smb: client: fix race with concurrent opens in unlink(2)

[ Upstream commit 0af1561b2d60bab2a2b00720a5c7b292ecc549ec ]

According to some logs reported by customers, CIFS client might end up
reporting unlinked files as existing in stat(2) due to concurrent
opens racing with unlink(2).

Besides sending the removal request to the server, the unlink process
could involve closing any deferred close as well as marking all
existing open handles as deleted to prevent them from deferring
closes, which increases the race window for potential concurrent
opens.

Fix this by unhashing the dentry in cifs_unlink() to prevent any
subsequent opens.  Any open attempts, while we're still unlinking,
will block on parent's i_rwsem.

Reported-by: Jay Shin <[email protected]>
Signed-off-by: Paulo Alcantara (Red Hat) <[email protected]>
Reviewed-by: David Howells <[email protected]>
Cc: Al Viro <[email protected]>
Cc: [email protected]
Signed-off-by: Steve French <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
(cherry picked from commit bc1427a48371808378ef0c8cc1d21a27352fbeb1)

Author: pcacjr

fs/smb/client/inode.c

@@ -1856,15 +1856,24 @@ int cifs_unlink(struct inode *dir, struct dentry *dentry)
 	struct cifs_sb_info *cifs_sb = CIFS_SB(sb);
 	struct tcon_link *tlink;
 	struct cifs_tcon *tcon;
+	__u32 dosattr = 0, origattr = 0;
 	struct TCP_Server_Info *server;
 	struct iattr *attrs = NULL;
-	__u32 dosattr = 0, origattr = 0;
+	bool rehash = false;
 
 	cifs_dbg(FYI, "cifs_unlink, dir=0x%p, dentry=0x%p\n", dir, dentry);
 
 	if (unlikely(cifs_forced_shutdown(cifs_sb)))
 		return -EIO;
 
+	/* Unhash dentry in advance to prevent any concurrent opens */
+	spin_lock(&dentry->d_lock);
+	if (!d_unhashed(dentry)) {
+		__d_drop(dentry);
+		rehash = true;
+	}
+	spin_unlock(&dentry->d_lock);
+
 	tlink = cifs_sb_tlink(cifs_sb);
 	if (IS_ERR(tlink))
 		return PTR_ERR(tlink);
@@ -1915,7 +1924,8 @@ int cifs_unlink(struct inode *dir, struct dentry *dentry)
 			cifs_drop_nlink(inode);
 		}
 	} else if (rc == -ENOENT) {
-		d_drop(dentry);
+		if (simple_positive(dentry))
+			d_delete(dentry);
 	} else if (rc == -EBUSY) {
 		if (server->ops->rename_pending_delete) {
 			rc = server->ops->rename_pending_delete(full_path,
@@ -1968,6 +1978,8 @@ int cifs_unlink(struct inode *dir, struct dentry *dentry)
 	kfree(attrs);
 	free_xid(xid);
 	cifs_put_tlink(tlink);
+	if (rehash)
+		d_rehash(dentry);
 	return rc;
 }