Re: [PATCH] ring-buffer: Fix race between ring_buffer_subbuf_order_set() and readers

[Vincent Donnefort <vdonnefort@google.com>]

On Fri, Aug 14, 2026 at 10:42:08AM -0400, Steven Rostedt wrote:
> On Fri, 14 Aug 2026 01:16:13 +0900
> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > 
> > When ring_buffer_subbuf_order_set() updates buffer->subbuf_order, it
> > previously modified buffer->subbuf_order before clearing the cached
> > per-CPU free_page entries. Furthermore, clearing cpu_buffer->free_page
> > was done under cpu_buffer->reader_lock, whereas
> > ring_buffer_alloc_read_page() protects cpu_buffer->free_page using
> > arch_spin_lock(&cpu_buffer->lock).
> > 
> > Because ring_buffer_alloc_read_page(), ring_buffer_free_read_page(),
> > and ring_buffer_read_page() checked buffer->subbuf_order locklessly
> > before accessing reader resources, a TOCTOU race allowed a concurrent
> > reader to obtain, cache, or swap a page allocated under an outdated order
> > while tagging bpage->order with the new order. This allowed undersized
> > pages to be swapped into the ring buffer, leading to heap buffer overflows,
> > or caused free_pages() to be called with an invalid order.
> > 
> > Fix this by:
> > 1. Flushing and freeing all per-CPU cached free_page entries under
> >    arch_spin_lock(&cpu_buffer->lock) using old_order before modifying
> >    buffer->subbuf_order.
> > 2. Protecting bpage->order assignment under
> >    arch_spin_lock(&cpu_buffer->lock) in ring_buffer_alloc_read_page().
> > 3. Moving the buffer->subbuf_order validation inside
> >    arch_spin_lock(&cpu_buffer->lock) in ring_buffer_free_read_page().
> > 4. Re-validating buffer->subbuf_order inside reader_lock in
> >    ring_buffer_read_page().
> > 5. Protecting cpu_buffer->free_page extraction with
> >    arch_spin_lock(&cpu_buffer->lock) in ring_buffer_subbuf_order_set().
> > 
> > Fixes: 2808e31ec12e ("ring-buffer: Add interface for configuring trace sub buffer size")
> > Assisted-by: Antigravity:gemini-3.6-flash
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > ---
> 
> 
> Please rebase on top of ring-buffer/for-next, as I added Vincent's patches to that.
> 
> >  kernel/trace/ring_buffer.c |   35 +++++++++++++++++++++++++++++------
> >  1 file changed, 29 insertions(+), 6 deletions(-)
> > 
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index 2667992f0aa2..6d180689ad59 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -6957,11 +6957,11 @@ ring_buffer_alloc_read_page(struct trace_buffer *buffer, int cpu)
> >  	if (!bpage)
> >  		return ERR_PTR(-ENOMEM);
> >  
> > -	bpage->order = buffer->subbuf_order;
> >  	cpu_buffer = buffer->buffers[cpu];
> >  	local_irq_save(flags);
> >  	arch_spin_lock(&cpu_buffer->lock);
> >  
> > +	bpage->order = buffer->subbuf_order;
> 
> This is still needed.

I am not sure, the lock is per-cpu_buffer but subbuf_order. is global to
trace_buffer?

> 
> >  	if (cpu_buffer->free_page) {
> >  		bpage->data = cpu_buffer->free_page;
> >  		cpu_buffer->free_page = NULL;
> > @@ -7010,13 +7010,13 @@ void ring_buffer_free_read_page(struct trace_buffer *buffer, int cpu,
> >  	 * is different from the subbuffer order of the buffer -
> >  	 * we can't reuse it
> >  	 */
> > -	if (page_ref_count(page) > 1 || data_page->order != buffer->subbuf_order)
> > +	if (page_ref_count(page) > 1)
> >  		goto out;
> >  
> >  	local_irq_save(flags);
> >  	arch_spin_lock(&cpu_buffer->lock);
> >  
> > -	if (!cpu_buffer->free_page) {
> > +	if (data_page->order == buffer->subbuf_order && !cpu_buffer->free_page) {
> 
> Swap the order please. It has to check both to continue and if one fails it
> will not continue. Checking for cpu_buffer->free_page to be NULL first is
> the quicker check. And also the more likely one to fail.
> 
> >  		cpu_buffer->free_page = dpage;
> >  		dpage = NULL;
> >  	}
> > @@ -7094,15 +7094,15 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
> >  	if (!data_page || !data_page->data)
> >  		return -1;
> >  
> > -	if (data_page->order != buffer->subbuf_order)
> > -		return -1;
> > -
> >  	dpage = data_page->data;
> >  	if (!dpage)
> >  		return -1;
> >  
> >  	guard(raw_spinlock_irqsave)(&cpu_buffer->reader_lock);
> >  
> > +	if (data_page->order != buffer->subbuf_order)
> > +		return -1;
> > +

I have modified this as part of 

tracing: Fix subbuf resize races with trace_pipe_raw readers 

> >  	reader = rb_get_reader_page(cpu_buffer);
> >  	if (!reader)
> >  		return -1;
> > @@ -7350,6 +7350,27 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
> >  	/* Make sure all commits have finished */
> >  	synchronize_rcu();
> >  
> > +	/* Flush any cached free_page allocated with old_order */
> > +	for_each_buffer_cpu(buffer, cpu) {
> > +		struct buffer_data_page *old_free;
> > +		unsigned long flags;
> > +
> > +		if (!cpumask_test_cpu(cpu, buffer->cpumask))
> > +			continue;
> > +
> > +		cpu_buffer = buffer->buffers[cpu];
> > +
> > +		local_irq_save(flags);
> > +		arch_spin_lock(&cpu_buffer->lock);
> > +		old_free = cpu_buffer->free_page;
> > +		cpu_buffer->free_page = NULL;
> > +		arch_spin_unlock(&cpu_buffer->lock);
> > +		local_irq_restore(flags);
> > +
> > +		if (old_free)
> > +			free_pages((unsigned long)old_free, old_order);
> > +	}
> 
> Honestly, this should be a separate patch. The first part of this patch is
> data races with adding and freeing, but this is about changes to the size.
> 
> > +
> >  	buffer->subbuf_order = order;
> >  	buffer->subbuf_size = psize - BUF_PAGE_HDR_SIZE;
> >  
> > @@ -7431,8 +7452,10 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
> >  		cpu_buffer->nr_pages = cpu_buffer->nr_pages_to_update;
> >  		cpu_buffer->nr_pages_to_update = 0;
> >  
> > +		arch_spin_lock(&cpu_buffer->lock);
> >  		old_free_data_page = cpu_buffer->free_page;
> >  		cpu_buffer->free_page = NULL;
> > +		arch_spin_unlock(&cpu_buffer->lock);
> 
> This is already fixed by Vincent (and I would have asked this to be a
> separate patch too if it hadn't).
> 
> -- Steve
> 
> >  
> >  		rb_head_page_activate(cpu_buffer);
> >  
> 

-- 
Vincent