[PATCH 4.19 2/4] ext4: prevent creating duplicate encrypted filenames

[Eric Biggers]

From: Eric Biggers <ebiggers at google.com>

commit 75d18cd1868c2aee43553723872c35d7908f240f 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: 4461471107b7 ("ext4 crypto: enable filename encryption")
Cc: stable at vger.kernel.org
Link: https://lore.kernel.org/r/20201118075609.120337-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers at google.com>
---
 fs/ext4/namei.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 4191552880bd7..3c238006870d3 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2106,6 +2106,9 @@ 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;
+
 	retval = ext4_fname_setup_filename(dir, &dentry->d_name, 0, &fname);
 	if (retval)
 		return retval;
-- 
2.29.2