[PATCH nf] netfilter: conntrack: Avoid nf_ct_helper_hash uses after free

[Florent Revest]

If register_nf_conntrack_bpf() fails (for example, if the .BTF section
contains an invalid entry), nf_conntrack_init_start() calls
nf_conntrack_helper_fini() as part of its cleanup path and
nf_ct_helper_hash gets freed.

Further netfilter modules like netfilter_conntrack_ftp don't check
whether nf_conntrack initialized correctly and call
nf_conntrack_helpers_register() which accesses the freed
nf_ct_helper_hash and causes a uaf.

This patch guards nf_conntrack_helper_register() from accessing
freed/uninitialized nf_ct_helper_hash maps and fixes a boot-time
use-after-free.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 12f7a505331e ("netfilter: add user-space connection tracking helper infrastructure")
Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
---
 net/netfilter/nf_conntrack_helper.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/netfilter/nf_conntrack_helper.c b/net/netfilter/nf_conntrack_helper.c
index 0c4db2f2ac43..f22691f83853 100644
--- a/net/netfilter/nf_conntrack_helper.c
+++ b/net/netfilter/nf_conntrack_helper.c
@@ -360,6 +360,9 @@ int nf_conntrack_helper_register(struct nf_conntrack_helper *me)
 	BUG_ON(me->expect_class_max >= NF_CT_MAX_EXPECT_CLASSES);
 	BUG_ON(strlen(me->name) > NF_CT_HELPER_NAME_LEN - 1);
 
+	if (!nf_ct_helper_hash)
+		return -ENOENT;
+
 	if (me->expect_policy->max_expected > NF_CT_EXPECT_MAX_CNT)
 		return -EINVAL;
 
@@ -515,4 +518,5 @@ int nf_conntrack_helper_init(void)
 void nf_conntrack_helper_fini(void)
 {
 	kvfree(nf_ct_helper_hash);
+	nf_ct_helper_hash = NULL;
 }
-- 
2.41.0.162.gfafddb0af9-goog