[SRU][J][PATCH 8/9] 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 2b781d1d4f933990318bcc5c68fb75a717379e42 linux-5.15.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 adc50991a6d75..a22557bdfe66a 100644
--- a/crypto/af_alg.c
+++ b/crypto/af_alg.c
@@ -595,8 +595,8 @@ 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