Re: [PATCH v4 1/9] ring-buffer: Free cpu_buffer->free_page with subbuf_order

[Vincent Donnefort <vdonnefort@google.com>]

On Wed, Aug 12, 2026 at 03:50:37PM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [Critical] Concurrent execution of sub-buffer order resizing (`ring_buffer_subbuf_order_set`) and read operations (`ring_buffer_alloc_read_page`, `ring_buffer_read_page`) can lead to out-of-bounds writes, use-after-free, and page allocator corruption due to mismatched synchronization.
> --
> 
> --- Patch [1]: [PATCH v4 1/9] ring-buffer: Free cpu_buffer->free_page with subbuf_order ---
> commit 4100674474d6bd293fb872fecb17fc781ca58502
> Author: Vincent Donnefort <vdonnefort@google.com>
> 
> ring-buffer: Free cpu_buffer->free_page with subbuf_order
>     
> When sub-buffers use an order greater than 0, cpu_buffer->free_page is
> allocated with subbuf_order. Use the correct order for
> cpu_buffer->free_page.
> 
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index 2667992f0aa29..a3d28b2e2c94a 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -2631,7 +2631,7 @@ static void rb_free_cpu_buffer(struct ring_buffer_per_cpu *cpu_buffer)
> >  		free_buffer_page(bpage);
> >  	}
> >  
> > -	free_page((unsigned long)cpu_buffer->free_page);
> > +	free_pages((unsigned long)cpu_buffer->free_page, cpu_buffer->buffer->subbuf_order);
> 
> [Severity: Critical]
> This isn't a bug introduced by this patch, but is it possible for a race
> condition between resizing and reading to lead to memory corruption?
> 
> If thread A changes the sub-buffer size via
> /sys/kernel/tracing/buffer_subbuf_size_kb, ring_buffer_subbuf_order_set()
> updates buffer->subbuf_order locklessly globally before tearing down the CPU
> buffers and clearing their free_page.
> 
> If thread B concurrently reads /sys/kernel/tracing/per_cpu/cpu0/trace_pipe_raw
> via tracing_buffers_read(), it observes the new order and calls
> ring_buffer_alloc_read_page().
> 
> Because ring_buffer_subbuf_order_set() hasn't yet cleared
> cpu_buffer->free_page for this CPU (they use distinct locks), thread B pops
> the old page from free_page but tags it with the new order:
> 
> kernel/trace/ring_buffer.c:ring_buffer_alloc_read_page() {
>     ...
>     bpage->order = buffer->subbuf_order;
>     ...
>     if (cpu_buffer->free_page) {
>         bpage->data = cpu_buffer->free_page;
>         cpu_buffer->free_page = NULL;
>     }
>     ...
> }
> 
> Thread B then calls ring_buffer_read_page(). The lockless check passes
> because both match the new order:
> 
> kernel/trace/ring_buffer.c:ring_buffer_read_page() {
>     ...
>     if (data_page->order != buffer->subbuf_order)
>         return -1;
>     ...
>     reader->page = data_page->data;
> }
> 
> Could this allow the undersized old page to be swapped into the ring buffer
> as the reader_page, leading to an out-of-bounds write when a writer attempts
> to write large events? 
> 
> It seems it could also cause page allocator corruption when a smaller page
> is later passed to free_pages() with a larger order.
> 
> >  	kfree(cpu_buffer);
> >  }
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260812153311.2328812-1-vdonnefort@google.com?part=1

All that is hopefully fixed in one of the later patch of the series

-- 
Vincent