Re: [syzbot] [ext4?] general protection fault in ext4_finish_bio

[David Howells <dhowells@redhat.com>]

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

    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: c662b043cdca ("crypto: af_alg/hash: Support MSG_SPLICE_PAGES")
    Reported-by: syzbot+689ec3afb1ef07b766b2@syzkaller.appspotmail.com
    Link: https://lore.kernel.org/r/000000000000554b8205ffdea64e@google.com/
    Signed-off-by: David Howells <dhowells@redhat.com>
    cc: Herbert Xu <herbert@gondor.apana.org.au>
    cc: Paolo Abeni <pabeni@redhat.com>
    cc: "David S. Miller" <davem@davemloft.net>
    cc: Eric Dumazet <edumazet@google.com>
    cc: Jakub Kicinski <kuba@kernel.org>
    cc: linux-crypto@vger.kernel.org
    cc: netdev@vger.kernel.org

diff --git a/crypto/algif_hash.c b/crypto/algif_hash.c
index 0ab43e149f0e..82c44d4899b9 100644
--- a/crypto/algif_hash.c
+++ b/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);