Re: [PATCH ipsec] xfrm: avoid using skb->mac_len to decide if mac header is shown

[Florian Westphal <fw@strlen.de>]

En-Wei Wu <en-wei.wu@canonical.com> wrote:
> When we use Intel WWAN with xfrm, our system always hangs after
> browsing websites for a few seconds. The error message shows that
> it is a slab-out-of-bounds error:
> 
> The reason is that the eth_hdr(skb) inside the if statement evaluated
> to an unexpected address with skb->mac_header = ~0U (indicating there
> is no MAC header). The unreliability of skb->mac_len causes the if
> statement to become true even if there is no MAC header inside the
> skb data buffer.
> 
> Replace the skb->mac_len in the if statement with the more reliable macro
> skb_mac_header_was_set(skb) fixes this issue.
> 
> Fixes: b3284df1c86f ("xfrm: remove input2 indirection from xfrm_mode")

No, that just moved code around.

> Signed-off-by: En-Wei Wu <en-wei.wu@canonical.com>
> ---
>  net/xfrm/xfrm_input.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/xfrm/xfrm_input.c b/net/xfrm/xfrm_input.c
> index 749e7eea99e4..93b261340105 100644
> --- a/net/xfrm/xfrm_input.c
> +++ b/net/xfrm/xfrm_input.c
> @@ -251,7 +251,7 @@ static int xfrm4_remove_tunnel_encap(struct xfrm_state *x, struct sk_buff *skb)
>  
>  	skb_reset_network_header(skb);
>  	skb_mac_header_rebuild(skb);
> -	if (skb->mac_len)
> +	if (skb_mac_header_was_set(skb))
>  		eth_hdr(skb)->h_proto = skb->protocol;

I think you will need to check both, else you restore the bug fixed in
87cdf3148b11 ("xfrm: Verify MAC header exists before overwriting
eth_hdr(skb)->h_proto").