Date:   Wed, 14 Jun 2017 17:24:53 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-fscrypt@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] ext4: forbid encrypting root directory

Hi Andreas,

On Wed, Jun 14, 2017 at 06:00:33PM -0600, Andreas Dilger wrote:
> On Jun 14, 2017, at 5:17 PM, Eric Biggers <ebiggers3@...il.com> wrote:
> > 
> > From: Eric Biggers <ebiggers@...gle.com>
> > 
> > Currently it's possible to encrypt all files and directories on an ext4
> > filesystem by deleting everything, including lost+found, then setting an
> > encryption policy on the root directory.  However, this is incompatible
> > with e2fsck because e2fsck expects to find, create, and/or write to
> > lost+found and does not have access to any encryption keys.  Especially
> > problematic is that if e2fsck can't find lost+found, it will create it
> > without regard for whether the root directory is encrypted.  This is
> > wrong for obvious reasons, and it causes a later run of e2fsck to
> > consider the lost+found directory entry to be corrupted.
> > 
> > It's not clear it would be useful to support encrypting the root
> > directory, because in that scenario dm-crypt might as well be used
> > instead.
> 
> The benefit of per-file encryption over dm-crypt is that it isn't an
> all-or-nothing usage case like dm-crypt (e.g. there could be different
> users/keys for different subdirectories), and secure file delete (as
> mentioned earlier) works properly with per-file encryption, and doesn't
> work at all with dm-crypt.
> 

Well, encrypting the root directory *is* the all-or-nothing use case --- all
files and directories on the filesystem would be encrypted with the same key,
and by design it cannot be overridden for individual files/directories.

"Secure file delete" would be an advantage if/when it's implemented, though.

> > But in any case, it's broken currently and must not be allowed; so
> > start returning an error if userspace tries to set an encryption
> > policy on the root directory.
> > 
> > For now do this in ext4 only, because f2fs and ubifs do not appear to
> > have the lost+found requirement.  We could move it into
> > fscrypt_ioctl_set_policy() later if desired, though.
> 
> What about a special case to handle an unencrypted lost+found inode?
> 
> There was also a patch series a while ago to explicitly add lost+found
> into the superblock so that it could be found even if the root directory
> was corrupted, and to allow flexibility in whether it is always shown in
> the root directory or not (e.g. allowing ".lost+found" or similar).
> 

It could be done if the lost+found inode was not linked to from any directory
and instead had its inode number stored in the superblock so that e2fsck could
still find it.  However, if e2fsck put files in a lost+found directory that
doesn't exist in the filesystem directory structure, how would users retrieve
those files?  Would users be required to run a special e2fsprogs command to
list/read/delete the files in lost+found?

> > +	/*
> > +	 * Encrypting the root directory is not allowed because e2fsck expects
> > +	 * lost+found to exist and be unencrypted, and encrypting the root
> > +	 * directory would imply encrypting the lost+found directory as well as
> > +	 * the filename "lost+found" itself.
> > +	 */
> > +	if (inode->i_ino == EXT4_ROOT_INO)
> > +		return -EBUSY;
> 
> Why -EBUSY and not -EPERM?
> 

No strong reason; I had in mind that EBUSY is returned when running a command
like 'rmdir /mnt'.  Probably EPERM would be better.

Eric

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