udp: properly support MSG_PEEK with truncated buffers

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@google.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 8841e98..ac14ae4 100644 (file)
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1271,6 +1271,7 @@ int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
        int peeked, off = 0;
        int err;
        int is_udplite = IS_UDPLITE(sk);
+       bool checksum_valid = false;
        bool slow;
 
        if (flags & MSG_ERRQUEUE)
@@ -1296,11 +1297,12 @@ 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_msg(skb, sizeof(struct udphdr),
                                            msg, copied);
        else {

diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 9da3287..00775ee 100644 (file)
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -402,6 +402,7 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
        int peeked, off = 0;
        int err;
        int is_udplite = IS_UDPLITE(sk);
+       bool checksum_valid = false;
        int is_udp4;
        bool slow;
 
@@ -433,11 +434,12 @@ 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_msg(skb, sizeof(struct udphdr),
                                            msg, copied);
        else {