[PATCH] ring-buffer: fix incorrect boundary check order

[Jeongjun Park <aha310510@gmail.com>]

If there is a case where the variable s is greater than or equal to nr_subbufs
before entering the loop, oob read or use-after-free will occur. This problem
occurs because the variable s is used as an index to dereference the
struct page before the variable value range check. This logic prevents the
wrong address value from being copied to the pages array through the subsequent
range check, but oob read still occurs, so the code needs to be modified.

Signed-off-by: Jeongjun Park <aha310510@gmail.com>
---
 kernel/trace/ring_buffer.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 60210fb5b211..6804ab126802 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7059,7 +7059,7 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 	}
 
 	while (p < nr_pages) {
-		struct page *page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
+		struct page *page;
 		int off = 0;
 
 		if (WARN_ON_ONCE(s >= nr_subbufs)) {
@@ -7067,6 +7067,8 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 			goto out;
 		}
 
+		page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
+
 		for (; off < (1 << (subbuf_order)); off++, page++) {
 			if (p >= nr_pages)
 				break;
--