From 47980b6dbf83961eec1c1363ea986e9c06ff8054 Mon Sep 17 00:00:00 2001 From: Florian Westphal Date: Thu, 14 May 2026 14:21:57 +0200 Subject: netfilter: nf_conntrack_gre: fix gre keymap list corruption Quoting reporter: A race between GRE keymap insertion and destruction can corrupt the kernel list or use a freed object. `nf_ct_gre_keymap_add()` publishes a new keymap pointer before the embedded `list_head` is linked, while `nf_ct_gre_keymap_destroy()` can concurrently delete and free that same object. An unprivileged user can reach this through the PPTP conntrack helper by racing PPTP control messages or helper teardown, leading to KASAN-detectable list corruption/UAF in kernel context. ## Root Cause Analysis `exp_gre()` installs GRE expectations for a PPTP control flow and then adds two GRE keymap entries [..] The add path publishes `ct_pptp_info->keymap[dir]` before linking the embedded list node [..] Concurrent teardown deletes that partially initialized object. Make add/destroy symmetric: install both, destroy both while under lock. Furthermore, we should refuse to publish a new mapping in case ct is going away, else we may leak the allocation. The "retrans" detection is strange: existing mapping is checked for key equality with the new mapping, then for "is on the list" via list walk. But I can't see how an existing keymap entry can be NOT on list. Change this to only check if we're asked to map same tuple again -- if so, skip re-install, else signal failure. Last, add a bug trap for the keymap list; it has to be empty when namespace is going away. Reported-by: Leo Lin Signed-off-by: Florian Westphal --- include/linux/netfilter/nf_conntrack_proto_gre.h | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) (limited to 'include/linux') diff --git a/include/linux/netfilter/nf_conntrack_proto_gre.h b/include/linux/netfilter/nf_conntrack_proto_gre.h index 9ee7014400e8..ad5563f0f864 100644 --- a/include/linux/netfilter/nf_conntrack_proto_gre.h +++ b/include/linux/netfilter/nf_conntrack_proto_gre.h @@ -18,9 +18,10 @@ struct nf_ct_gre_keymap { struct rcu_head rcu; }; -/* add new tuple->key_reply pair to keymap */ -int nf_ct_gre_keymap_add(struct nf_conn *ct, enum ip_conntrack_dir dir, - struct nf_conntrack_tuple *t); +/* add tuple->key_reply pairs to keymap */ +bool nf_ct_gre_keymap_add(struct nf_conn *ct, + const struct nf_conntrack_tuple *orig, + const struct nf_conntrack_tuple *repl); /* delete keymap entries */ void nf_ct_gre_keymap_destroy(struct nf_conn *ct); -- cgit v1.2.3