> > Subject: [PATCH RFC] xfrm: Fix NULL pointer dereference when skb_dst_force
> > clears the dst_entry.
> >
> > 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-l...@genoetigt.de>
> > Reported-by: Kristian Evensen <kristian.even...@gmail.com>
> > Reported-by: Wolfgang Walter <li...@stwm.de>
> > Signed-off-by: Steffen Klassert <steffen.klass...@secunet.com>
> > ---
> > 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 89b178a78dc7..36d15a38ce5e 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 7c5e8978aeaa..626e0f4d1749 100644
> > --- a/net/xfrm/xfrm_policy.c
> > +++ b/net/xfrm/xfrm_policy.c
> > @@ -2548,6 +2548,10 @@ 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)) {
>
> This patch fixes the problem here.
>
> XfrmFwdHdrError gets around 80 at the very beginning and remains so. Probably
> this happens when some route are changed/set then.
>
> Regards and thanks,
Same here, we're now running stable for ~6 hours, XfrmFwdHdrError is at 220.
This is less than 1 lost packet per minute, which seems to be okay for now.
