Commit 0152eee6 authored Nov 22, 2018 by Steffen Klassert

xfrm: Fix NULL pointer dereference in xfrm_input when skb_dst_force clears the dst_entry.



Since commit 222d7dbd ("net: prevent dst uses after free")
skb_dst_force() might clear the dst_entry attached to the skb.
The xfrm code doesn'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 case the dst_entry was cleared. We also
move the skb_dst_force() to a codepath that is not used when
the transformation was offloaded, because in this case we
don't have a dst_entry attached to the skb.

The output and forwarding path was already fixed by
commit 9e143793 ("xfrm: Fix NULL pointer dereference when
skb_dst_force clears the dst_entry.")

Fixes: 222d7dbd ("net: prevent dst uses after free")
Reported-by: Jean-Philippe Menil <jpmenil@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

parent ca92e173

net/xfrm/xfrm_input.c

+6 −1

Original line number	Diff line number	Diff line
		@@ -346,6 +346,12 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)

		skb->sp->xvec[skb->sp->len++] = x;

		skb_dst_force(skb);
		if (!skb_dst(skb)) {
		XFRM_INC_STATS(net, LINUX_MIB_XFRMINERROR);
		goto drop;
		}

		lock:
		spin_lock(&x->lock);

		@@ -385,7 +391,6 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
		XFRM_SKB_CB(skb)->seq.input.low = seq;
		XFRM_SKB_CB(skb)->seq.input.hi = seq_hi;

		skb_dst_force(skb);
		dev_hold(skb->dev);

		if (crypto_done)