Re: [PATCH net v3] net/openvswitch: check Ethernet header length in key_extract()

[Eelco Chaudron]

On 24 Jul 2026, at 1:37, Cen Zhang (Microsoft) 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.
>
>   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: AutonomousCodeSecurity@xxxxxxxxxxxxx
> Signed-off-by: Cen Zhang (Microsoft) <blbllhy@xxxxxxxxx>

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

> ---
> 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/20260721143602.64677-1-blbllhy@xxxxxxxxx
>
>  net/openvswitch/flow.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/net/openvswitch/flow.c b/net/openvswitch/flow.c
> index 66366982f604..fc4002353e17 100644
> --- a/net/openvswitch/flow.c
> +++ b/net/openvswitch/flow.c
> @@ -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;

>
>  	/* 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);
> -- 
> 2.53.0