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    Subject[PATCH AUTOSEL 4.14 11/61] xfrm: Fix NULL pointer dereference when skb_dst_force clears the dst_entry.
    Date
    From: Steffen Klassert <steffen.klassert@secunet.com>

    [ Upstream commit 9e1437937807b0122e8da1ca8765be2adca9aee6 ]

    Since commit 222d7dbd258d ("net: prevent dst uses after free")
    skb_dst_force() might clear the dst_entry attached to the skb.
    The xfrm code don't expect this to happen, so we crash with
    a NULL pointer dereference in this case. Fix it by checking
    skb_dst(skb) for NULL after skb_dst_force() and drop the packet
    in cast the dst_entry was cleared.

    Fixes: 222d7dbd258d ("net: prevent dst uses after free")
    Reported-by: Tobias Hommel <netdev-list@genoetigt.de>
    Reported-by: Kristian Evensen <kristian.evensen@gmail.com>
    Reported-by: Wolfgang Walter <linux@stwm.de>
    Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    net/xfrm/xfrm_output.c | 4 ++++
    net/xfrm/xfrm_policy.c | 4 ++++
    2 files changed, 8 insertions(+)

    diff --git a/net/xfrm/xfrm_output.c b/net/xfrm/xfrm_output.c
    index 35610cc881a9..c47660fba498 100644
    --- a/net/xfrm/xfrm_output.c
    +++ b/net/xfrm/xfrm_output.c
    @@ -101,6 +101,10 @@ static int xfrm_output_one(struct sk_buff *skb, int err)
    spin_unlock_bh(&x->lock);

    skb_dst_force(skb);
    + if (!skb_dst(skb)) {
    + XFRM_INC_STATS(net, LINUX_MIB_XFRMOUTERROR);
    + goto error_nolock;
    + }

    if (xfrm_offload(skb)) {
    x->type_offload->encap(x, skb);
    diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
    index 2fb7a78308e1..37c32e73aaef 100644
    --- a/net/xfrm/xfrm_policy.c
    +++ b/net/xfrm/xfrm_policy.c
    @@ -2550,6 +2550,10 @@ int __xfrm_route_forward(struct sk_buff *skb, unsigned short family)
    }

    skb_dst_force(skb);
    + if (!skb_dst(skb)) {
    + XFRM_INC_STATS(net, LINUX_MIB_XFRMFWDHDRERROR);
    + return 0;
    + }

    dst = xfrm_lookup(net, skb_dst(skb), &fl, NULL, XFRM_LOOKUP_QUEUE);
    if (IS_ERR(dst)) {
    --
    2.17.1
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