[PATCH net-next] 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 always making a pass of the loop, even in the case that no data
is provided to the sendmsg().

Fix also extract_iter_to_sg() to handle a zero-length iterator by returning
0 immediately.

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>
Tested-by: syzbot+13a08c0bf4d212766c3c@xxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: syzbot+14234ccf6d0ef629ec1a@xxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: syzbot+4e2e47f32607d0f72d43@xxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: syzbot+472626bb5e7c59fb768f@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 |   21 +++++----------------
 lib/scatterlist.c   |    2 +-
 2 files changed, 6 insertions(+), 17 deletions(-)

diff --git a/crypto/algif_hash.c b/crypto/algif_hash.c
index dfb048cefb60..1176533a55c9 100644
--- a/crypto/algif_hash.c
+++ b/crypto/algif_hash.c
@@ -83,26 +83,14 @@ static int hash_sendmsg(struct socket *sock, struct msghdr *msg,
 
 	ctx->more = false;
 
-	while (msg_data_left(msg)) {
+	do {
 		ctx->sgl.sgt.sgl = ctx->sgl.sgl;
 		ctx->sgl.sgt.nents = 0;
 		ctx->sgl.sgt.orig_nents = 0;
 
 		err = -EIO;
 		npages = iov_iter_npages(&msg->msg_iter, max_pages);
-		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);
+		sg_init_table(ctx->sgl.sgl, max_t(size_t, npages, 1));
 
 		ctx->sgl.need_unpin = iov_iter_extract_will_pin(&msg->msg_iter);
 
@@ -111,7 +99,8 @@ static int hash_sendmsg(struct socket *sock, struct msghdr *msg,
 		if (err < 0)
 			goto unlock_free;
 		len = err;
-		sg_mark_end(ctx->sgl.sgt.sgl + ctx->sgl.sgt.nents - 1);
+		if (len > 0)
+			sg_mark_end(ctx->sgl.sgt.sgl + ctx->sgl.sgt.nents - 1);
 
 		if (!msg_data_left(msg)) {
 			err = hash_alloc_result(sk, ctx);
@@ -148,7 +137,7 @@ static int hash_sendmsg(struct socket *sock, struct msghdr *msg,
 
 		copied += len;
 		af_alg_free_sg(&ctx->sgl);
-	}
+	} while (msg_data_left(msg));
 
 	ctx->more = msg->msg_flags & MSG_MORE;
 	err = 0;
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index e97d7060329e..77a7b18ee751 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -1340,7 +1340,7 @@ ssize_t extract_iter_to_sg(struct iov_iter *iter, size_t maxsize,
 			   struct sg_table *sgtable, unsigned int sg_max,
 			   iov_iter_extraction_t extraction_flags)
 {
-	if (maxsize == 0)
+	if (!maxsize || !iter->count)
 		return 0;
 
 	switch (iov_iter_type(iter)) {