Commit 0992d67b authored Oct 31, 2020 by Guillaume Nault Committed by Jakub Kicinski Nov 03, 2020

mpls: drop skb's dst in mpls_forward()



Commit 394de110 ("net: Added pointer check for
dst->ops->neigh_lookup in dst_neigh_lookup_skb") added a test in
dst_neigh_lookup_skb() to avoid a NULL pointer dereference. The root
cause was the MPLS forwarding code, which doesn't call skb_dst_drop()
on incoming packets. That is, if the packet is received from a
collect_md device, it has a metadata_dst attached to it that doesn't
implement any dst_ops function.

To align the MPLS behaviour with IPv4 and IPv6, let's drop the dst in
mpls_forward(). This way, dst_neigh_lookup_skb() doesn't need to test
->neigh_lookup any more. Let's keep a WARN condition though, to
document the precondition and to ease detection of such problems in the
future.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/f8c2784c13faa54469a2aac339470b1049ca6b63.1604102750.git.gnault@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>

parent 6d89076e

include/net/dst.h

+5 −7

Original line number	Diff line number	Diff line
		@@ -400,13 +400,11 @@ static inline struct neighbour dst_neigh_lookup(const struct dst_entry dst, co
		static inline struct neighbour dst_neigh_lookup_skb(const struct dst_entry dst,
		struct sk_buff *skb)
		{
		struct neighbour *n = NULL;
		struct neighbour *n;

		if (WARN_ON_ONCE(!dst->ops->neigh_lookup))
		return NULL;

		/* The packets from tunnel devices (eg bareudp) may have only
		* metadata in the dst pointer of skb. Hence a pointer check of
		* neigh_lookup is needed.
		*/
		if (dst->ops->neigh_lookup)
		n = dst->ops->neigh_lookup(dst, skb, NULL);

		return IS_ERR(n) ? NULL : n;

net/mpls/af_mpls.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -377,6 +377,8 @@ static int mpls_forward(struct sk_buff skb, struct net_device dev,
		if (!pskb_may_pull(skb, sizeof(*hdr)))
		goto err;

		skb_dst_drop(skb);

		/* Read and decode the label */
		hdr = mpls_hdr(skb);
		dec = mpls_entry_decode(hdr);

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