[PATCH net v2] amt: don't read the IP source address from a reallocated skb header

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

amt_update_handler() caches iph = ip_hdr(skb) and then calls
pskb_may_pull(). pskb_may_pull() can reallocate the skb head: the new
head is allocated and the old one is freed. The cached iph is not
refreshed, so the following tunnel lookup reads iph->saddr from the
freed head. On an AMT relay this lookup runs for every incoming
membership update, before the update's nonce and response MAC are
validated.

The sibling handlers amt_multicast_data_handler() and
amt_membership_query_handler() re-read ip_hdr() after the pull and are
not affected; only amt_update_handler() keeps the pre-pull pointer.

Snapshot the source address before the pulls and match against the
snapshot.

The stale read was confirmed by instrumentation rather than a sanitizer:
after the head is reallocated the comparison reads from the freed old
head. KASAN does not flag it because the skb head is released through
the page-fragment free path, which is not poisoned on free.

Fixes: cbc21dc1cfe9 ("amt: add data plane of amt interface")
Acked-by: Taehee Yoo <ap420073@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
v2: per Taehee Yoo's review
    (https://lore.kernel.org/all/CAMArcTWCg4x1bxrzr+XHc_FqbzJELCMu+tE=x8Jhewgr-_A3Rw@mail.gmail.com/):
    - retag the subject as [PATCH net] (this is a bug fix);
    - drop Cc: stable -- the Fixes tag is enough for the stable backport
      process to pick it up;
    - carry Taehee Yoo's Acked-by.
    No code change from v1.
    v1: https://lore.kernel.org/all/20260614155539.3106537-1-michael.bommarito@gmail.com/

Confirmed on x86_64 by instrumenting the comparison: with the update
packet built so the first pskb_may_pull() reallocates the head (it pulls
bytes out of a page fragment with no tailroom), the read runs against
the freed old head -- the head pointer moves and the old page's refcount
is 0. Neither generic KASAN nor arm64 HW-tag KASAN reports it: page-
fragment frees are not synchronously poisoned, and under MTE the freed
page keeps a tag matching the stale pointer, so this class of stale-
header read escapes the usual fuzzing oracles. On a live relay the freed
head is also exposed to reuse by later skb allocations.

  amtdbg: cmp reads iph=...e000 (skb->head=...384380) stale_head=1 ref=0

A KUnit covering the re-read can follow separately.

 drivers/net/amt.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/amt.c b/drivers/net/amt.c
index f2f3139..af6e28d 100644
--- a/drivers/net/amt.c
+++ b/drivers/net/amt.c
@@ -2455,8 +2455,10 @@ static bool amt_update_handler(struct amt_dev *amt, struct sk_buff *skb)
 	struct ethhdr *eth;
 	struct iphdr *iph;
 	int len, hdr_size;
+	__be32 saddr;
 
 	iph = ip_hdr(skb);
+	saddr = iph->saddr;
 
 	hdr_size = sizeof(*amtmu) + sizeof(struct udphdr);
 	if (!pskb_may_pull(skb, hdr_size))
@@ -2472,7 +2474,7 @@ static bool amt_update_handler(struct amt_dev *amt, struct sk_buff *skb)
 	skb_reset_network_header(skb);
 
 	list_for_each_entry_rcu(tunnel, &amt->tunnel_list, list) {
-		if (tunnel->ip4 == iph->saddr) {
+		if (tunnel->ip4 == saddr) {
 			if ((amtmu->nonce == tunnel->nonce &&
 			     amtmu->response_mac == tunnel->mac)) {
 				mod_delayed_work(amt_wq, &tunnel->gc_wq,
base-commit: 5200f5f493f79f14bbdc349e402a40dfb32f23c8
-- 
2.53.0