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

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 21 Jan 2021 23:15:22 +0300
Denis Efremov <efremov@linux.com> wrote:

> On 1/21/21 10:09 PM, Steven Rostedt wrote:
> > On Thu, 21 Jan 2021 17:30:40 +0300
> > Denis Efremov <efremov@linux.com> wrote:
> >   
> >> Hi,
> >>
> >> This patch (CVE-2020-27825) was tagged with
> >> Fixes: b23d7a5f4a07a ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
> >>
> >> I'm not an expert here but it seems like b23d7a5f4a07a only refactored
> >> ring_buffer_reset_cpu() by introducing reset_disabled_cpu_buffer() without
> >> significant changes. Hence, mutex_lock(&buffer->mutex)/mutex_unlock(&buffer->mutex)
> >> can be backported further than b23d7a5f4a07a~ and to all LTS kernels. Is
> >> b23d7a5f4a07a the actual cause of the bug?
> >>  
> > 
> > Ug, that looks to be a mistake. Looking back at the thread about this:
> > 
> >   https://lore.kernel.org/linux-arm-msm/20200915141304.41fa7c30@gandalf.local.home/  
> 
> I see from the link that it was planned to backport the patch to LTS kernels:
> 
> > Actually we are seeing issue in older kernel like 4.19/4.14/5.4 and there below patch was not 
> > present in stable branches:
> > Commit b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")  
> 
> The point is that it's not backported yet. Maybe because of Fixes tag. I've discovered
> this while trying to formalize CVE-2020-27825 bug in cvehound
> https://github.com/evdenis/cvehound/blob/master/cvehound/cve/CVE-2020-27825.cocci
> 
> I think that the backport to the 4.4+ should be something like:
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 547a3a5ac57b..2171b377bbc1 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -4295,6 +4295,8 @@ void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu)
>  	if (!cpumask_test_cpu(cpu, buffer->cpumask))
>  		return;
>  
> +	mutex_lock(&buffer->mutex);
> +
>  	atomic_inc(&buffer->resize_disabled);
>  	atomic_inc(&cpu_buffer->record_disabled);
>  
> @@ -4317,6 +4319,8 @@ void ring_buffer_reset_cpu(struct ring_buffer *buffer, int cpu)
>  
>  	atomic_dec(&cpu_buffer->record_disabled);
>  	atomic_dec(&buffer->resize_disabled);
> +
> +	mutex_unlock(&buffer->mutex);
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);
>  

That could possibly work.

-- Steve