[PATCH v2 bpf 2/2] bpf: net: Avoid incorrect bpf_sk_reuseport_detach call

[Martin KaFai Lau <kafai@xxxxxx>]

bpf_sk_reuseport_detach is currently called when sk->sk_user_data
is not NULL.  It is incorrect because sk->sk_user_data may not be
managed by the bpf's reuseport_array.  It has been reported in [1] that,
the bpf_sk_reuseport_detach() which is called from udp_lib_unhash() has
corrupted the sk_user_data managed by l2tp.

This patch solves it by using another bit (defined as SK_USER_DATA_BPF)
of the sk_user_data pointer value.  It marks that a sk_user_data is
managed/owned by BPF.

The patch depends on a PTRMASK introduced in
commit f1ff5ce2cd5e ("net, sk_msg: Clear sk_user_data pointer on clone if tagged").

[ Note: sk->sk_user_data is used by bpf's reuseport_array only when a sk is
  added to the bpf's reuseport_array.
  i.e. doing setsockopt(SO_REUSEPORT) and having "sk->sk_reuseport == 1"
  alone will not stop sk->sk_user_data being used by other means. ]

[1]: https://lore.kernel.org/netdev/20200706121259.GA20199@xxxxxxxxxxx/

Reported-by: James Chapman <jchapman@xxxxxxxxxxx>
Cc: James Chapman <jchapman@xxxxxxxxxxx>
Fixes: 5dc4c4b7d4e8 ("bpf: Introduce BPF_MAP_TYPE_REUSEPORT_SOCKARRAY")
Signed-off-by: Martin KaFai Lau <kafai@xxxxxx>
---
 include/net/sock.h           | 3 ++-
 kernel/bpf/reuseport_array.c | 5 +++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 3428619faae4..1183507df95b 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -533,7 +533,8 @@ enum sk_pacing {
  * be copied.
  */
 #define SK_USER_DATA_NOCOPY	1UL
-#define SK_USER_DATA_PTRMASK	~(SK_USER_DATA_NOCOPY)
+#define SK_USER_DATA_BPF	2UL	/* Managed by BPF */
+#define SK_USER_DATA_PTRMASK	~(SK_USER_DATA_NOCOPY | SK_USER_DATA_BPF)
 
 /**
  * sk_user_data_is_nocopy - Test if sk_user_data pointer must not be copied
diff --git a/kernel/bpf/reuseport_array.c b/kernel/bpf/reuseport_array.c
index a95bc8d7e812..cae9d505e04a 100644
--- a/kernel/bpf/reuseport_array.c
+++ b/kernel/bpf/reuseport_array.c
@@ -24,7 +24,7 @@ void bpf_sk_reuseport_detach(struct sock *sk)
 
 	write_lock_bh(&sk->sk_callback_lock);
 	sk_user_data = (uintptr_t)sk->sk_user_data;
-	if (sk_user_data) {
+	if (sk_user_data & SK_USER_DATA_BPF) {
 		struct sock __rcu **socks;
 
 		socks = (void *)(sk_user_data & SK_USER_DATA_PTRMASK);
@@ -309,7 +309,8 @@ int bpf_fd_reuseport_array_update_elem(struct bpf_map *map, void *key,
 	if (err)
 		goto put_file_unlock;
 
-	sk_user_data = (uintptr_t)&array->ptrs[index] | SK_USER_DATA_NOCOPY;
+	sk_user_data = (uintptr_t)&array->ptrs[index] | SK_USER_DATA_NOCOPY |
+		SK_USER_DATA_BPF;
 	WRITE_ONCE(nsk->sk_user_data, (void *)sk_user_data);
 	rcu_assign_pointer(array->ptrs[index], nsk);
 	free_osk = osk;
-- 
2.24.1