ext4 crypto: use dget_parent() in ext4_d_revalidate()

This avoids potential problems caused by a race where the inode gets
renamed out from its parent directory and the parent directory is
deleted while ext4_d_revalidate() is running.

Fixes: 28b4c263961c
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

fs/ext4/crypto.c

@@ -478,13 +478,16 @@ uint32_t ext4_validate_encryption_key_size(uint32_t mode, uint32_t size)
  */
 static int ext4_d_revalidate(struct dentry *dentry, unsigned int flags)
 {
-	struct inode *dir = d_inode(dentry->d_parent);
-	struct ext4_crypt_info *ci = EXT4_I(dir)->i_crypt_info;
+	struct dentry *dir;
+	struct ext4_crypt_info *ci;
 	int dir_has_key, cached_with_key;
 
-	if (!ext4_encrypted_inode(dir))
+	dir = dget_parent(dentry);
+	if (!ext4_encrypted_inode(d_inode(dir))) {
+		dput(dir);
 		return 0;
-
+	}
+	ci = EXT4_I(d_inode(dir))->i_crypt_info;
 	if (ci && ci->ci_keyring_key &&
 	    (ci->ci_keyring_key->flags & ((1 << KEY_FLAG_INVALIDATED) |
 					  (1 << KEY_FLAG_REVOKED) |
@@ -494,6 +497,7 @@ static int ext4_d_revalidate(struct dentry *dentry, unsigned int flags)
 	/* this should eventually be an flag in d_flags */
 	cached_with_key = dentry->d_fsdata != NULL;
 	dir_has_key = (ci != NULL);
+	dput(dir);
 
 	/*
 	 * If the dentry was cached without the key, and it is a