Re: [RFC PATCH 0/6] Allow setting file birth time with utimensat()

David Disseldorp
2019-02-15T14:14:19Z

On Thu, 14 Feb 2019 22:59:47 -0800, Omar Sandoval wrote:

> On Fri, Feb 15, 2019 at 11:16:57AM +1100, Dave Chinner wrote:
> > On Thu, Feb 14, 2019 at 03:14:29PM -0800, Omar Sandoval wrote:
> > > On Fri, Feb 15, 2019 at 09:06:26AM +1100, Dave Chinner wrote:
...
> > > > Inode create time is forensic metadata in XFS - information we use
> > > > for sequence of event and inode lifetime analysis during examination
> > > > of broken filesystem images and systems that have been broken into.
> > > > Just because it's exposed to userspace via statx(), it doesn't mean
> > > > that it is information that users should be allowed to change. i.e.
> > > > allowing users to be able to change the create time on files makes
> > > > it completely useless for the purpose it was added to XFS for...
> > > >
> > > > And allowing root to change the create time doesn't really help,
> > > > because once you've broken into a system, this makes it really easy
> > > > to cover tracks
> > >
> > > If the threat model is that the attacker has root, then they can
> > > overwrite the timestamp on disk anyways, no?
> >
> > Modifying the block devicee under an active filesystem is fraught
> > with danger, and there's no guarantee it will work if the metadata
> > being modified is still active in memory. Corrupting the filesystem
> > is a sure way to get noticed....
> >
> > > > (e.g. we can't find files that were created and
> > > > unlinked during the break in window anymore) and lay false
> > > > trails....
> > >
> > > Fair point, although there's still ctime during the break-in window,
> >
> > Unless you're smart enough to know how to trigger S_NOCMTIME or
> > FMODE_NOCMTIME....
> >
> > > which I assume you'd be looking for anyways since files modified during
> > > the break-in window are also of interest.

I'm not sure I follow the forensics use-case for immutable btime. I'd
expect dm-verity or selinux/apparmor audits to do a better job for those
worried about this kind of attack.

> > ... and then that also can't be guaranteed. :/
> >
> > > I see a few options, none of which are particularly nice:
> > >
> > > 1. Filesystems like XFS could choose not to support setting btime even
> > > if they support reading it.
> > > 2. XFS could add a second, writeable btime which is used for
> > > statx/utimes when available (it would fit in di_pad2...).
> > > 3. We could add a btime_writable sysctl/mount option/mkfs option.
> >
> > 4. create time remains a read-only field, and btrfs grows its own
> > special interface to twiddle it in btrfs-recv if it really is
> > necessary.
>
> I'm curious to hear what the ext4/f2fs/CIFS developers think. If no one
> else wants btime to be mutable, then I might as well make it
> Btrfs-specific. That is, assuming we reach consensus on the Btrfs side
> that btrfs receive should set btime.

Samba currently uses a user.DOSATTRIB xattr for tracking creation time.
IMO a mutable btime accessible via statx would be useful for
cross-protocol interoperability.

Cheers, David