomic_long_cmpxchg(&event->refcount, 1, 0) != 1,
+				     "unexpected event refcount: %ld; ptr=%p\n",
+				     atomic_long_read(&event->refcount), event)) {
+		/* leak to avoid use-after-free */
+		return;
+	}
+
+	_free_event(event);
+}
+
+/*
  * Remove user event from the owner task.
  */
 static void perf_remove_from_owner(struct perf_event *event)
@@ -14184,7 +14200,7 @@ inherit_event(struct perf_event *parent_event,
 
 	pmu_ctx = find_get_pmu_context(child_event->pmu, child_ctx, child_event);
 	if (IS_ERR(pmu_ctx)) {
-		put_event(child_event);
+		free_event(child_event);
 		return ERR_CAST(pmu_ctx);
 	}
 	child_event->pmu_ctx = pmu_ctx;
@@ -14199,7 +14215,7 @@ inherit_event(struct perf_event *parent_event,
 	if (is_orphaned_event(parent_event) ||
 	    !atomic_long_inc_not_zero(&parent_event->refcount)) {
 		mutex_unlock(&parent_event->child_mutex);
-		put_event(child_event);
+		free_event(child_event);
 		return NULL;
 	}