[PATCH V2] ipsec: be careful of non existing mac headers

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Niccolo Belli reported ipsec crashes in case we handle a frame without
mac header (atm in his case)

Before copying mac header, better make sure it is present.

Bugzilla reference:  https://bugzilla.kernel.org/show_bug.cgi?id=42809

Reported-by: Niccolò Belli <darkbasic@xxxxxxxxxxxxxxx>
Tested-by: Niccolò Belli <darkbasic@xxxxxxxxxxxxxxx>
Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
---
V2: added skb_mac_header_rebuild() helper as David suggested.

 include/linux/skbuff.h       |   10 ++++++++++
 net/ipv4/xfrm4_mode_beet.c   |    5 +----
 net/ipv4/xfrm4_mode_tunnel.c |    6 ++----
 net/ipv6/xfrm6_mode_beet.c   |    6 +-----
 net/ipv6/xfrm6_mode_tunnel.c |    6 ++----
 5 files changed, 16 insertions(+), 17 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 50db9b0..ae86ade 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1465,6 +1465,16 @@ static inline void skb_set_mac_header(struct sk_buff *skb, const int offset)
 }
 #endif /* NET_SKBUFF_DATA_USES_OFFSET */
 
+static inline void skb_mac_header_rebuild(struct sk_buff *skb)
+{
+	if (skb_mac_header_was_set(skb)) {
+		const unsigned char *old_mac = skb_mac_header(skb);
+
+		skb_set_mac_header(skb, -skb->mac_len);
+		memmove(skb_mac_header(skb), old_mac, skb->mac_len);
+	}
+}
+
 static inline int skb_checksum_start_offset(const struct sk_buff *skb)
 {
 	return skb->csum_start - skb_headroom(skb);
diff --git a/net/ipv4/xfrm4_mode_beet.c b/net/ipv4/xfrm4_mode_beet.c
index 6341818..e3db3f9 100644
--- a/net/ipv4/xfrm4_mode_beet.c
+++ b/net/ipv4/xfrm4_mode_beet.c
@@ -110,10 +110,7 @@ static int xfrm4_beet_input(struct xfrm_state *x, struct sk_buff *skb)
 
 	skb_push(skb, sizeof(*iph));
 	skb_reset_network_header(skb);
-
-	memmove(skb->data - skb->mac_len, skb_mac_header(skb),
-		skb->mac_len);
-	skb_set_mac_header(skb, -skb->mac_len);
+	skb_mac_header_rebuild(skb);
 
 	xfrm4_beet_make_header(skb);
 
diff --git a/net/ipv4/xfrm4_mode_tunnel.c b/net/ipv4/xfrm4_mode_tunnel.c
index 534972e..ed4bf11 100644
--- a/net/ipv4/xfrm4_mode_tunnel.c
+++ b/net/ipv4/xfrm4_mode_tunnel.c
@@ -66,7 +66,6 @@ static int xfrm4_mode_tunnel_output(struct xfrm_state *x, struct sk_buff *skb)
 
 static int xfrm4_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
 {
-	const unsigned char *old_mac;
 	int err = -EINVAL;
 
 	if (XFRM_MODE_SKB_CB(skb)->protocol != IPPROTO_IPIP)
@@ -84,10 +83,9 @@ static int xfrm4_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
 	if (!(x->props.flags & XFRM_STATE_NOECN))
 		ipip_ecn_decapsulate(skb);
 
-	old_mac = skb_mac_header(skb);
-	skb_set_mac_header(skb, -skb->mac_len);
-	memmove(skb_mac_header(skb), old_mac, skb->mac_len);
 	skb_reset_network_header(skb);
+	skb_mac_header_rebuild(skb);
+
 	err = 0;
 
 out:
diff --git a/net/ipv6/xfrm6_mode_beet.c b/net/ipv6/xfrm6_mode_beet.c
index a81ce94..9949a35 100644
--- a/net/ipv6/xfrm6_mode_beet.c
+++ b/net/ipv6/xfrm6_mode_beet.c
@@ -80,7 +80,6 @@ static int xfrm6_beet_output(struct xfrm_state *x, struct sk_buff *skb)
 static int xfrm6_beet_input(struct xfrm_state *x, struct sk_buff *skb)
 {
 	struct ipv6hdr *ip6h;
-	const unsigned char *old_mac;
 	int size = sizeof(struct ipv6hdr);
 	int err;
 
@@ -90,10 +89,7 @@ static int xfrm6_beet_input(struct xfrm_state *x, struct sk_buff *skb)
 
 	__skb_push(skb, size);
 	skb_reset_network_header(skb);
-
-	old_mac = skb_mac_header(skb);
-	skb_set_mac_header(skb, -skb->mac_len);
-	memmove(skb_mac_header(skb), old_mac, skb->mac_len);
+	skb_mac_header_rebuild(skb);
 
 	xfrm6_beet_make_header(skb);
 
diff --git a/net/ipv6/xfrm6_mode_tunnel.c b/net/ipv6/xfrm6_mode_tunnel.c
index 261e6e6..9f2095b 100644
--- a/net/ipv6/xfrm6_mode_tunnel.c
+++ b/net/ipv6/xfrm6_mode_tunnel.c
@@ -63,7 +63,6 @@ static int xfrm6_mode_tunnel_output(struct xfrm_state *x, struct sk_buff *skb)
 static int xfrm6_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
 {
 	int err = -EINVAL;
-	const unsigned char *old_mac;
 
 	if (XFRM_MODE_SKB_CB(skb)->protocol != IPPROTO_IPV6)
 		goto out;
@@ -80,10 +79,9 @@ static int xfrm6_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
 	if (!(x->props.flags & XFRM_STATE_NOECN))
 		ipip6_ecn_decapsulate(skb);
 
-	old_mac = skb_mac_header(skb);
-	skb_set_mac_header(skb, -skb->mac_len);
-	memmove(skb_mac_header(skb), old_mac, skb->mac_len);
 	skb_reset_network_header(skb);
+	skb_mac_header_rebuild(skb);
+
 	err = 0;
 
 out:


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