[PATCH v2] ring-buffer: Take cpus_read_lock() when changing the subbuf order

[David Carlier <devnexen@gmail.com>]

A CPU coming online for the first time gets its per CPU buffer allocated
by trace_rb_cpu_prepare(), which then adds it to buffer->cpumask. If that
happens while ring_buffer_subbuf_order_set() is running, the CPU shows up
after the allocation loop and the install loop walks it with an empty
new_pages list, corrupting the buffer.

Take cpus_read_lock() as ring_buffer_resize() does.

Fixes: f9b94daa542a ("ring-buffer: Set new size of the ring buffer sub page")
Cc: stable@vger.kernel.org
Signed-off-by: David Carlier <devnexen@gmail.com>
---

Notes:
    v2:
     - Reword the changelog: offlined CPUs stay in ->cpumask, so only a CPU
       coming online for the first time can appear mid-flight (Vincent)
     - Drop the comment above guard(cpus_read_lock)() (Vincent)
    
    v1: https://lore.kernel.org/linux-trace-kernel/20260914182850.21449-1-devnexen@gmail.com/

 kernel/trace/ring_buffer.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 04bb94c29f58..e78fb4bb2195 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7464,6 +7464,8 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 	if (psize > RB_WRITE_MASK + 1)
 		return -EINVAL;
 
+	guard(cpus_read_lock)();
+
 	/* prevent another thread from changing buffer sizes */
 	guard(mutex)(&buffer->mutex);
 
-- 
2.55.0