net/openvswitch: check Ethernet header length in key_extract()

PATCH Superseded Cen Zhang (Microsoft) 3 participants 4 messages Jul 23 – Jul 25

Subsystems (3)

Networking Network drivers Openvswitch

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Cen Zhang (Microsoft)

author

2026-07-23 23:38 UTC #0 lore raw

PATCH v3 1/1 net/openvswitch: check Ethernet header length in key_extract()

When a packet arrives on an ARPHRD_NONE device (e.g. TUN), ovs_flow_key_extract() trusts the user-provided skb->protocol field: if it is ETH_P_TEB, the packet is classified as MAC_PROTO_ETHERNET and key_extract() is called without ensuring the skb has ETH_HLEN (14) bytes of linear data. key_extract() unconditionally pulls 2 * ETH_ALEN bytes for MAC addresses and parse_ethertype() pulls 2 more, either of which triggers a kernel BUG in __skb_pull() when the linear area is too small.

kernel BUG at include/linux/skbuff.h:2848!

  RIP: 0010:key_extract+0xa7e/0xd90 net/openvswitch/flow.c:933
  ovs_flow_key_extract+0x419/0xa70
  ovs_vport_receive+0x222/0x390
  netdev_frame_hook+0x3e0/0x630
  tun_get_user+0x2d0c/0x38e0

Fixed by adding checks in key_extract() before pulling the Ethernet header.

Fixes 217ac77a3c25 ("openvswitch: allow L3 netdev ports")

Reported-by [email protected]

Signed-off-by Cen Zhang (Microsoft) <[email protected]>

---
v3: Use check_header() per review, fix format issue.
v2: Moved the check into key_extract() per Ilya Maximets.

Link https://lore.kernel.org/all/[email protected] open here

 net/openvswitch/flow.c | 5 +++++
 1 file changed, 5 insertions(+)

net/openvswitch/flow.c +5 -0

index 66366982f604..fc4002353e17 100644
@@ -911,6 +911,7 @@ static int key_extract_l3l4(struct sk_buff *skb, struct sw_flow_key *key)
 static int key_extract(struct sk_buff *skb, struct sw_flow_key *key)
 {
 	struct ethhdr *eth;
+	int err;
 
 	/* Flags are always used as part of stats */
 	key->tp.flags = 0;

@@ -926,6 +927,10 @@ static int key_extract(struct sk_buff *skb, struct sw_flow_key *key)
 		skb_reset_network_header(skb);
 		key->eth.type = skb->protocol;
 	} else {
+		err = check_header(skb, ETH_HLEN);
+		if (unlikely(err))
+			return err;
+
 		eth = eth_hdr(skb);
 		ether_addr_copy(key->eth.src, eth->h_source);
 		ether_addr_copy(key->eth.dst, eth->h_dest);

Signature · 1 line

2.53.0

Eelco Chaudron

maintainer

2026-07-24 10:12 UTC #1 lore raw

Quoted 15 lines · Cen Zhang (Microsoft)

When a packet arrives on an ARPHRD_NONE device (e.g. TUN), ovs_flow_key_extract() trusts the user-provided skb->protocol field: if it is ETH_P_TEB, the packet is classified as MAC_PROTO_ETHERNET and key_extract() is called without ensuring the skb has ETH_HLEN (14) bytes of linear data. key_extract() unconditionally pulls 2 * ETH_ALEN bytes for MAC addresses and parse_ethertype() pulls 2 more, either of which triggers a kernel BUG in __skb_pull() when the linear area is too small.
kernel BUG at include/linux/skbuff.h:2848!
  RIP: 0010:key_extract+0xa7e/0xd90 net/openvswitch/flow.c:933
  ovs_flow_key_extract+0x419/0xa70
  ovs_vport_receive+0x222/0x390
  netdev_frame_hook+0x3e0/0x630
  tun_get_user+0x2d0c/0x38e0

Quoted 6 lines

Fixed by adding checks in key_extract() before pulling the Ethernet header.
Fixes 217ac77a3c25 ("openvswitch: allow L3 netdev ports")
Reported-by [email protected]
Signed-off-by Cen Zhang (Microsoft) <[email protected]>

Hi Cen,

Thanks for the new revision.

The doc comment for key_extract() also needs updating. It currently states:

* The caller must ensure that skb->len >= ETH_HLEN.

The comment should be removed or replaced with a note that the function returns -EINVAL when the frame is too short to contain an Ethernet header.

One more comment below.

