linux-kernel - [PATCH] trace/ring_buffer: Fix locking order in ring_buffer

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Message-Id: <20251005141636.12405-2-ssranevjti@gmail.com>
Date: Sun,  5 Oct 2025 19:46:36 +0530
From: ssrane_b23@...vjti.ac.in
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	syzbot+c530b4d95ec5cd4f33a7@...kaller.appspotmail.com,
	Shaurya Rane <ssrane_b23@...vjti.ac.in>
Subject: [PATCH] trace/ring_buffer: Fix locking order in ring_buffer_map()

From: Shaurya Rane <ssrane_b23@...vjti.ac.in>

The kernel's lockdep validator detected a circular locking dependency
in ring_buffer_map(). The function was acquiring the per-CPU
'cpu_buffer->mapping_lock' before the global 'buffer->mutex'.

This violates the established locking hierarchy where 'buffer->mutex'
should be acquired first, leading to a potential deadlock.

Fix this by reordering the mutex acquisition to lock 'buffer->mutex'
before 'cpu_buffer->mapping_lock', satisfying the lockdep requirements
and preventing the deadlock.

Reported-by: syzbot+c530b4d95ec5cd4f33a7@...kaller.appspotmail.com

Signed-off-by: Shaurya Rane <ssrane_b23@...vjti.ac.in>
---
 kernel/trace/ring_buffer.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 43460949ad3f..82c3d5d2dcf6 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7222,9 +7222,10 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
 
 	if (!cpumask_test_cpu(cpu, buffer->cpumask))
 		return -EINVAL;
-
+	
 	cpu_buffer = buffer->buffers[cpu];
-
+	
+	guard(mutex)(&buffer->mutex);
 	guard(mutex)(&cpu_buffer->mapping_lock);
 
 	if (cpu_buffer->user_mapped) {
-- 
2.34.1