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

[Steven Rostedt <rostedt@goodmis.org>]

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 reader to obtain, cache, or swap a page allocated under an outdated order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.

>  	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;
> +
>  	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);
>