[PATCH] ring-buffer: fix array bounds checking

[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.

Fixes: 117c39200d9d ("ring-buffer: Introducing ring-buffer mapping functions")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
---
 kernel/trace/ring_buffer.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 7e257e855dd1..83da74bf7bd6 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -6994,9 +6994,9 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 {
 	unsigned long nr_subbufs, nr_pages, nr_vma_pages, pgoff = vma->vm_pgoff;
 	unsigned int subbuf_pages, subbuf_order;
-	struct page **pages;
+	struct page **pages, *page;
 	int p = 0, s = 0;
-	int err;
+	int err, off;
 
 	/* Refuse MP_PRIVATE or writable mappings */
 	if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
@@ -7055,14 +7055,14 @@ 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]);
-		int off = 0;
-
 		if (WARN_ON_ONCE(s >= nr_subbufs)) {
 			err = -EINVAL;
 			goto out;
 		}
 
+		page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
+		off = 0;
+
 		for (; off < (1 << (subbuf_order)); off++, page++) {
 			if (p >= nr_pages)
 				break;
--