FAILED: patch "[PATCH] crypto: algif_skcipher - rewind rounded output bytes" failed to apply to 5.10-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x 2f90facc7b94934653eb7ebfadcc25d503b89196
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107061933-failed-5.10-2f90facc7b94@kernel.org' --subject-prefix 'PATCH 5.10.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 2f90facc7b94934653eb7ebfadcc25d503b89196 Mon Sep 17 00:00:00 2001
From: Daehyeon Ko <4ncienth@gmail.com>
Date: Sun, 4 Oct 2026 14:09:46 +0900
Subject: [PATCH] crypto: algif_skcipher - rewind rounded output bytes

af_alg_get_rsgl() advances the receive iterator by the unrounded RX
scatterlist length.  For a continuing skcipher request,
_skcipher_recvmsg() then rounds its local length down to a whole chunksize
without restoring the iterator.

A later chunk starts after bytes which the provider never wrote, while the
syscall returns only the processed length.  When copy_splice_read()
supplies non-zeroed pages and publishes that return as a dense prefix, the
gap exposes stale bytes from a previous page lifetime.

With unprivileged xts(aes), a 31-byte MSG_MORE request followed by one
final byte advances a 47-byte destination while returning 32 bytes.  A
hardened v7.2-rc5 image retained all 15 seeded stale bytes in 4,829 of
4,829 records.  With init_on_alloc enabled, all 72,345 gap bytes observed
were zero.

Rewind the iterator by the tail excluded during rounding, before rejecting
a zero processed length.  The crypto request uses only the rounded length;
af_alg_free_resources() releases the entire RX list before another receive
iteration.

The fixed target-like kernel returned byte-identical dense 48-byte output
in 4,771 of 4,771 rounds.  The unmodified kernel returned the vulnerable
32-byte short record in 2,409 of 2,409 rounds.

Fixes: e870456d8e7c ("crypto: algif_skcipher - overhaul memory management")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
---
 crypto/algif_skcipher.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/crypto/algif_skcipher.c b/crypto/algif_skcipher.c
index 1e61fe6e24b99..291dbd6989325 100644
--- a/crypto/algif_skcipher.c
+++ b/crypto/algif_skcipher.c
@@ -135,12 +135,16 @@ static int _skcipher_recvmsg(struct socket *sock, struct msghdr *msg,
 	 * full block size buffers.
 	 */
 	if (ctx->more || len < ctx->used) {
-		if (len < bs) {
+		size_t excess = len % bs;
+
+		len -= excess;
+		iov_iter_revert(&msg->msg_iter, excess);
+
+		if (!len) {
 			err = -EINVAL;
 			goto free;
 		}
 
-		len -= len % bs;
 		cflags |= CRYPTO_SKCIPHER_REQ_NOTFINAL;
 	}
 
-- 
2.53.0