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>

1 files changed, 4 insertions, 2 deletions

diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 9da3287a3923..00775ee27d86 100644
--- 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 {