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

[David Carlier <devnexen@gmail.com>]

A CPU coming online during ring_buffer_subbuf_order_set() can be added
to buffer->cpumask after the allocation loop, and the install loop then
uses its 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>
---
 kernel/trace/ring_buffer.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 04bb94c29f58..445931bc9ae0 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7464,6 +7464,12 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
 	if (psize > RB_WRITE_MASK + 1)
 		return -EINVAL;
 
+	/*
+	 * Keep CPUs from coming online while changing the order to
+	 * synchronize with new per CPU buffers being created.
+	 */
+	guard(cpus_read_lock)();
+
 	/* prevent another thread from changing buffer sizes */
 	guard(mutex)(&buffer->mutex);
 
-- 
2.55.0