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

PATCH Applied Cen Zhang (Microsoft) 3 participants 3 messages Jul 30 – Aug 4

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Networking Network drivers Openvswitch
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Cen Zhang (Microsoft)

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#0 lore raw

PATCH v5 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 calling check_header() in key_extract() before accessing the Ethernet header.

Fixes 217ac77a3c25 ("openvswitch: allow L3 netdev ports")
Reported-by [email protected]
Reviewed-by Eelco Chaudron <[email protected]>
Signed-off-by Cen Zhang (Microsoft) <[email protected]>

---
v5: Separate err declaration and initialization per review.
v4: Update documentation and move variables into the Ethernet block.
v3: Use check_header() per review, fix format issue.
v2: Moved the check into key_extract() per Ilya Maximets.

Link https://lore.kernel.org/r/[email protected] open here
 net/openvswitch/flow.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)
net/openvswitch/flow.c +7 -4
index 66366982f604..46c1d66aad8c 100644
@@ -889,8 +889,6 @@ static int key_extract_l3l4(struct sk_buff *skb, struct sw_flow_key *key)
  * Ethernet header
  * @key: output flow key
  *
- * The caller must ensure that skb->len >= ETH_HLEN.
- *
  * Initializes @skb header fields as follows:
  *
  *    - skb->mac_header: the L2 header.

@@ -910,8 +908,6 @@ 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;
-
 	/* Flags are always used as part of stats */
 	key->tp.flags = 0;
 

@@ -926,6 +922,13 @@ static int key_extract(struct sk_buff *skb, struct sw_flow_key *key)
 		skb_reset_network_header(skb);
 		key->eth.type = skb->protocol;
 	} else {
+		struct ethhdr *eth;
+		int err;
+
+		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
maintainer Reviewed-by on v5 1/1
#1 lore raw
Quoted 22 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

Fixed by calling check_header() in key_extract() before accessing the Ethernet header.

Fixes 217ac77a3c25 ("openvswitch: allow L3 netdev ports")
Reported-by [email protected]
Reviewed-by Eelco Chaudron <[email protected]>
Signed-off-by Cen Zhang (Microsoft) <[email protected]>
Quoted 6 lines

---
v5: Separate err declaration and initialization per review.
v4: Update documentation and move variables into the Ethernet block.
v3: Use check_header() per review, fix format issue.
v2: Moved the check into key_extract() per Ilya Maximets.

Link https://lore.kernel.org/r/[email protected] open here
Reviewed-by Ilya Maximets <[email protected]>
CI Applied
#2 lore raw
In reply to Cen Zhang (Microsoft)
This patch was applied to netdev/net.git (main) by Paolo Abeni <[email protected]>:

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni <[email protected]>:

Quoted 9 lines · On Thu, 30 Jul 2026 18:20:06 -0400 you wrote:

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.

[...]

Here is the summary with links:
- [net,v5] net/openvswitch: check Ethernet header length in key_extract()

    https://git.kernel.org/netdev/net/c/cf6f8b29befb

You are awesome, thank you!

Signature · 2 lines
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