linux-kernel - [PATCH] ring-buffer: Check for NULL cpu_buffer in ring_buffer_wake

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Date:   Tue, 1 Nov 2022 19:10:09 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     LKML <linux-kernel@...r.kernel.org>,
        Linux trace kernel <linux-trace-kernel@...r.kernel.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Roland Ruckerbauer <roland.rucky@...il.com>,
        Takashi Iwai <tiwai@...e.de>, regressions@...ts.linux.dev,
        Steven Noonan <steven.noonan@...il.com>
Subject: [PATCH] ring-buffer: Check for NULL cpu_buffer in
 ring_buffer_wake_waiters()

From: "Steven Rostedt (Google)" <rostedt@...dmis.org>

On some machines the number of listed CPUs may be bigger than the actual
CPUs that exist. The tracing subsystem allocates a per_cpu directory with
access to the per CPU ring buffer via a cpuX file. But to save space, the
ring buffer will only allocate buffers for online CPUs, even though the
CPU array will be as big as the nr_cpu_ids.

With the addition of waking waiters on the ring buffer when closing the
file, the ring_buffer_wake_waiters() now needs to make sure that the
buffer is allocated (with the irq_work allocated with it) before trying to
wake waiters, as it will cause a NULL pointer dereference.

While debugging this, I added a NULL check for the buffer itself (which is
OK to do), and also NULL pointer checks against buffer->buffers (which is
not fine, and will WARN) as well as making sure the CPU number passed in
is within the nr_cpu_ids (which is also not fine if it isn't).

Link: https://lore.kernel.org/all/87h6zklb6n.wl-tiwai@suse.de/
Link: https://lore.kernel.org/all/CAM6Wdxc0KRJMXVAA0Y=u6Jh2V=uWB-_Fn6M4xRuNppfXzL1mUg@mail.gmail.com/

Cc: stable@...r.kernel.org
Bugzilla: https://bugzilla.opensuse.org/show_bug.cgi?id=1204705
Reported-by: Takashi Iwai <tiwai@...e.de>
Reported-by: Roland Ruckerbauer <roland.rucky@...il.com>
Fixes: f3ddb74ad079 ("tracing: Wake up ring buffer waiters on closing of the file")
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/ring_buffer.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 199759c73519..9712083832f4 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -937,6 +937,9 @@ void ring_buffer_wake_waiters(struct trace_buffer *buffer, int cpu)
 	struct ring_buffer_per_cpu *cpu_buffer;
 	struct rb_irq_work *rbwork;

+	if (!buffer)
+		return;
+
 	if (cpu == RING_BUFFER_ALL_CPUS) {

 		/* Wake up individual ones too. One level recursion */
@@ -945,7 +948,15 @@ void ring_buffer_wake_waiters(struct trace_buffer *buffer, int cpu)

 		rbwork = &buffer->irq_work;
 	} else {
+		if (WARN_ON_ONCE(!buffer->buffers))
+			return;
+		if (WARN_ON_ONCE(cpu >= nr_cpu_ids))
+			return;
+
 		cpu_buffer = buffer->buffers[cpu];
+		/* The CPU buffer may not have been initialized yet */
+		if (!cpu_buffer)
+			return;
 		rbwork = &cpu_buffer->irq_work;
 	}

-- 
2.35.1