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

[Omar Sandoval <osandov@osandov.com>]

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:
> > > On Thu, Feb 14, 2019 at 02:00:07AM -0800, Omar Sandoval wrote:
> > > > From: Omar Sandoval <osandov@fb.com>
> > > > 
> > > > Hi,
> > > > 
> > > > Since statx was added in 4.11, userspace has had an interface for
> > > > reading btime (file creation time), but no way to set it. This RFC patch
> > > > series adds support for changing btime with utimensat(). Patch 1 adds
> > > > the VFS infrastructure, patch 2 adds the support to utimensat() with a
> > > > new flag, and the rest of the patches add filesystem support; I excluded
> > > > CIFS for now because I don't have a CIFS setup to test it on.
> > > > 
> > > > Updating btime is useful for at least a couple of use cases:
> > > > 
> > > > - Backup/restore programs (my motivation for this feature is btrfs send)
> > > > - File servers which interoperate with operating systems that allow
> > > >   updating file creation time, including Mac OS [1] and Windows [2]
> > > 
> > > So you're adding an interface that allows users to change the create
> > > time of files without needing any privileges?
> > 
> > I think it'd be reasonable to make this a privileged operation. I didn't
> > for this initial submission for a couple of reasons:
> > 
> > 1. The precedent on Mac OS and Windows is that this isn't a privileged
> >    operation.
> 
> Don't really care about them. Interop file servers that support these
> operations on other OSs will need to be storing this info in xattrs
> because they have to work on filesystems that don't support btime.
> 
> > 2. I knew there would be different opinions on this either way I went.
> 
> Yup.
> 
> > > 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.
> 
> ... 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.

> I'm still not convinced that even backup/restore should be doing this,
> because there's so much other metadata that is unique even on
> restored files that it doesn't really make any sense to me to lie
> about it being created in the past....

I suppose it depends on how you interpret btime: if it's strictly
filesystem metadata, then it makes sense that it should be immutable; if
it's metadata for the user's own purposes, then we should allow setting
it.