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

[Ilya Maximets <i.maximets@ovn.org>]

On 7/21/26 4:36 PM, 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 check_header(skb, ETH_HLEN) in ovs_flow_key_extract()
> before calling key_extract().
> 
> Fixes: 217ac77a3c25 ("openvswitch: allow L3 netdev ports")
> Reported-by: AutonomousCodeSecurity@microsoft.com
> Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
> ---
>  net/openvswitch/flow.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/net/openvswitch/flow.c b/net/openvswitch/flow.c
> index 66366982f604..a0f9d788d284 100644
> --- a/net/openvswitch/flow.c
> +++ b/net/openvswitch/flow.c
> @@ -1035,6 +1035,13 @@ int ovs_flow_key_extract(const struct ip_tunnel_info *tun_info,
>  		return res;
>  	key->mac_proto = res;
>  
> +	if (res == MAC_PROTO_ETHERNET) {
> +		int err = check_header(skb, ETH_HLEN);
> +
> +		if (unlikely(err))
> +			return err;
> +	}
I'd suggest moving this check closer to the place where we're pulling
the header, i.e. into the key_extract() function itself.

Best regards, Ilya Maximets.