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 --- 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 1e61fe6e24b9..291dbd698932 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; }