Re: [PATCH] ring-buffer: Check for NULL cpu_buffer in ring_buffer_wake_waiters()

From: Google
Date: Wed Nov 02 2022 - 22:54:53 EST


On Tue, 1 Nov 2022 19:10:09 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
>
> 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@xxxxxxx/
> Link: https://lore.kernel.org/all/CAM6Wdxc0KRJMXVAA0Y=u6Jh2V=uWB-_Fn6M4xRuNppfXzL1mUg@xxxxxxxxxxxxxx/
>

This looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Thanks,

> Cc: stable@xxxxxxxxxxxxxxx
> Bugzilla: https://bugzilla.opensuse.org/show_bug.cgi?id=1204705
> Reported-by: Takashi Iwai <tiwai@xxxxxxx>
> Reported-by: Roland Ruckerbauer <roland.rucky@xxxxxxxxx>
> Fixes: f3ddb74ad079 ("tracing: Wake up ring buffer waiters on closing of the file")
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> ---
> 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
>


--
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>