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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <000000000000b2585a05fdeb8379@google.com>
References: <000000000000b2585a05fdeb8379@google.com>
To:     syzbot <syzbot+6efc50cc1f8d718d6cb7@syzkaller.appspotmail.com>
Cc:     dhowells@redhat.com, davem@davemloft.net,
        herbert@gondor.apana.org.au, kuba@kernel.org,
        linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [crypto?] KASAN: slab-out-of-bounds Read in extract_iter_to_sg
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Date:   Wed, 14 Jun 2023 16:51:42 +0100
Message-ID: <1657853.1686757902@warthog.procyon.org.uk>
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#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.g=
it main

    crypto: af_alg/hash: Fix recvmsg() after sendmsg(MSG_MORE)
    =

    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 u=
p 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 mess=
age
    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 retu=
rning
    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@syzkaller.appspotmail.com
    Link: https://lore.kernel.org/r/000000000000b928f705fdeb873a@google.co=
m/
    Signed-off-by: David Howells <dhowells@redhat.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

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 ms=
ghdr *msg,
 =

 	ctx->more =3D false;
 =

-	while (msg_data_left(msg)) {
+	do {
 		ctx->sgl.sgt.sgl =3D ctx->sgl.sgl;
 		ctx->sgl.sgt.nents =3D 0;
 		ctx->sgl.sgt.orig_nents =3D 0;
 =

 		err =3D -EIO;
 		npages =3D iov_iter_npages(&msg->msg_iter, max_pages);
-		if (npages =3D=3D 0)
-			goto unlock_free;
-
-		if (npages > ARRAY_SIZE(ctx->sgl.sgl)) {
-			err =3D -ENOMEM;
-			ctx->sgl.sgt.sgl =3D
-				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 =3D iov_iter_extract_will_pin(&msg->msg_iter);
 =

@@ -111,7 +99,8 @@ static int hash_sendmsg(struct socket *sock, struct msg=
hdr *msg,
 		if (err < 0)
 			goto unlock_free;
 		len =3D 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 =3D hash_alloc_result(sk, ctx);
@@ -148,7 +137,7 @@ static int hash_sendmsg(struct socket *sock, struct ms=
ghdr *msg,
 =

 		copied +=3D len;
 		af_alg_free_sg(&ctx->sgl);
-	}
+	} while (msg_data_left(msg));
 =

 	ctx->more =3D msg->msg_flags & MSG_MORE;
 	err =3D 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, si=
ze_t maxsize,
 			   struct sg_table *sgtable, unsigned int sg_max,
 			   iov_iter_extraction_t extraction_flags)
 {
-	if (maxsize =3D=3D 0)
+	if (!maxsize || !iter->count)
 		return 0;
 =

 	switch (iov_iter_type(iter)) {