Re: [BUG] xfrm: unable to handle kernel NULL pointer dereference

[Steffen Klassert <steffen.klassert@secunet.com>]

On Fri, Nov 16, 2018 at 08:12:46PM +0100, Steffen Klassert wrote:
> On Fri, Nov 16, 2018 at 08:48:00PM +0200, Lennert Buytenhek wrote:
> > On Sat, Nov 10, 2018 at 08:34:34PM +0100, Jean-Philippe Menil wrote:
> > 
> > > we're seeing unexpected crashes from kernel 4.15 to 4.18.17, using
> > > IPsec VTI interfaces, on several vpn hosts, since upgrade from 4.4.
> > 
> > I looked into this with Jean-Philippe, and it appears to be crashing
> > on a NULL pointer dereference in the inlined xfrm_policy_check() call
> > in vti_rcv_cb(), and specifically on the skb_dst(skb) dereference in
> > __xfrm_policy_check2():
> > 
> > 	return  (!net->xfrm.policy_count[dir] && !skb->sp) ||
> > 		(skb_dst(skb)->flags & DST_NOPOLICY) ||		<=====
> > 		__xfrm_policy_check(sk, ndir, skb, family);
> > 
> > Commit 9e1437937807 ("xfrm: Fix NULL pointer dereference when
> > skb_dst_force clears the dst_entry.") fixes a very similar problem on
> > the output and forward paths, but our issue seems to be triggering on
> > the input path.
> 
> Yes, this is the same problem. skb_dst_force() does not
> really force a refcount anymore, it might clear the dst
> pointer instead (maybe this function should be renamed).

I plan to apply this patch to the ipsec tree:

[PATCH RFC] xfrm: Fix NULL pointer dereference in xfrm_input 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. 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 9e1437937807 ("xfrm: Fix NULL pointer dereference when
skb_dst_force clears the dst_entry.")

Fixes: 222d7dbd258d ("net: prevent dst uses after free")
Reported-by: Jean-Philippe Menil <jpmenil@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
---
 net/xfrm/xfrm_input.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/xfrm/xfrm_input.c b/net/xfrm/xfrm_input.c
index 684c0bc01e2c..d5635908587f 100644
--- a/net/xfrm/xfrm_input.c
+++ b/net/xfrm/xfrm_input.c
@@ -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)
-- 
2.17.1