crypto: algif_hash - Fix race between MORE and non-MORE sends

The 'MSG_MORE' state of the previous sendmsg() is fetched without the
socket lock held, so two sendmsg calls can race.  This can be seen with a
large sendfile() as that now does a series of sendmsg() calls, and if a
write() comes in on the same socket at an inopportune time, it can flip the
state.

Fix this by moving the fetch of ctx->more inside the socket lock.

Fixes: c662b04 ("crypto: af_alg/hash: Support MSG_SPLICE_PAGES")
Reported-by: [email protected]
Link: https://lore.kernel.org/r/[email protected]/
Signed-off-by: David Howells <[email protected]>
Tested-by: [email protected]
cc: Herbert Xu <[email protected]>
cc: Paolo Abeni <[email protected]>
cc: "David S. Miller" <[email protected]>
cc: Eric Dumazet <[email protected]>
cc: Jakub Kicinski <[email protected]>
cc: [email protected]
cc: [email protected]
Signed-off-by: Herbert Xu <[email protected]>

Author: dhowells

crypto/algif_hash.c

@@ -68,13 +68,15 @@ static int hash_sendmsg(struct socket *sock, struct msghdr *msg,
 	struct hash_ctx *ctx = ask->private;
 	ssize_t copied = 0;
 	size_t len, max_pages, npages;
-	bool continuing = ctx->more, need_init = false;
+	bool continuing, need_init = false;
 	int err;
 
 	max_pages = min_t(size_t, ALG_MAX_PAGES,
 			  DIV_ROUND_UP(sk->sk_sndbuf, PAGE_SIZE));
 
 	lock_sock(sk);
+	continuing = ctx->more;
+
 	if (!continuing) {
 		/* Discard a previous request that wasn't marked MSG_MORE. */
 		hash_free_result(sk, ctx);