[SRU][N][PATCH 8/8] crypto: af_alg - Fix page reassignment overflow in af_alg_pull_tsgl

[Massimiliano Pellizzer]

From: Herbert Xu <herbert at gondor.apana.org.au>

commit 31d00156e50ecad37f2cb6cbf04aaa9a260505ef upstream.

When page reassignment was added to af_alg_pull_tsgl the original
loop wasn't updated so it may try to reassign one more page than
necessary.

Add the check to the reassignment so that this does not happen.

Also update the comment which still refers to the obsolete offset
argument.

Reported-by: syzbot+d23888375c2737c17ba5 at syzkaller.appspotmail.com
Fixes: e870456d8e7c ("crypto: algif_skcipher - overhaul memory management")
Signed-off-by: Herbert Xu <herbert at gondor.apana.org.au>
Signed-off-by: Eric Biggers <ebiggers at kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from commit c8369a6d62f5abde9cbd4b62c45bf4b996be2468 linux-6.12.y)
CVE-2026-31431
Signed-off-by: Massimiliano Pellizzer <massimiliano.pellizzer at canonical.com>
---
 crypto/af_alg.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/crypto/af_alg.c b/crypto/af_alg.c
index cab04b009d03..2c1ec61cfb56 100644
--- a/crypto/af_alg.c
+++ b/crypto/af_alg.c
@@ -702,8 +702,8 @@ void af_alg_pull_tsgl(struct sock *sk, size_t used, struct scatterlist *dst)
 			 * Assumption: caller created af_alg_count_tsgl(len)
 			 * SG entries in dst.
 			 */
-			if (dst) {
-				/* reassign page to dst after offset */
+			if (dst && plen) {
+				/* reassign page to dst */
 				get_page(page);
 				sg_set_page(dst + j, page, plen, sg[i].offset);
 				j++;
-- 
2.53.0