[PATCH 2/6] ring-buffer: Prevent subbuf order change when resizing is disabled

[Vincent Donnefort <vdonnefort@google.com>]

Because ring_buffer_subbuf_order_set() frees buffer pages, we can't
allow it when resizing is disabled. A non-consuming reader is at risk of
use-after-free (rb_advance_iter()).

Return -EBUSY on resize_disabled, matching ring_buffer_resize()
behaviour.

Fixes: f9b94daa542a ("ring-buffer: Set new size of the ring buffer sub page")
Reported-by: syzbot+e0cc44465d6bae735679@syzkaller.appspotmail.com
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index afe75ad2bbf2..9bc467c4dbbb 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7360,7 +7360,7 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 
 		cpu_buffer = buffer->buffers[cpu];
 
-		if (cpu_buffer->mapped) {
+		if (atomic_read(&cpu_buffer->resize_disabled)) {
 			err = -EBUSY;
 			goto error;
 		}
-- 
2.55.0.654.g21b8a5bc05-goog