On Fri, 14 Aug 2026 01:16:13 +0900 "Masami Hiramatsu (Google)" <[email protected]> wrote:
> From: Masami Hiramatsu (Google) <[email protected]> > > 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 reader to obtain, cache, or swap a page allocated under an outdated order 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) <[email protected]> > --- 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); >
