netdev - Re: [PATCH net] ipv4: ip_gre: Handle skb_pull() failure in ipgre

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Date: Mon, 27 Nov 2023 10:55:01 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Shigeru Yoshida <syoshida@...hat.com>, 
 davem@...emloft.net, 
 dsahern@...nel.org, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com
Cc: netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 Shigeru Yoshida <syoshida@...hat.com>
Subject: Re: [PATCH net] ipv4: ip_gre: Handle skb_pull() failure in
 ipgre_xmit()

Shigeru Yoshida wrote:
> In ipgre_xmit(), skb_pull() may fail even if pskb_inet_may_pull() returns
> true. For example, applications can create a malformed packet that causes
> this problem with PF_PACKET.

It may fail because because pskb_inet_may_pull does not account for
tunnel->hlen.

Is that what you are referring to with malformed packet? Can you
eloborate a bit on in which way the packet has to be malformed to
reach this?

FYI: I had a quick look at the IPv6 equivalent code.
ip6gre_tunnel_xmit is sufficiently different. It makes sense that this
is an IPv4 only patch.

> This patch fixes the problem by dropping skb and returning from the
> function if skb_pull() fails.
> 
> Fixes: c54419321455 ("GRE: Refactor GRE tunneling code.")
> Signed-off-by: Shigeru Yoshida <syoshida@...hat.com>
> ---
>  net/ipv4/ip_gre.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
> index 22a26d1d29a0..95efa97cb84b 100644
> --- a/net/ipv4/ip_gre.c
> +++ b/net/ipv4/ip_gre.c
> @@ -643,7 +643,8 @@ static netdev_tx_t ipgre_xmit(struct sk_buff *skb,
>  		/* Pull skb since ip_tunnel_xmit() needs skb->data pointing
>  		 * to gre header.
>  		 */
> -		skb_pull(skb, tunnel->hlen + sizeof(struct iphdr));
> +		if (!skb_pull(skb, tunnel->hlen + sizeof(struct iphdr)))
> +			goto free_skb;
>  		skb_reset_mac_header(skb);
>  
>  		if (skb->ip_summed == CHECKSUM_PARTIAL &&
> -- 
> 2.41.0
>