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: bf63e250 ("crypto: af_alg: Support MSG_SPLICE_PAGES")
Reported-by: syzbot+6efc50cc1f8d718d6cb7@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/000000000000b2585a05fdeb8379@google.com/Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: syzbot+6efc50cc1f8d718d6cb7@syzkaller.appspotmail.com
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-crypto@vger.kernel.org
cc: netdev@vger.kernel.org
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

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;