Re: [PATCH net] amt: linearize skb in amt_rcv() to fix use-after-free of cached headers

On Tue, Jul 21, 2026 at 5:10 PM Eric Dumazet [off-list ref] wrote:

On Wed, Jul 22, 2026 at 1:28 AM Xiang Mei (Microsoft) [off-list ref] wrote:

quoted

The AMT message handlers cache pointers into the skb linear area (grec,
iph, ip6h, eth) and keep using them across ip_mc_may_pull() /
pskb_may_pull(). On a non-linear skb (e.g. a reassembled fragmented AMT
membership update) such a pull can reallocate skb->head via
pskb_expand_head(), leaving those pointers dangling into freed memory.

Linearize the skb on entry to amt_rcv() so no later pull reallocates the
head. This is a no-op for the common linear skb, and covers all handlers
at once. Drop the packet if linearization fails.

  BUG: KASAN: slab-use-after-free in amt_igmpv3_report_handler (drivers/net/amt.c:2023)
  Read of size 4 at addr ffff888013fdd71c by task exploit
  Call Trace:
   <IRQ>
   amt_igmpv3_report_handler (drivers/net/amt.c:2023)
   amt_update_handler (drivers/net/amt.c:2503)
   amt_rcv (drivers/net/amt.c:2846)
   udp_queue_rcv_one_skb (net/ipv4/udp.c:2389)
   udp_unicast_rcv_skb (net/ipv4/udp.c:2580)
   ip_protocol_deliver_rcu (net/ipv4/ip_input.c:207)
   ip_local_deliver_finish (net/ipv4/ip_input.c:242)
   ip_local_deliver (net/ipv4/ip_input.c:262)
   ip_rcv (net/ipv4/ip_input.c:612)
   __netif_receive_skb_one_core (net/core/dev.c:6212)

Fixes: cbc21dc1cfe9 ("amt: add data plane of amt interface")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Xiang Mei (Microsoft) <redacted>
---
 drivers/net/amt.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/amt.c b/drivers/net/amt.c
index 951dd10e192b..2bce99d45d2c 100644
--- a/drivers/net/amt.c
+++ b/drivers/net/amt.c

@@ -2785,6 +2785,10 @@ static int amt_rcv(struct sock *sk, struct sk_buff *skb)
        }

        skb->dev = amt->dev;
+       if (skb_linearize(skb)) {
+               err = true;
+               goto drop;
+       }
        iph = ip_hdr(skb);
        type = amt_parse_type(skb);
        if (type == -1) {
--

2.43.0

You are essentially killing the performance and usability of this module.

pw-bot: cr

Thanks for your review. I focus too much on a small and elegant patch,
ignoring the functionality, Lol.
I'll revise after understanding the performance concerns.

Xiang