[PATCH bpf v3] bpf: Fix netns reference imbalance in conntrack kfuncs

The opts argument of the BPF conntrack kfuncs can point to a shared
map value.  __bpf_nf_ct_lookup() and __bpf_nf_ct_alloc_entry() read
opts->netns_id separately when acquiring and releasing the network
namespace reference.

The reference imbalance can occur as follows:

  CPU 0                                  CPU 1
  read opts->netns_id (-1)
  skip get_net_ns_by_id()
                                         write opts->netns_id (id)
  read opts->netns_id (id)
  put_net(net) /* no matching get */

The reverse transition leaks the reference.  Repeating the unmatched put
can destroy a live namespace and crash later users.

The kernel reported:

  Oops: general protection fault, probably for non-canonical address
  KASAN: null-ptr-deref in range [0x00000000000000e8-0x00000000000000ef]
  RIP: 0010:bpf_prog_test_run_xdp+0x52c/0x1700
  Call Trace:
   __sys_bpf+0x1662/0x50c0
   __x64_sys_bpf+0x73/0xb0
   do_syscall_64+0xf9/0x540
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
  Kernel panic - not syncing: Fatal exception

Snapshot every input field of opts with READ_ONCE() before validating or
using it.  The netns_id snapshot keeps the namespace get/put pair
balanced, while the other snapshots keep the remaining options from
changing partway through an invocation.  The individual reads can still
observe an inconsistent combination during a concurrent update, but each
selected field value remains stable for that invocation.

Fixes: aed8ee7feb44 ("net: netfilter: Deduplicate code in bpf_{xdp,skb}_ct_lookup")
Fixes: d7e79c97c00c ("net: netfilter: Add kfuncs to allocate and insert CT")
Signed-off-by: Chengfeng Ye <[email protected]>
---
Changes in v3:
- Inline the one-use reserved-byte READ_ONCE() checks instead of
  copying them into a local array.
- Reorder the new local declarations in reverse-xmas-tree order.

Link: https://lore.kernel.org/bpf/[email protected]/ [v2]

Changes in v2:
- Snapshot l4proto, ct_zone_id, ct_zone_dir, and the reserved bytes in
  addition to netns_id, as requested in review.
- Rebase onto bpf/master.

Link: https://lore.kernel.org/bpf/[email protected]/ [v1]

Please queue this fix for stable kernels.

 net/netfilter/nf_conntrack_bpf.c | 72 +++++++++++++++++++++-----------
 1 file changed, 48 insertions(+), 24 deletions(-)

diff --git a/net/netfilter/nf_conntrack_bpf.c b/net/netfilter/nf_conntrack_bpf.c
index f98d1d4b42c3..c2df7c948281 100644
--- a/net/netfilter/nf_conntrack_bpf.c
+++ b/net/netfilter/nf_conntrack_bpf.c
@@ -122,42 +122,54 @@ __bpf_nf_ct_alloc_entry(struct net *net, struct bpf_sock_tuple *bpf_tuple,
 	struct nf_conntrack_tuple otuple, rtuple;
 	struct nf_conntrack_zone ct_zone;
 	struct nf_conn *ct;
+	u8 ct_zone_dir = 0;
+	u16 ct_zone_id;
+	s32 netns_id;
+	u8 l4proto;
 	int err;
 
 	if (!(opts_len == NF_BPF_CT_OPTS_SZ || opts_len == 12))
 		return ERR_PTR(-EINVAL);
+
+	netns_id = READ_ONCE(opts->netns_id);
+	l4proto = READ_ONCE(opts->l4proto);
+	ct_zone_id = READ_ONCE(opts->ct_zone_id);
 	if (opts_len == NF_BPF_CT_OPTS_SZ) {
-		if (opts->reserved[0] || opts->reserved[1] || opts->reserved[2])
+		ct_zone_dir = READ_ONCE(opts->ct_zone_dir);
+		if (READ_ONCE(opts->reserved[0]) ||
+		    READ_ONCE(opts->reserved[1]) ||
+		    READ_ONCE(opts->reserved[2]))
 			return ERR_PTR(-EINVAL);
 	} else {
-		if (opts->ct_zone_id)
+		if (ct_zone_id)
 			return ERR_PTR(-EINVAL);
 	}
 
-	if (unlikely(opts->netns_id < BPF_F_CURRENT_NETNS))
+	if (unlikely(netns_id < BPF_F_CURRENT_NETNS))
 		return ERR_PTR(-EINVAL);
 
-	err = bpf_nf_ct_tuple_parse(bpf_tuple, tuple_len, opts->l4proto,
+	err = bpf_nf_ct_tuple_parse(bpf_tuple, tuple_len, l4proto,
 				    IP_CT_DIR_ORIGINAL, &otuple);
 	if (err < 0)
 		return ERR_PTR(err);
 
-	err = bpf_nf_ct_tuple_parse(bpf_tuple, tuple_len, opts->l4proto,
+	err = bpf_nf_ct_tuple_parse(bpf_tuple, tuple_len, l4proto,
 				    IP_CT_DIR_REPLY, &rtuple);
 	if (err < 0)
 		return ERR_PTR(err);
 
