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    Subject[PATCH 3.10 028/180] udp: properly support MSG_PEEK with truncated buffers
    Date
    From: Eric Dumazet <edumazet@google.com>

    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@google.com>
    Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    [ luis: backported to 3.16: adjusted context ]
    Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
    Signed-off-by: Charles (Chas) Williams <ciwillia@brocade.com>
    Signed-off-by: Willy Tarreau <w@1wt.eu>
    ---
    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 63b536b..68174e4 100644
    --- a/net/ipv4/udp.c
    +++ b/net/ipv4/udp.c
    @@ -1208,6 +1208,7 @@ int udp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
    int peeked, off = 0;
    int err;
    int is_udplite = IS_UDPLITE(sk);
    + bool checksum_valid = false;
    bool slow;

    if (flags & MSG_ERRQUEUE)
    @@ -1233,11 +1234,12 @@ 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 3046d02..d234e6f 100644
    --- a/net/ipv6/udp.c
    +++ b/net/ipv6/udp.c
    @@ -370,6 +370,7 @@ int udpv6_recvmsg(struct kiocb *iocb, struct sock *sk,
    int peeked, off = 0;
    int err;
    int is_udplite = IS_UDPLITE(sk);
    + bool checksum_valid = false;
    int is_udp4;
    bool slow;

    @@ -401,11 +402,12 @@ 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 {
    --
    2.8.0.rc2.1.gbe9624a
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