Cheers,

Eelco

Quoted 13 lines

---
v3: Use check_header() per review, fix format issue.
v2: Moved the check into key_extract() per Ilya Maximets.
Link https://lore.kernel.org/all/[email protected] open here
 net/openvswitch/flow.c | 5 +++++
 1 file changed, 5 insertions(+)
net/openvswitch/flow.c +0 -0
index 66366982f604..fc4002353e17 100644

net/openvswitch/flow.c static int key_extract_l3l4(struct sk_buff *skb, struct sw_flow_key *key) · line 914

Quoted 4 lines

net/openvswitch/flow.c +1 -0

@@ -911,6 +911,7 @@ static int key_extract_l3l4(struct sk_buff *skb, struct sw_flow_key *key)
 static int key_extract(struct sk_buff *skb, struct sw_flow_key *key)
 {
 	struct ethhdr *eth;
+	int err;

Since err is only used in the else block, it should be scoped there:

	} else {
		int err = check_header(skb, ETH_HLEN);

		if (unlikely(err))
			return err;

Quoted 16 lines

net/openvswitch/flow.c +4 -0

 	/* Flags are always used as part of stats */
 	key->tp.flags = 0;

@@ -926,6 +927,10 @@ static int key_extract(struct sk_buff *skb, struct sw_flow_key *key)
 		skb_reset_network_header(skb);
 		key->eth.type = skb->protocol;
 	} else {
+		err = check_header(skb, ETH_HLEN);
+		if (unlikely(err))
+			return err;
+
 		eth = eth_hdr(skb);
 		ether_addr_copy(key->eth.src, eth->h_source);
 		ether_addr_copy(key->eth.dst, eth->h_dest);

Signature · 1 line

2.53.0

Ilya Maximets

maintainer

2026-07-24 12:34 UTC #2 lore raw

Quoted 65 lines · Eelco Chaudron

Quoted 15 lines · Cen Zhang (Microsoft)
When a packet arrives on an ARPHRD_NONE device (e.g. TUN), ovs_flow_key_extract() trusts the user-provided skb->protocol field: if it is ETH_P_TEB, the packet is classified as MAC_PROTO_ETHERNET and key_extract() is called without ensuring the skb has ETH_HLEN (14) bytes of linear data. key_extract() unconditionally pulls 2 * ETH_ALEN bytes for MAC addresses and parse_ethertype() pulls 2 more, either of which triggers a kernel BUG in __skb_pull() when the linear area is too small.
kernel BUG at include/linux/skbuff.h:2848!
  RIP: 0010:key_extract+0xa7e/0xd90 net/openvswitch/flow.c:933
  ovs_flow_key_extract+0x419/0xa70
  ovs_vport_receive+0x222/0x390
  netdev_frame_hook+0x3e0/0x630
  tun_get_user+0x2d0c/0x38e0
Quoted 6 lines
Fixed by adding checks in key_extract() before pulling the Ethernet header.
Fixes 217ac77a3c25 ("openvswitch: allow L3 netdev ports")
Reported-by [email protected]
Signed-off-by Cen Zhang (Microsoft) <[email protected]>
Hi Cen,
Thanks for the new revision.
The doc comment for key_extract() also needs updating. It currently states:
* The caller must ensure that skb->len >= ETH_HLEN.
The comment should be removed or replaced with a note that the function returns -EINVAL when the frame is too short to contain an Ethernet header.
One more comment below.
Cheers,
Eelco
Quoted 13 lines
---
v3: Use check_header() per review, fix format issue.
v2: Moved the check into key_extract() per Ilya Maximets.
Link https://lore.kernel.org/all/[email protected] open here
 net/openvswitch/flow.c | 5 +++++
 1 file changed, 5 insertions(+)
net/openvswitch/flow.c +0 -0
index 66366982f604..fc4002353e17 100644
net/openvswitch/flow.c static int key_extract_l3l4(struct sk_buff *skb, struct sw_flow_key *key) · line 914
Quoted 4 lines
net/openvswitch/flow.c +1 -0
@@ -911,6 +911,7 @@ static int key_extract_l3l4(struct sk_buff *skb, struct sw_flow_key *key)
 static int key_extract(struct sk_buff *skb, struct sw_flow_key *key)
 {
 	struct ethhdr *eth;
+	int err;
Since err is only used in the else block, it should be scoped there:

Quoted 5 lines

	} else {
		int err = check_header(skb, ETH_HLEN);

		if (unlikely(err))
			return err;

The 'eth' is also only used in the 'else' block, but defined at the top. It doesn't matter where, but I think, we should keep them together.

Quoted 18 lines

Quoted 17 lines

net/openvswitch/flow.c +4 -0

 	/* Flags are always used as part of stats */
 	key->tp.flags = 0;

@@ -926,6 +927,10 @@ static int key_extract(struct sk_buff *skb, struct sw_flow_key *key)
 		skb_reset_network_header(skb);
 		key->eth.type = skb->protocol;
 	} else {
+		err = check_header(skb, ETH_HLEN);
+		if (unlikely(err))
+			return err;
+
 		eth = eth_hdr(skb);
 		ether_addr_copy(key->eth.src, eth->h_source);
 		ether_addr_copy(key->eth.dst, eth->h_dest);

Signature · 1 line

2.53.0

Cen Zhang (Microsoft)

author

2026-07-25 04:34 UTC #3 lore raw

Thanks for the reviews. I've sent v4 addressing the comments.