fscrypt: don't ignore minor_hash when hash is 0

commit 77f30bf upstream.

When initializing a no-key name, fscrypt_fname_disk_to_usr() sets the
minor_hash to 0 if the (major) hash is 0.

This doesn't make sense because 0 is a valid hash code, so we shouldn't
ignore the filesystem-provided minor_hash in that case.  Fix this by
removing the special case for 'hash == 0'.

This is an old bug that appears to have originated when the encryption
code in ext4 and f2fs was moved into fs/crypto/.  The original ext4 and
f2fs code passed the hash by pointer instead of by value.  So
'if (hash)' actually made sense then, as it was checking whether a
pointer was NULL.  But now the hashes are passed by value, and
filesystems just pass 0 for any hashes they don't have.  There is no
need to handle this any differently from the hashes actually being 0.

It is difficult to reproduce this bug, as it only made a difference in
the case where a filename's 32-bit major hash happened to be 0.
However, it probably had the largest chance of causing problems on
ubifs, since ubifs uses minor_hash to do lookups of no-key names, in
addition to using it as a readdir cookie.  ext4 only uses minor_hash as
a readdir cookie, and f2fs doesn't use minor_hash at all.

Fixes: 0b81d07 ("fs crypto: move per-file encryption from f2fs tree to fs/crypto")
Cc: <stable@vger.kernel.org> # v4.6+
Link: https://lore.kernel.org/r/20210527235236.2376556-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/crypto/fname.c

@@ -344,13 +344,9 @@ int fscrypt_fname_disk_to_usr(const struct inode *inode,
 		     offsetof(struct fscrypt_nokey_name, sha256));
 	BUILD_BUG_ON(BASE64_CHARS(FSCRYPT_NOKEY_NAME_MAX) > NAME_MAX);
 
-	if (hash) {
-		nokey_name.dirhash[0] = hash;
-		nokey_name.dirhash[1] = minor_hash;
-	} else {
-		nokey_name.dirhash[0] = 0;
-		nokey_name.dirhash[1] = 0;
-	}
+	nokey_name.dirhash[0] = hash;
+	nokey_name.dirhash[1] = minor_hash;
+
 	if (iname->len <= sizeof(nokey_name.bytes)) {
 		memcpy(nokey_name.bytes, iname->name, iname->len);
 		size = offsetof(struct fscrypt_nokey_name, bytes[iname->len]);