net: convert sock.sk_refcnt from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

This patch uses refcount_inc_not_zero() instead of
atomic_inc_not_zero_hint() due to absense of a _hint()
version of refcount API. If the hint() version must
be used, we might need to revisit API.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 7c2e21e..c885254 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2847,7 +2847,7 @@ static int unix_seq_show(struct seq_file *seq, void *v)
 
 		seq_printf(seq, "%pK: %08X %08X %08X %04X %02X %5lu",
 			s,
-			atomic_read(&s->sk_refcnt),
+			refcount_read(&s->sk_refcnt),
 			0,
 			s->sk_state == TCP_LISTEN ? __SO_ACCEPTCON : 0,
 			s->sk_type,