[PATCH] xfrm: fix compat ALLOCSPI request use-after-free

[David Lee <david.lee@trailofbits.com>]

xfrm_state_netlink() builds the ALLOCSPI response with
dump_one_state(), which already calls alloc_compat() with the response
skb and header.

xfrm_alloc_userspi() then calls alloc_compat() again, but passes the
original request skb and its header. For a compat request, the
translator therefore interprets the 228-byte compat xfrm_userspi_info
as the 232-byte native layout and reads four bytes past the declared
payload. It also publishes the translated child through the request's
frag_list.

A multicast clone of the request shares skb_shared_info and can observe
that child. xfrm_user_rcv_msg() frees it after the request handler
returns, racing a compat receiver which may still be copying from it and
resulting in a use-after-free.

Remove the redundant conversion. The response keeps its correct compat
translation from dump_one_state(), and no child is attached to the
inbound request.

Fixes: 5f3eea6b7e8f ("xfrm/compat: Attach xfrm dumps to 64=>32 bit translator")
Bug found and triaged by OpenAI Security Research and 
validated by Trail of Bits.

Assisted-by: Codex:gpt-5.6-sol gpt-5.5-cyber
Signed-off-by: Kyle Zeng <kylebot@openai.com>
---
Trail of Bits has a reproducer for this bug that triggers a
KASAN use-after-free and can share if needed.

 net/xfrm/xfrm_user.c | 12 ------------
 1 file changed, 12 deletions(-)

diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
index d6db63304ba6bd5bbd9c09c665170936e1107ced..a7f35a123648d3946aae88674c7bead5666c9eb3 100644
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -1877,7 +1877,6 @@ static int xfrm_alloc_userspi(struct sk_buff *skb, struct nlmsghdr *nlh,
 	struct net *net = sock_net(skb->sk);
 	struct xfrm_state *x;
 	struct xfrm_userspi_info *p;
-	struct xfrm_translator *xtr;
 	struct sk_buff *resp_skb;
 	xfrm_address_t *daddr;
 	int family;
@@ -1943,17 +1942,6 @@ static int xfrm_alloc_userspi(struct sk_buff *skb, struct nlmsghdr *nlh,
 		goto out;
 	}
 
-	xtr = xfrm_get_translator();
-	if (xtr) {
-		err = xtr->alloc_compat(skb, nlmsg_hdr(skb));
-
-		xfrm_put_translator(xtr);
-		if (err) {
-			kfree_skb(resp_skb);
-			goto out;
-		}
-	}
-
 	err = nlmsg_unicast(xfrm_net_nlsk(net, skb), resp_skb, NETLINK_CB(skb).portid);
 
 out:
-- 
2.53.0