On Fri, 14 Aug 2026 15:53:43 +0100 Vincent Donnefort <[email protected]> wrote:
> 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)" <[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 reader to obtain, cache, or swap a page allocated under an outdated order 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. > > I am not sure, the lock is per-cpu_buffer but subbuf_order. is global to > trace_buffer? Yeah, we don't need this. I got new ring_buffer_read_page() ensures the data_page->order == cpu_buffer->reader_page->order. This is a bit tricky but it allows us to asynchronously allocate reader_page in different order outside of locking. (and if free_page exists, it is copied with its order, so it is safe.) > > > > > > 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 Got it. Yours changes the data structure. Mine is for older kernels as a minimal change. > > > > 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. Since Vincent's patch[1] handles data page pointer with its order, we don't this part anymore. [1] ring-buffer: Make cpu_buffer::free_page a buffer_data_read_page So I confirmed Vincent's patches with Fixed tag are fixing the racing problem in the different way. Thank you, -- Masami Hiramatsu (Google) <[email protected]>
