[PATCH 2.6.32 08/42] udp: properly support MSG_PEEK with truncated buffers

[Willy Tarreau]

2.6.32-longterm review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@xxxxxxxxxx>

commit 197c949e7798fbf28cfadc69d9ca0c2abbf93191 upstream.

Backport of this upstream commit into stable kernels :
89c22d8c3b27 ("net: Fix skb csum races when peeking")
exposed a bug in udp stack vs MSG_PEEK support, when user provides
a buffer smaller than skb payload.

In this case,
skb_copy_and_csum_datagram_iovec(skb, sizeof(struct udphdr),
                                 msg->msg_iov);
returns -EFAULT.

This bug does not happen in upstream kernels since Al Viro did a great
job to replace this into :
skb_copy_and_csum_datagram_msg(skb, sizeof(struct udphdr), msg);
This variant is safe vs short buffers.

For the time being, instead reverting Herbert Xu patch and add back
skb->ip_summed invalid changes, simply store the result of
udp_lib_checksum_complete() so that we avoid computing the checksum a
second time, and avoid the problematic
skb_copy_and_csum_datagram_iovec() call.

This patch can be applied on recent kernels as it avoids a double
checksumming, then backported to stable kernels as a bug fix.

Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Acked-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
(cherry picked from commit 18a6eba2eabbcb50a78210b16f7dd43d888a537b)
Signed-off-by: Willy Tarreau <w@xxxxxx>
---
 net/ipv4/udp.c | 6 ++++--
 net/ipv6/udp.c | 6 ++++--
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 3ae286b..f12e404 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -941,6 +941,7 @@ int udp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 	int peeked;
 	int err;
 	int is_udplite = IS_UDPLITE(sk);
+	bool checksum_valid = false;
 
 	if (flags & MSG_ERRQUEUE)
 		return ip_recv_error(sk, msg, len, addr_len);
@@ -965,11 +966,12 @@ try_again:
 	 */
 
 	if (copied < ulen || UDP_SKB_CB(skb)->partial_cov) {
-		if (udp_lib_checksum_complete(skb))
+		checksum_valid = !udp_lib_checksum_complete(skb);
+		if (!checksum_valid)
 			goto csum_copy_err;
 	}
 
-	if (skb_csum_unnecessary(skb))
+	if (checksum_valid || skb_csum_unnecessary(skb))
 		err = skb_copy_datagram_iovec(skb, sizeof(struct udphdr),
 					      msg->msg_iov, copied);
 	else {
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 0b023f3..3f8b3b6 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -198,6 +198,7 @@ int udpv6_recvmsg(struct kiocb *iocb, struct sock *sk,
 	int peeked;
 	int err;
 	int is_udplite = IS_UDPLITE(sk);
+	bool checksum_valid = false;
 	int is_udp4;
 
 	if (flags & MSG_ERRQUEUE)
@@ -225,11 +226,12 @@ try_again:
 	 */
 
 	if (copied < ulen || UDP_SKB_CB(skb)->partial_cov) {
-		if (udp_lib_checksum_complete(skb))
+		checksum_valid = !udp_lib_checksum_complete(skb);
+		if (!checksum_valid)
 			goto csum_copy_err;
 	}
 
-	if (skb_csum_unnecessary(skb))
+	if (checksum_valid || skb_csum_unnecessary(skb))
 		err = skb_copy_datagram_iovec(skb, sizeof(struct udphdr),
 					      msg->msg_iov, copied       );
 	else {
-- 
1.7.12.2.21.g234cd45.dirty