[PATCH v2] smb: fix a possible data race in cifs_can_echo()

[Tuo Li <islituo@gmail.com>]

The struct field TCP_Server_Info.tcpStatus is often protected by the lock
srv_lock when is accessed. Here is an example in __cifs_reconnect():

  spin_lock(&server->srv_lock);
  if (server->tcpStatus != CifsExiting)
    server->tcpStatus = CifsNeedNegotiate;
  spin_unlock(&server->srv_lock);

However, the variable server->tcpStatus is accessed without holding the
lock server->srv_lock in cifs_can_echo():

  if (server->tcpStatus == CifsGood)
    return true;

To fix this possible data race, a lock and unlock pair is added when
accessing the variable server->tcpStatus.

Reported-by: BassCheck <bass@buaa.edu.cn>
Signed-off-by: Tuo Li <islituo@gmail.com>
---
v2:
* Release the lock server->srv_lock in the false branch.
---
 fs/smb/client/smb1ops.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/smb/client/smb1ops.c b/fs/smb/client/smb1ops.c
index 7d1b3fc014d9..5120241d3c0e 100644
--- a/fs/smb/client/smb1ops.c
+++ b/fs/smb/client/smb1ops.c
@@ -1049,8 +1049,12 @@ cifs_dir_needs_close(struct cifsFileInfo *cfile)
 static bool
 cifs_can_echo(struct TCP_Server_Info *server)
 {
-	if (server->tcpStatus == CifsGood)
+	spin_lock(&server->srv_lock);
+	if (server->tcpStatus == CifsGood) {
+		spin_unlock(&server->srv_lock);
 		return true;
+	}
+	spin_unlock(&server->srv_lock);
 
 	return false;
 }
-- 
2.34.1