ext4: prevent creating duplicate encrypted filenames

commit 75d18cd upstream.

As described in "fscrypt: add fscrypt_is_nokey_name()", it's possible to
create a duplicate filename in an encrypted directory by creating a file
concurrently with adding the directory's encryption key.

Fix this bug on ext4 by rejecting no-key dentries in ext4_add_entry().

Note that the duplicate check in ext4_find_dest_de() sometimes prevented
this bug.  However in many cases it didn't, since ext4_find_dest_de()
doesn't examine every dentry.

Fixes: 4461471 ("ext4 crypto: enable filename encryption")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20201118075609.120337-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/ext4/namei.c

@@ -2195,6 +2195,9 @@ static int ext4_add_entry(handle_t *handle, struct dentry *dentry,
 	if (!dentry->d_name.len)
 		return -EINVAL;
 
+	if (fscrypt_is_nokey_name(dentry))
+		return -ENOKEY;
+
 #ifdef CONFIG_UNICODE
 	if (sb_has_strict_encoding(sb) && IS_CASEFOLDED(dir) &&
 	    sb->s_encoding && utf8_validate(sb->s_encoding, &dentry->d_name))