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udp: properly support MSG_PEEK with truncated buffers 

Backport of this upstream commit into stable kernels :
89c22d8c3b ("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>
This commit is contained in:
Eric Dumazet 2015-12-30 08:51:12 -05:00 committed by David S. Miller
parent 815bc580fe
commit 197c949e77
2 changed files with 8 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
net/ipv4/udp.c
View File

@ -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 {

6
net/ipv6/udp.c
View File

@ -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 {