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udp: properly support MSG_PEEK with truncated buffers
Backport of this upstream commit into stable kernels :
89c22d8 ("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>
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Eric Dumazet authored and davem330 committed Jan 4, 2016
1 parent 815bc58 commit 197c949
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Showing 2 changed files with 8 additions and 4 deletions.
6 changes: 4 additions & 2 deletions net/ipv4/udp.c
Expand Up @@ -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)
Expand All @@ -1296,11 +1297,12 @@ int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
*/

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 {
Expand Down
6 changes: 4 additions & 2 deletions net/ipv6/udp.c
Expand Up @@ -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;

Expand Down Expand Up @@ -433,11 +434,12 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
*/

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 {
Expand Down

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