[for-next][PATCH 6/9] ring-buffer: Fix subbuf resize race with ring_buffer_alloc_read_page()

[Steven Rostedt]

From: Vincent Donnefort <vdonnefort@xxxxxxxxxx>

ring_buffer_alloc_read_page() is racy with ring_buffer_subbuf_order_set,
it can allocate a reader page with an outdated order. This isn't a big
issue, the user can still re-allocate a new reader page and try again.

However, what is more problematic is if the value of subbuf_order
changes in the middle of ring_buffer_alloc_read_page(). In that case,
bpage->order might not match the actual allocated memory.

Use bpage->order for the allocation to prevent this race.

Cc: stable@xxxxxxxxxxxxxxx
Link: https://patch.msgid.link/20260813131152.3589632-6-vdonnefort@xxxxxxxxxx
Fixes: bce761d75745 ("ring-buffer: Read and write to ring buffers with custom sub buffer size")
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Signed-off-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/trace/ring_buffer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index e436793ed5c1..796c10319d6a 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7018,7 +7018,7 @@ ring_buffer_alloc_read_page(struct trace_buffer *buffer, int cpu)
 	if (bpage->data) {
 		rb_init_data_page(bpage->data);
 	} else {
-		bpage->data = alloc_cpu_data(cpu, cpu_buffer->buffer->subbuf_order);
+		bpage->data = alloc_cpu_data(cpu, bpage->order);
 		if (!bpage->data) {
 			kfree(bpage);
 			return ERR_PTR(-ENOMEM);
-- 
2.53.0