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fscrypt: add fscrypt_is_nokey_name()

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commit 159e1de201b6fca10bfec50405a3b53a561096a8 upstream.

It's possible to create a duplicate filename in an encrypted directory
by creating a file concurrently with adding the encryption key.

Specifically, sys_open(O_CREAT) (or sys_mkdir(), sys_mknod(), or
sys_symlink()) can lookup the target filename while the directory's
encryption key hasn't been added yet, resulting in a negative no-key
dentry.  The VFS then calls ->create() (or ->mkdir(), ->mknod(), or
->symlink()) because the dentry is negative.  Normally, ->create() would
return -ENOKEY due to the directory's key being unavailable.  However,
if the key was added between the dentry lookup and ->create(), then the
filesystem will go ahead and try to create the file.

If the target filename happens to already exist as a normal name (not a
no-key name), a duplicate filename may be added to the directory.

In order to fix this, we need to fix the filesystems to prevent
->create(), ->mkdir(), ->mknod(), and ->symlink() on no-key names.
(->rename() and ->link() need it too, but those are already handled
correctly by fscrypt_prepare_rename() and fscrypt_prepare_link().)

In preparation for this, add a helper function fscrypt_is_nokey_name()
that filesystems can use to do this check.  Use this helper function for
the existing checks that fs/crypto/ does for rename and link.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20201118075609.120337-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Eric Biggers 2020-11-17 23:56:05 -08:00 committed by Greg Kroah-Hartman
parent 3b7c17a814
commit 2da473e59e
2 changed files with 37 additions and 2 deletions
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5
fs/crypto/hooks.c
View File

@ -61,7 +61,7 @@ int __fscrypt_prepare_link(struct inode *inode, struct inode *dir,
return err;
/* ... in case we looked up no-key name before key was added */
if (dentry->d_flags & DCACHE_NOKEY_NAME)
if (fscrypt_is_nokey_name(dentry))
return -ENOKEY;
if (!fscrypt_has_permitted_context(dir, inode))
@ -86,7 +86,8 @@ int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry,
return err;
/* ... in case we looked up no-key name(s) before key was added */
if ((old_dentry->d_flags | new_dentry->d_flags) & DCACHE_NOKEY_NAME)
if (fscrypt_is_nokey_name(old_dentry) ||
fscrypt_is_nokey_name(new_dentry))
return -ENOKEY;
if (old_dir != new_dir) {

34
include/linux/fscrypt.h
View File

@ -111,6 +111,35 @@ static inline void fscrypt_handle_d_move(struct dentry *dentry)
dentry->d_flags &= ~DCACHE_NOKEY_NAME;
}
/**
* fscrypt_is_nokey_name() - test whether a dentry is a no-key name
* @dentry: the dentry to check
*
* This returns true if the dentry is a no-key dentry. A no-key dentry is a
* dentry that was created in an encrypted directory that hasn't had its
* encryption key added yet. Such dentries may be either positive or negative.
*
* When a filesystem is asked to create a new filename in an encrypted directory
* and the new filename's dentry is a no-key dentry, it must fail the operation
* with ENOKEY. This includes ->create(), ->mkdir(), ->mknod(), ->symlink(),
* ->rename(), and ->link(). (However, ->rename() and ->link() are already
* handled by fscrypt_prepare_rename() and fscrypt_prepare_link().)
*
* This is necessary because creating a filename requires the directory's
* encryption key, but just checking for the key on the directory inode during
* the final filesystem operation doesn't guarantee that the key was available
* during the preceding dentry lookup. And the key must have already been
* available during the dentry lookup in order for it to have been checked
* whether the filename already exists in the directory and for the new file's
* dentry not to be invalidated due to it incorrectly having the no-key flag.
*
* Return: %true if the dentry is a no-key name
*/
static inline bool fscrypt_is_nokey_name(const struct dentry *dentry)
{
return dentry->d_flags & DCACHE_NOKEY_NAME;
}
/* crypto.c */
void fscrypt_enqueue_decrypt_work(struct work_struct *);
@ -244,6 +273,11 @@ static inline void fscrypt_handle_d_move(struct dentry *dentry)
{
}
static inline bool fscrypt_is_nokey_name(const struct dentry *dentry)
{
return false;
}
/* crypto.c */
static inline void fscrypt_enqueue_decrypt_work(struct work_struct *work)
{