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

[Michael Bommarito <michael.bommarito@gmail.com>]

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