Re: [PATCH v1] trace: Fix race in trace_open and buffer resize call

[Gaurav Kohli]

Hi Steven,

please let us know, if below looks good to you or need modifications.

Thanks
Gaurav

On 9/24/2020 7:25 PM, Gaurav Kohli wrote:
> Below race can come, if trace_open and resize of
> cpu buffer is running parallely on different cpus
> CPUX                                CPUY
> 				    ring_buffer_resize
> 				    atomic_read(&buffer->resize_disabled)
> tracing_open
> tracing_reset_online_cpus
> ring_buffer_reset_cpu
> rb_reset_cpu
> 				    rb_update_pages
> 				    remove/insert pages
> resetting pointer
> This race can cause data abort or some times infinte loop in
> rb_remove_pages and rb_insert_pages while checking pages
> for sanity.
>
> Signed-off-by: Gaurav Kohli <gkohli@xxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> Changes since v0:
>    -Addressed Steven's review comments.
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 93ef0ab..15bf28b 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -4866,6 +4866,9 @@ void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu)
>   	if (!cpumask_test_cpu(cpu, buffer->cpumask))
>   		return;
>   
> +	/* prevent another thread from changing buffer sizes */
> +	mutex_lock(&buffer->mutex);
> +
>   	atomic_inc(&cpu_buffer->resize_disabled);
>   	atomic_inc(&cpu_buffer->record_disabled);
>   
> @@ -4876,6 +4879,8 @@ void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu)
>   
>   	atomic_dec(&cpu_buffer->record_disabled);
>   	atomic_dec(&cpu_buffer->resize_disabled);
> +
> +	mutex_unlock(&buffer->mutex);
>   }
>   EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);
>   
> @@ -4889,6 +4894,9 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
>   	struct ring_buffer_per_cpu *cpu_buffer;
>   	int cpu;
>   
> +	/* prevent another thread from changing buffer sizes */
> +	mutex_lock(&buffer->mutex);
> +
>   	for_each_online_buffer_cpu(buffer, cpu) {
>   		cpu_buffer = buffer->buffers[cpu];
>   
> @@ -4907,6 +4915,8 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
>   		atomic_dec(&cpu_buffer->record_disabled);
>   		atomic_dec(&cpu_buffer->resize_disabled);
>   	}
> +
> +	mutex_unlock(&buffer->mutex);
>   }
>   
>   /**

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