Re: [PATCH net v3] ipv4: ip_gre: Fix drops of small packets in ipgre_xmit

[Eric Dumazet]

On Wed, Sep 25, 2024 at 1:53 AM Anton Danilov
<littlesmilingcloud@xxxxxxxxx> wrote:
>
> Regression Description:
>
> Depending on the options specified for the GRE tunnel device, small
> packets may be dropped. This occurs because the pskb_network_may_pull
> function fails due to the packet's insufficient length.
>
> For example, if only the okey option is specified for the tunnel device,
> original (before encapsulation) packets smaller than 28 bytes (including
> the IPv4 header) will be dropped. This happens because the required
> length is calculated relative to the network header, not the skb->head.
>
> Here is how the required length is computed and checked:
>
> * The pull_len variable is set to 28 bytes, consisting of:
>   * IPv4 header: 20 bytes
>   * GRE header with Key field: 8 bytes
>
> * The pskb_network_may_pull function adds the network offset, shifting
> the checkable space further to the beginning of the network header and
> extending it to the beginning of the packet. As a result, the end of
> the checkable space occurs beyond the actual end of the packet.
>
> Instead of ensuring that 28 bytes are present in skb->head, the function
> is requesting these 28 bytes starting from the network header. For small
> packets, this requested length exceeds the actual packet size, causing
> the check to fail and the packets to be dropped.
>
> This issue affects both locally originated and forwarded packets in
> DMVPN-like setups.
>
> How to reproduce (for local originated packets):
>
>   ip link add dev gre1 type gre ikey 1.9.8.4 okey 1.9.8.4 \
>           local <your-ip> remote 0.0.0.0
>
>   ip link set mtu 1400 dev gre1
>   ip link set up dev gre1
>   ip address add 192.168.13.1/24 dev gre1
>   ip neighbor add 192.168.13.2 lladdr <remote-ip> dev gre1
>   ping -s 1374 -c 10 192.168.13.2
>   tcpdump -vni gre1
>   tcpdump -vni <your-ext-iface> 'ip proto 47'
>   ip -s -s -d link show dev gre1
>
> Solution:
>
> Use the pskb_may_pull function instead the pskb_network_may_pull.
>
> Fixes: 80d875cfc9d3 ("ipv4: ip_gre: Avoid skb_pull() failure in ipgre_xmit()")
>
> Signed-off-by: Anton Danilov <littlesmilingcloud@xxxxxxxxx>

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>