linux-kernel - Re: [PATCH 2/2] ring-buffer: Fix race between integrity check and readers

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Date:	Fri, 30 Nov 2012 11:21:19 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	linux-kernel@...r.kernel.org, stable <stable@...r.kernel.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 2/2] ring-buffer: Fix race between integrity check and
 readers

On Fri, 2012-11-30 at 11:12 -0500, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@...hat.com>
> 
> The function rb_check_pages() was added to make sure the ring buffer's
> pages were sane. This check is done when the ring buffer size is modified
> as well as when the iterator is released (closing the "trace" file),
> as that was considered a non fast path and a good place to do a sanity
> check.
> 
> The problem is that the check does not have any locks around it.
> If one process were to read the trace file, and another were to read
> the raw binary file, the check could happen while the reader is reading
> the file.
> 
> The issues with this is that the check requires to clear the HEAD page
> before doing the full check and it restores it afterward. But readers
> require the HEAD page to exist before it can read the buffer, otherwise
> it gives a nasty warning and disables the buffer.
> 
> By adding the reader lock around the check, this keeps the race from
> happening.
> 
> Cc: stable@...r.kernel.org # 3.6

Again, quilt failed to Cc stable :-(

-- Steve

> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> ---
>  kernel/trace/ring_buffer.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index ec01803..4cb5e51 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -3783,12 +3783,17 @@ void
>  ring_buffer_read_finish(struct ring_buffer_iter *iter)
>  {
>  	struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
> +	unsigned long flags;
>  
>  	/*
>  	 * Ring buffer is disabled from recording, here's a good place
> -	 * to check the integrity of the ring buffer. 
> +	 * to check the integrity of the ring buffer.
> +	 * Must prevent readers from trying to read, as the check
> +	 * clears the HEAD page and readers require it.
>  	 */
> +	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
>  	rb_check_pages(cpu_buffer);
> +	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>  
>  	atomic_dec(&cpu_buffer->record_disabled);
>  	atomic_dec(&cpu_buffer->buffer->resize_disabled);


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