linux-kernel - [PATCH 5.1 30/98] net: correct udp zerocopy refcnt also when zerocopy only on append

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Date:   Thu, 20 Jun 2019 19:56:57 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, syzbot <syzkaller@...glegroups.com>,
        Willem de Bruijn <willemb@...gle.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH 5.1 30/98] net: correct udp zerocopy refcnt also when zerocopy only on append

From: Willem de Bruijn <willemb@...gle.com>

[ Upstream commit 522924b583082f51b8a2406624a2f27c22119b20 ]

The below patch fixes an incorrect zerocopy refcnt increment when
appending with MSG_MORE to an existing zerocopy udp skb.

  send(.., MSG_ZEROCOPY | MSG_MORE);	// refcnt 1
  send(.., MSG_ZEROCOPY | MSG_MORE);	// refcnt still 1 (bar frags)

But it missed that zerocopy need not be passed at the first send. The
right test whether the uarg is newly allocated and thus has extra
refcnt 1 is not !skb, but !skb_zcopy.

  send(.., MSG_MORE);			// <no uarg>
  send(.., MSG_ZEROCOPY);		// refcnt 1

Fixes: 100f6d8e09905 ("net: correct zerocopy refcnt with udp MSG_MORE")
Reported-by: syzbot <syzkaller@...glegroups.com>
Signed-off-by: Willem de Bruijn <willemb@...gle.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/ipv4/ip_output.c  |    2 +-
 net/ipv6/ip6_output.c |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -923,7 +923,7 @@ static int __ip_append_data(struct sock
 		uarg = sock_zerocopy_realloc(sk, length, skb_zcopy(skb));
 		if (!uarg)
 			return -ENOBUFS;
-		extra_uref = !skb;	/* only extra ref if !MSG_MORE */
+		extra_uref = !skb_zcopy(skb);	/* only ref on new uarg */
 		if (rt->dst.dev->features & NETIF_F_SG &&
 		    csummode == CHECKSUM_PARTIAL) {
 			paged = true;
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1344,7 +1344,7 @@ emsgsize:
 		uarg = sock_zerocopy_realloc(sk, length, skb_zcopy(skb));
 		if (!uarg)
 			return -ENOBUFS;
-		extra_uref = !skb;	/* only extra ref if !MSG_MORE */
+		extra_uref = !skb_zcopy(skb);	/* only ref on new uarg */
 		if (rt->dst.dev->features & NETIF_F_SG &&
 		    csummode == CHECKSUM_PARTIAL) {
 			paged = true;