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    Subject[PATCH 3.19.y-ckt 085/102] net: Fix skb_set_peeked use-after-free bug
    Date
    3.19.8-ckt7 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Herbert Xu <herbert@gondor.apana.org.au>

    commit a0a2a6602496a45ae838a96db8b8173794b5d398 upstream.

    The commit 738ac1ebb96d02e0d23bc320302a6ea94c612dec ("net: Clone
    skb before setting peeked flag") introduced a use-after-free bug
    in skb_recv_datagram. This is because skb_set_peeked may create
    a new skb and free the existing one. As it stands the caller will
    continue to use the old freed skb.

    This patch fixes it by making skb_set_peeked return the new skb
    (or the old one if unchanged).

    Fixes: 738ac1ebb96d ("net: Clone skb before setting peeked flag")
    Reported-by: Brenden Blanco <bblanco@plumgrid.com>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
    Tested-by: Brenden Blanco <bblanco@plumgrid.com>
    Reviewed-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Kamal Mostafa <kamal@canonical.com>
    ---
    net/core/datagram.c | 13 +++++++------
    1 file changed, 7 insertions(+), 6 deletions(-)

    diff --git a/net/core/datagram.c b/net/core/datagram.c
    index 91dbcfa..f5cce20 100644
    --- a/net/core/datagram.c
    +++ b/net/core/datagram.c
    @@ -131,12 +131,12 @@ out_noerr:
    goto out;
    }

    -static int skb_set_peeked(struct sk_buff *skb)
    +static struct sk_buff *skb_set_peeked(struct sk_buff *skb)
    {
    struct sk_buff *nskb;

    if (skb->peeked)
    - return 0;
    + return skb;

    /* We have to unshare an skb before modifying it. */
    if (!skb_shared(skb))
    @@ -144,7 +144,7 @@ static int skb_set_peeked(struct sk_buff *skb)

    nskb = skb_clone(skb, GFP_ATOMIC);
    if (!nskb)
    - return -ENOMEM;
    + return ERR_PTR(-ENOMEM);

    skb->prev->next = nskb;
    skb->next->prev = nskb;
    @@ -157,7 +157,7 @@ static int skb_set_peeked(struct sk_buff *skb)
    done:
    skb->peeked = 1;

    - return 0;
    + return skb;
    }

    /**
    @@ -229,8 +229,9 @@ struct sk_buff *__skb_recv_datagram(struct sock *sk, unsigned int flags,
    continue;
    }

    - error = skb_set_peeked(skb);
    - if (error)
    + skb = skb_set_peeked(skb);
    + error = PTR_ERR(skb);
    + if (IS_ERR(skb))
    goto unlock_err;

    atomic_inc(&skb->users);
    --
    1.9.1


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