bpf: skmsg, fix psock create on existing kcm/tls port

Before using the psock returned by sk_psock_get() when adding it to a
sockmap we need to ensure it is actually a sockmap based psock.
Previously we were only checking this after incrementing the reference
counter which was an error. This resulted in a slab-out-of-bounds
error when the psock was not actually a sockmap type.

This moves the check up so the reference counter is only used
if it is a sockmap psock.

Eric reported the following KASAN BUG,

BUG: KASAN: slab-out-of-bounds in atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
BUG: KASAN: slab-out-of-bounds in refcount_inc_not_zero_checked+0x97/0x2f0 lib/refcount.c:120
Read of size 4 at addr ffff88019548be58 by task syz-executor4/22387

CPU: 1 PID: 22387 Comm: syz-executor4 Not tainted 4.19.0-rc7+ torvalds#264
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1c4/0x2b4 lib/dump_stack.c:113
 print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
 check_memory_region_inline mm/kasan/kasan.c:260 [inline]
 check_memory_region+0x13e/0x1b0 mm/kasan/kasan.c:267
 kasan_check_read+0x11/0x20 mm/kasan/kasan.c:272
 atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
 refcount_inc_not_zero_checked+0x97/0x2f0 lib/refcount.c:120
 sk_psock_get include/linux/skmsg.h:379 [inline]
 sock_map_link.isra.6+0x41f/0xe30 net/core/sock_map.c:178
 sock_hash_update_common+0x19b/0x11e0 net/core/sock_map.c:669
 sock_hash_update_elem+0x306/0x470 net/core/sock_map.c:738
 map_update_elem+0x819/0xdf0 kernel/bpf/syscall.c:818

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Fixes: 604326b ("bpf, sockmap: convert to generic sk_msg interface")

include/linux/skmsg.h

@@ -270,11 +270,6 @@ static inline struct sk_psock *sk_psock(const struct sock *sk)
  return rcu_dereference_sk_user_data(sk);
 }
 
-static inline bool sk_has_psock(struct sock *sk)
-{
- return sk_psock(sk) != NULL && sk->sk_prot->recvmsg == tcp_bpf_recvmsg;
-}
-
 static inline void sk_psock_queue_msg(struct sk_psock *psock,
  struct sk_msg *msg)
 {
@@ -374,6 +369,26 @@ static inline bool sk_psock_test_state(const struct sk_psock *psock,
  return test_bit(bit, &psock->state);
 }
 
+static inline struct sk_psock *sk_psock_get_checked(struct sock *sk)
+{
+ struct sk_psock *psock;
+
+ rcu_read_lock();
+ psock = sk_psock(sk);
+ if (psock) {
+ if (sk->sk_prot->recvmsg != tcp_bpf_recvmsg) {
+ psock = ERR_PTR(-EBUSY);
+ goto out;
+ }
+
+ if (!refcount_inc_not_zero(&psock->refcnt))
+ psock = ERR_PTR(-EBUSY);
+ }
+out:
+ rcu_read_unlock();
+ return psock;
+}
+
 static inline struct sk_psock *sk_psock_get(struct sock *sk)
 {
  struct sk_psock *psock;

net/core/sock_map.c

@@ -175,12 +175,13 @@ static int sock_map_link(struct bpf_map *map, struct sk_psock_progs *progs,
  }
  }
 
- psock = sk_psock_get(sk);
+ psock = sk_psock_get_checked(sk);
+ if (IS_ERR(psock)) {
+ ret = PTR_ERR(psock);
+ goto out_progs;
+ }
+
  if (psock) {
- if (!sk_has_psock(sk)) {
- ret = -EBUSY;
- goto out_progs;
- }
  if ((msg_parser && READ_ONCE(psock->progs.msg_parser)) ||
  (skb_progs && READ_ONCE(psock->progs.skb_parser))) {
  sk_psock_put(sk, psock);