linux-kernel - [PATCH net-next] crypto: Fix af_alg_sendmsg(MSG_SPLICE

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Date:   Thu, 15 Jun 2023 22:08:54 +0100
From:   David Howells <dhowells@...hat.com>
To:     netdev@...r.kernel.org
cc:     dhowells@...hat.com,
        syzbot+6efc50cc1f8d718d6cb7@...kaller.appspotmail.com,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH net-next] crypto: Fix af_alg_sendmsg(MSG_SPLICE_PAGES) sglist limit

When af_alg_sendmsg() calls extract_iter_to_sg(), it passes MAX_SGL_ENTS as
the maximum number of elements that may be written to, but some of the
elements may already have been used (as recorded in sgl->cur), so
extract_iter_to_sg() may end up overrunning the scatterlist.

Fix this to limit the number of elements to "MAX_SGL_ENTS - sgl->cur".

Note: It probably makes sense in future to alter the behaviour of
extract_iter_to_sg() to stop if "sgtable->nents >= sg_max" instead, but
this is a smaller fix for now.

The bug causes errors looking something like:

BUG: KASAN: slab-out-of-bounds in sg_assign_page include/linux/scatterlist.h:109 [inline]
BUG: KASAN: slab-out-of-bounds in sg_set_page include/linux/scatterlist.h:139 [inline]
BUG: KASAN: slab-out-of-bounds in extract_bvec_to_sg lib/scatterlist.c:1183 [inline]
BUG: KASAN: slab-out-of-bounds in extract_iter_to_sg lib/scatterlist.c:1352 [inline]
BUG: KASAN: slab-out-of-bounds in extract_iter_to_sg+0x17a6/0x1960 lib/scatterlist.c:1339

Fixes: bf63e250c4b1 ("crypto: af_alg: Support MSG_SPLICE_PAGES")
Reported-by: syzbot+6efc50cc1f8d718d6cb7@...kaller.appspotmail.com
Link: https://lore.kernel.org/r/000000000000b2585a05fdeb8379@google.com/
Signed-off-by: David Howells <dhowells@...hat.com>
Tested-by: syzbot+6efc50cc1f8d718d6cb7@...kaller.appspotmail.com
cc: Herbert Xu <herbert@...dor.apana.org.au>
cc: "David S. Miller" <davem@...emloft.net>
cc: Eric Dumazet <edumazet@...gle.com>
cc: Jakub Kicinski <kuba@...nel.org>
cc: Paolo Abeni <pabeni@...hat.com>
cc: Jens Axboe <axboe@...nel.dk>
cc: Matthew Wilcox <willy@...radead.org>
cc: linux-crypto@...r.kernel.org
cc: netdev@...r.kernel.org
---
 crypto/af_alg.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/crypto/af_alg.c b/crypto/af_alg.c
index 7d4b6016b83d..cdb1dcc5dd1a 100644
--- a/crypto/af_alg.c
+++ b/crypto/af_alg.c
@@ -1043,7 +1043,7 @@ int af_alg_sendmsg(struct socket *sock, struct msghdr *msg, size_t size,
 			};
 
 			plen = extract_iter_to_sg(&msg->msg_iter, len, &sgtable,
-						  MAX_SGL_ENTS, 0);
+						  MAX_SGL_ENTS - sgl->cur, 0);
 			if (plen < 0) {
 				err = plen;
 				goto unlock;