[PATCH] 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 risizing 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 8e2485bb3aa8..414bca2121a7 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7358,7 +7358,7 @@ 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;
 		}

base-commit: 075b74841bd0065a3bda3440873c747938e69b68
-- 
2.55.0.571.g244d577d93-goog