Re: [RFC] odd check in ceph_encode_encrypted_dname()

[Luis Henriques <luis@igalia.com>]

On Sat, Feb 15 2025, Al Viro wrote:

> On Fri, Feb 14, 2025 at 04:05:42PM +0000, Luis Henriques wrote:
>
>> So, IIRC, when encrypting the snapshot name (the "my-snapshot" string),
>> you'll use key from the original inode.  That's why we need to handle
>> snapshot names starting with '_' differently.  And why we have a
>> customized base64 encoding function.
>
> OK...  The reason I went looking at that thing was the race with rename()
> that can end up with UAF in ceph_mdsc_build_path().
>
> We copy the plaintext name under ->d_lock, but then we call
> ceph_encode_encrypted_fname() which passes dentry->d_name to
> ceph_encode_encrypted_dname() with no locking whatsoever.
>
> Have it race with rename and you've got a lot of unpleasantness.
>
> The thing is, we can have all ceph_encode_encrypted_dname() put the
> plaintext name into buf; that eliminates the need to have a separate
> qstr (or dentry, in case of ceph_encode_encrypted_fname()) argument and
> simplifies ceph_encode_encrypted_dname() while we are at it.
>
> Proposed fix in git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git #d_name
>
> WARNING: it's completely untested and needs review.  It's split in two commits
> (massage of ceph_encode_encrypted_dname(), then changing the calling conventions);
> both patches in followups.
>
> Please, review.

I've reviewed both patches and they seem to be OK, so feel free to add my

Reviewed-by: Luis Henriques <luis@igalia.com>

But as I said, I don't have a test environment at the moment.  I'm adding
Slava to CC with the hope that he may be able to run some fscrypt-specific
tests (including snapshots creation) against these patches.

Cheers,
-- 
Luís