-	if (opts->netns_id >= 0) {
-		net = get_net_ns_by_id(net, opts->netns_id);
+	if (netns_id >= 0) {
+		net = get_net_ns_by_id(net, netns_id);
 		if (unlikely(!net))
 			return ERR_PTR(-ENONET);
 	}
 
 	if (opts_len == NF_BPF_CT_OPTS_SZ) {
-		if (opts->ct_zone_dir == 0)
-			opts->ct_zone_dir = NF_CT_DEFAULT_ZONE_DIR;
-		nf_ct_zone_init(&ct_zone,
-				opts->ct_zone_id, opts->ct_zone_dir, 0);
+		if (ct_zone_dir == 0) {
+			ct_zone_dir = NF_CT_DEFAULT_ZONE_DIR;
+			opts->ct_zone_dir = ct_zone_dir;
+		}
+		nf_ct_zone_init(&ct_zone, ct_zone_id, ct_zone_dir, 0);
 	} else {
 		ct_zone = nf_ct_zone_dflt;
 	}
@@ -171,7 +183,7 @@ __bpf_nf_ct_alloc_entry(struct net *net, struct bpf_sock_tuple *bpf_tuple,
 	__nf_ct_set_timeout(ct, timeout * HZ);
 
 out:
-	if (opts->netns_id >= 0)
+	if (netns_id >= 0)
 		put_net(net);
 
 	return ct;
@@ -186,46 +198,58 @@ static struct nf_conn *__bpf_nf_ct_lookup(struct net *net,
 	struct nf_conntrack_tuple tuple;
 	struct nf_conntrack_zone ct_zone;
 	struct nf_conn *ct;
+	u8 ct_zone_dir = 0;
+	u16 ct_zone_id;
+	s32 netns_id;
+	u8 l4proto;
 	int err;
 
 	if (!opts || !bpf_tuple)
 		return ERR_PTR(-EINVAL);
 	if (!(opts_len == NF_BPF_CT_OPTS_SZ || opts_len == 12))
 		return ERR_PTR(-EINVAL);
+
+	netns_id = READ_ONCE(opts->netns_id);
+	l4proto = READ_ONCE(opts->l4proto);
+	ct_zone_id = READ_ONCE(opts->ct_zone_id);
 	if (opts_len == NF_BPF_CT_OPTS_SZ) {
-		if (opts->reserved[0] || opts->reserved[1] || opts->reserved[2])
+		ct_zone_dir = READ_ONCE(opts->ct_zone_dir);
+		if (READ_ONCE(opts->reserved[0]) ||
+		    READ_ONCE(opts->reserved[1]) ||
+		    READ_ONCE(opts->reserved[2]))
 			return ERR_PTR(-EINVAL);
 	} else {
-		if (opts->ct_zone_id)
+		if (ct_zone_id)
 			return ERR_PTR(-EINVAL);
 	}
-	if (unlikely(opts->l4proto != IPPROTO_TCP && opts->l4proto != IPPROTO_UDP))
+	if (unlikely(l4proto != IPPROTO_TCP && l4proto != IPPROTO_UDP))
 		return ERR_PTR(-EPROTO);
-	if (unlikely(opts->netns_id < BPF_F_CURRENT_NETNS))
+	if (unlikely(netns_id < BPF_F_CURRENT_NETNS))
 		return ERR_PTR(-EINVAL);
 
-	err = bpf_nf_ct_tuple_parse(bpf_tuple, tuple_len, opts->l4proto,
+	err = bpf_nf_ct_tuple_parse(bpf_tuple, tuple_len, l4proto,
 				    IP_CT_DIR_ORIGINAL, &tuple);
 	if (err < 0)
 		return ERR_PTR(err);
 
-	if (opts->netns_id >= 0) {
-		net = get_net_ns_by_id(net, opts->netns_id);
+	if (netns_id >= 0) {
+		net = get_net_ns_by_id(net, netns_id);
 		if (unlikely(!net))
 			return ERR_PTR(-ENONET);
 	}
 
 	if (opts_len == NF_BPF_CT_OPTS_SZ) {
-		if (opts->ct_zone_dir == 0)
-			opts->ct_zone_dir = NF_CT_DEFAULT_ZONE_DIR;
-		nf_ct_zone_init(&ct_zone,
-				opts->ct_zone_id, opts->ct_zone_dir, 0);
+		if (ct_zone_dir == 0) {
+			ct_zone_dir = NF_CT_DEFAULT_ZONE_DIR;
+			opts->ct_zone_dir = ct_zone_dir;
+		}
+		nf_ct_zone_init(&ct_zone, ct_zone_id, ct_zone_dir, 0);
 	} else {
 		ct_zone = nf_ct_zone_dflt;
 	}
 
 	hash = nf_conntrack_find_get(net, &ct_zone, &tuple);
-	if (opts->netns_id >= 0)
+	if (netns_id >= 0)
 		put_net(net);
 	if (!hash)
 		return ERR_PTR(-ENOENT);
-- 
2.43.0