From nobody Sat Dec 27 20:35:01 2025 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id AED5B1E493 for ; Sat, 16 Dec 2023 04:21:54 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8D64DC433CA; Sat, 16 Dec 2023 04:21:54 +0000 (UTC) Received: from rostedt by gandalf with local (Exim 4.97) (envelope-from ) id 1rEMCL-00000002yGk-2eXm; Fri, 15 Dec 2023 23:22:45 -0500 Message-ID: <20231216042245.415755764@goodmis.org> User-Agent: quilt/0.67 Date: Fri, 15 Dec 2023 23:22:27 -0500 From: Steven Rostedt To: linux-kernel@vger.kernel.org Cc: Masami Hiramatsu , Mark Rutland , Mathieu Desnoyers , Andrew Morton Subject: [for-linus][PATCH 13/15] ring-buffer: Fix 32-bit rb_time_read() race with rb_time_cmpxchg() References: <20231216042214.905262999@goodmis.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Mathieu Desnoyers The following race can cause rb_time_read() to observe a corrupted time stamp: rb_time_cmpxchg() [...] if (!rb_time_read_cmpxchg(&t->msb, msb, msb2)) return false; if (!rb_time_read_cmpxchg(&t->top, top, top2)) return false; __rb_time_read() [...] do { c =3D local_read(&t->cnt); top =3D local_read(&t->top); bottom =3D local_read(&t->bottom); msb =3D local_read(&t->msb); } while (c !=3D local_read(&t->cnt)); *cnt =3D rb_time_cnt(top); /* If top and msb counts don't match, this interrupted a write */ if (*cnt !=3D rb_time_cnt(msb)) return false; ^ this check fails to catch that "bottom" is still not updated. So the old "bottom" value is returned, which is wrong. Fix this by checking that all three of msb, top, and bottom 2-bit cnt values match. The reason to favor checking all three fields over requiring a specific update order for both rb_time_set() and rb_time_cmpxchg() is because checking all three fields is more robust to handle partial failures of rb_time_cmpxchg() when interrupted by nested rb_time_set(). Link: https://lore.kernel.org/lkml/20231211201324.652870-1-mathieu.desnoyer= s@efficios.com/ Link: https://lore.kernel.org/linux-trace-kernel/20231212193049.680122-1-ma= thieu.desnoyers@efficios.com Fixes: f458a1453424e ("ring-buffer: Test last update in 32bit version of __= rb_time_read()") Signed-off-by: Mathieu Desnoyers Signed-off-by: Steven Rostedt (Google) --- kernel/trace/ring_buffer.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index b8ab0557bd1b..f22a849da179 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -644,8 +644,8 @@ static inline bool __rb_time_read(rb_time_t *t, u64 *re= t, unsigned long *cnt) =20 *cnt =3D rb_time_cnt(top); =20 - /* If top and msb counts don't match, this interrupted a write */ - if (*cnt !=3D rb_time_cnt(msb)) + /* If top, msb or bottom counts don't match, this interrupted a write */ + if (*cnt !=3D rb_time_cnt(msb) || *cnt !=3D rb_time_cnt(bottom)) return false; =20 /* The shift to msb will lose its cnt bits */ --=20 2.42.0