[PATCH net-next v2] crypto: af_alg/hash: Fix recvmsg() after sendmsg(MSG_MORE)

[David Howells]

If an AF_ALG socket bound to a hashing algorithm is sent a zero-length
message with MSG_MORE set and then recvmsg() is called without first
sending another message without MSG_MORE set to end the operation, an oops
will occur because the crypto context and result doesn't now get set up in
advance because hash_sendmsg() now defers that as long as possible in the
hope that it can use crypto_ahash_digest() - and then because the message
is zero-length, it the data wrangling loop is skipped.

Fix this by handling zero-length sends at the top of the hash_sendmsg()
function.  If we're not continuing the previous sendmsg(), then just ignore
the send (hash_recvmsg() will invent something when called); if we are
continuing, then we finalise the request at this point if MSG_MORE is not
set to get any error here, otherwise the send is of no effect and can be
ignored.

Whilst we're at it, remove the code to create a kvmalloc'd scatterlist if
we get more than ALG_MAX_PAGES - this shouldn't happen.

Fixes: c662b043cdca ("crypto: af_alg/hash: Support MSG_SPLICE_PAGES")
Reported-by: syzbot+13a08c0bf4d212766c3c@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/000000000000b928f705fdeb873a@xxxxxxxxxx/
Reported-by: syzbot+14234ccf6d0ef629ec1a@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/000000000000c047db05fdeb8790@xxxxxxxxxx/
Reported-by: syzbot+4e2e47f32607d0f72d43@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/000000000000bcca3205fdeb87fb@xxxxxxxxxx/
Reported-by: syzbot+472626bb5e7c59fb768f@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/000000000000b55d8805fdeb8385@xxxxxxxxxx/
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Reported-and-tested-by: syzbot+6efc50cc1f8d718d6cb7@xxxxxxxxxxxxxxxxxxxxxxxxx
cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
cc: Eric Dumazet <edumazet@xxxxxxxxxx>
cc: Jakub Kicinski <kuba@xxxxxxxxxx>
cc: Paolo Abeni <pabeni@xxxxxxxxxx>
cc: Jens Axboe <axboe@xxxxxxxxx>
cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
cc: linux-crypto@xxxxxxxxxxxxxxx
cc: netdev@xxxxxxxxxxxxxxx
---
 crypto/algif_hash.c |   38 +++++++++++++++++++++++++-------------
 1 file changed, 25 insertions(+), 13 deletions(-)

diff --git a/crypto/algif_hash.c b/crypto/algif_hash.c
index dfb048cefb60..0ab43e149f0e 100644
--- a/crypto/algif_hash.c
+++ b/crypto/algif_hash.c
@@ -76,13 +76,30 @@ static int hash_sendmsg(struct socket *sock, struct msghdr *msg,
 
 	lock_sock(sk);
 	if (!continuing) {
-		if ((msg->msg_flags & MSG_MORE))
-			hash_free_result(sk, ctx);
+		/* Discard a previous request that wasn't marked MSG_MORE. */
+		hash_free_result(sk, ctx);
+		if (!msg_data_left(msg))
+			goto done; /* Zero-length; don't start new req */
 		need_init = true;
+	} else if (!msg_data_left(msg)) {
+		/*
+		 * No data - finalise the prev req if MSG_MORE so any error
+		 * comes out here.
+		 */
+		if (!(msg->msg_flags & MSG_MORE)) {
+			err = hash_alloc_result(sk, ctx);
+			if (err)
+				goto unlock_free;
+			ahash_request_set_crypt(&ctx->req, NULL,
+						ctx->result, 0);
+			err = crypto_wait_req(crypto_ahash_final(&ctx->req),
+					      &ctx->wait);
+			if (err)
+				goto unlock_free;
+		}
+		goto done_more;
 	}
 
-	ctx->more = false;
-
 	while (msg_data_left(msg)) {
 		ctx->sgl.sgt.sgl = ctx->sgl.sgl;
 		ctx->sgl.sgt.nents = 0;
@@ -93,15 +110,6 @@ static int hash_sendmsg(struct socket *sock, struct msghdr *msg,
 		if (npages == 0)
 			goto unlock_free;
 
-		if (npages > ARRAY_SIZE(ctx->sgl.sgl)) {
-			err = -ENOMEM;
-			ctx->sgl.sgt.sgl =
-				kvmalloc(array_size(npages,
-						    sizeof(*ctx->sgl.sgt.sgl)),
-					 GFP_KERNEL);
-			if (!ctx->sgl.sgt.sgl)
-				goto unlock_free;
-		}
 		sg_init_table(ctx->sgl.sgl, npages);
 
 		ctx->sgl.need_unpin = iov_iter_extract_will_pin(&msg->msg_iter);
@@ -150,7 +158,9 @@ static int hash_sendmsg(struct socket *sock, struct msghdr *msg,
 		af_alg_free_sg(&ctx->sgl);
 	}
 
+done_more:
 	ctx->more = msg->msg_flags & MSG_MORE;
+done:
 	err = 0;
 unlock:
 	release_sock(sk);
@@ -158,6 +168,8 @@ static int hash_sendmsg(struct socket *sock, struct msghdr *msg,
 
 unlock_free:
 	af_alg_free_sg(&ctx->sgl);
+	hash_free_result(sk, ctx);
+	ctx->more = false;
 	goto unlock;
 }