Re: [PATCH v4 4/9] ring-buffer: Fix subbuf resize race in ring_buffer_alloc_read_page()

[Vincent Donnefort <vdonnefort@google.com>]

On Wed, Aug 12, 2026 at 03:46:53PM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] The patch is an incomplete fix for a race condition in `ring_buffer_alloc_read_page()`. While it ensures newly allocated pages use `bpage->order`, it entirely misses the branch where `cpu_buffer->free_page` is reused. This allows an undersized cached page to be paired with a newer, larger `bpage->order`, leading to heap buffer overflows during reads and out-of-bounds frees when the page is released.
> --
> 
> --- Patch [4]: [PATCH v4 4/9] ring-buffer: Fix subbuf resize race in ring_buffer_alloc_read_page() ---
> commit 28754c82212e5979b25cf6fac815748ff021a277
> Author: Vincent Donnefort <vdonnefort@google.com>
> 
> ring-buffer: Fix subbuf resize race in ring_buffer_alloc_read_page()
> 
> ring_buffer_alloc_read_page() is racy with ring_buffer_subbuf_order_set,
> it can allocate a reader page with an outdated order. This isn't a big
> issue, the user can still re-allocate a new reader page and try again.
> 
> However, what is more problematic is if the value of subbuf_order
> changes in the middle of ring_buffer_alloc_read_page(). In that case,
> bpage->order might not match the actual allocated memory.
> 
> Use bpage->order for the allocation to prevent this race.
> 
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index 97449423d3a68..94552a433228d 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -7018,7 +7018,7 @@ ring_buffer_alloc_read_page(struct trace_buffer *buffer, int cpu)
> >  	if (bpage->data) {
> >  		rb_init_data_page(bpage->data);
> 
> [Severity: High]
> This is a pre-existing issue, but does this path leave bpage->order
> mismatched with the actual allocation size of cpu_buffer->free_page?
> 
> If ring_buffer_subbuf_order_set() updates buffer->subbuf_order to a larger
> size before clearing cpu_buffer->free_page, ring_buffer_alloc_read_page()
> might read the new larger order into bpage->order, but then claim an older,
> smaller cpu_buffer->free_page:

Not sure I follow here. the whole point is to read subbuf_order only once and
then make sure we always refer to bpage->order and not subbuf_order. 

I believe this is just fixed in the following patch.

> 
> kernel/trace/ring_buffer.c:ring_buffer_alloc_read_page() {
>     ...
> 	bpage->order = order;
>     ...
> 	if (cpu_buffer->free_page) {
> 		bpage->data = cpu_buffer->free_page;
> 		cpu_buffer->free_page = NULL;
> 	}
>     ...
> }
> 
> Could this lead to an overflow of dpage->data when reading events?
> 
> kernel/trace/ring_buffer.c:ring_buffer_read_page() {
>     ...
> 			memcpy(dpage->data + pos, rpage->data + rpos, event_size);
>     ...
> }
> 
> It also appears this could cause an out-of-bounds free of bpage->data when
> the page is released, since it frees using the mismatched bpage->order
> instead of the page's actual allocation order.
> 
> Should we also verify the cached page's actual order matches bpage->order
> before claiming it?
> 
> >  	} else {
> > -		bpage->data = alloc_cpu_data(cpu, cpu_buffer->buffer->subbuf_order);
> > +		bpage->data = alloc_cpu_data(cpu, bpage->order);
> >  		if (!bpage->data) {
> >  			kfree(bpage);
> >  			return ERR_PTR(-ENOMEM);
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260812153311.2328812-1-vdonnefort@google.com?part=4

-- 
Vincent