Re: [PATCH net] ila: reload IPv6 header after pskb_may_pull in checksum adjust

[Antoine Tenart <atenart@kernel.org>]

Hi Michael,

On Sat, Jul 11, 2026 at 11:06:48AM -0400, Michael Bommarito wrote:
> ila_csum_adjust_transport() caches ip6h = ipv6_hdr(skb) before calling
> pskb_may_pull(). On a non-linear skb whose transport header sits in a page
> fragment, pskb_may_pull() can call __pskb_pull_tail() / pskb_expand_head()
> and free the old skb head, leaving ip6h dangling; the following
> get_csum_diff(ip6h, p) then reads freed memory. ila_update_ipv6_locator()
> has the same pattern and additionally writes the new locator through the
> stale destination-address pointer.
> 
> Impact: a remote IPv6 packet routed through a configured ILA
> csum-adjust-transport route or receive-side mapping triggers a
> slab-use-after-free in ila_update_ipv6_locator() (KASAN). The route or
> mapping requires CAP_NET_ADMIN to configure, but trigger packets are
> unauthenticated once it exists.
> 
> Reload ip6h (and the derived iaddr) after each pskb_may_pull() before use,
> matching the transport-header reload the code already performs.
> 
> Fixes: 33f11d16142b ("ila: Create net/ipv6/ila directory")
> Cc: stable@vger.kernel.org
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
> ---
>  net/ipv6/ila/ila_common.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/net/ipv6/ila/ila_common.c b/net/ipv6/ila/ila_common.c
> index e71571455c8a0..acedc5a84e4d7 100644
> --- a/net/ipv6/ila/ila_common.c
> +++ b/net/ipv6/ila/ila_common.c
> @@ -85,6 +85,7 @@ static void ila_csum_adjust_transport(struct sk_buff *skb,
>  			struct tcphdr *th = (struct tcphdr *)
>  					(skb_network_header(skb) + nhoff);
>  
> +			ip6h = ipv6_hdr(skb);
>  			diff = get_csum_diff(ip6h, p);
>  			inet_proto_csum_replace_by_diff(&th->check, skb,
>  							diff, true, true);
> @@ -96,6 +97,7 @@ static void ila_csum_adjust_transport(struct sk_buff *skb,
>  					(skb_network_header(skb) + nhoff);
>  
>  			if (uh->check || skb->ip_summed == CHECKSUM_PARTIAL) {
> +				ip6h = ipv6_hdr(skb);
>  				diff = get_csum_diff(ip6h, p);
>  				inet_proto_csum_replace_by_diff(&uh->check, skb,
>  								diff, true, true);
> @@ -110,6 +112,7 @@ static void ila_csum_adjust_transport(struct sk_buff *skb,
>  			struct icmp6hdr *ih = (struct icmp6hdr *)
>  					(skb_network_header(skb) + nhoff);
>  
> +			ip6h = ipv6_hdr(skb);
>  			diff = get_csum_diff(ip6h, p);
>  			inet_proto_csum_replace_by_diff(&ih->icmp6_cksum, skb,
>  							diff, true, true);
> @@ -151,6 +154,9 @@ void ila_update_ipv6_locator(struct sk_buff *skb, struct ila_params *p,
>  		break;
>  	}
>  
> +	ip6h = ipv6_hdr(skb);
> +	iaddr = ila_a2i(&ip6h->daddr);

You should be able to reload the pointers only in the
ILA_CSUM_ADJUST_TRANSPORT case.

> +
>  	/* Now change destination address */
>  	iaddr->loc = p->locator;
>  }
> -- 
> 2.53.0
>