Re: [PATCH] net: tap: validate metadata and length for XDP buff before building up skb

[Si-Wei Liu <si-wei.liu@oracle.com>]

On 5/29/2024 7:26 PM, Jason Wang wrote:
> On Thu, May 30, 2024 at 8:54 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
>> The cited commit missed to check against the validity of the length
>> and various pointers on the XDP buff metadata in the tap_get_user_xdp()
>> path, which could cause a corrupted skb to be sent downstack. For
>> instance, tap_get_user() prohibits short frame which has the length
>> less than Ethernet header size from being transmitted, while the
>> skb_set_network_header() in tap_get_user_xdp() would set skb's
>> network_header regardless of the actual XDP buff data size. This
>> could either cause out-of-bound access beyond the actual length, or
>> confuse the underlayer with incorrect or inconsistent header length
>> in the skb metadata.
>>
>> Propose to drop any frame shorter than the Ethernet header size just
>> like how tap_get_user() does. While at it, validate the pointers in
>> XDP buff to avoid potential size overrun.
>>
>> Fixes: 0efac27791ee ("tap: accept an array of XDP buffs through sendmsg()")
>> Cc: jasowang@redhat.com
>> Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
>> ---
>>   drivers/net/tap.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
>> index bfdd3875fe86..69596479536f 100644
>> --- a/drivers/net/tap.c
>> +++ b/drivers/net/tap.c
>> @@ -1177,6 +1177,13 @@ static int tap_get_user_xdp(struct tap_queue *q, struct xdp_buff *xdp)
>>          struct sk_buff *skb;
>>          int err, depth;
>>
>> +       if (unlikely(xdp->data < xdp->data_hard_start ||
>> +                    xdp->data_end < xdp->data ||
>> +                    xdp->data_end - xdp->data < ETH_HLEN)) {
>> +               err = -EINVAL;
>> +               goto err;
>> +       }
> For ETH_HLEN check, is it better to do it in vhost-net?
Not sure. Initially I thought about this as well, but changed mind. 
Although the TUN_MSG_PTR interface was specifically customized for 
vhost-net in the kernel, could there be any userspace app do sendmsg() 
also with customized TUN_MSG_PTR control message over tap's fd? If 
that's possible in reality, I guess limiting the fix to only vhost-net 
in the kernel is narrow scoped.

Additionally, it seems just the skb delivery path in the tap driver (or 
tuntap) that populates the relevant skb field needs the ETH_HLEN check, 
the XDP fast path can just transmit or forward xdp buff as-is without 
having to check the (header) length of payload data. That said, it may 
break some guest applications that intentionally send out short frames 
(for test purpose?) if unconditionally drop all of them from the vhost-net.

> It seems tuntap suffers from this as well.
True, theoretically I can fix tuntap as well, but I don't have a setup 
to test out the code change thoroughly. Any volunteer here to do so 
(test it or fix it)?

>
> And for the check for other xdp fields, it deserves a BUG_ON() or at
> least WARN_ON() as they are set by vhost-net.
Hmmm, WARN_ON may be fine (I don't see userspace is prevented from 
fabricating such invalid addresses through the TUN_MSG_PTR uAPI).

-Siwei
>
> Thanks
>
>> +
>>          if (q->flags & IFF_VNET_HDR)
>>                  vnet_hdr_len = READ_ONCE(q->vnet_hdr_sz);
>>
>> --
>> 2.39.3
>>