Re: [PATCH 3/3] ima: use fs method to read integrity data (updated patch description)

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

On Mon, 2017-09-18 at 10:55 -0400, Mimi Zohar wrote:
> On Mon, 2017-09-18 at 12:13 +0200, Jan Kara wrote:
> > On Mon 18-09-17 10:19:25, Steven Whitehouse wrote:
> > > On 17/09/17 17:38, Al Viro wrote:
> > > >On Sun, Sep 17, 2017 at 09:34:01AM -0700, Linus Torvalds wrote:
> > > >>Now, I suspect most (all?) do, but that's a historical artifact rather
> > > >>than "design". In particular, the VFS layer used to do the locking for
> > > >>the filesystems, to guarantee the POSIX requirements (POSIX requires
> > > >>that writes be seen atomically).
> > > >>
> > > >>But that lock was pushed down into the filesystems, since some
> > > >>filesystems really wanted to have parallel writes (particularly for
> > > >>direct IO, where that POSIX serialization requirement doesn't exist).
> > > >>
> > > >>That's all many years ago, though. New filesystems are likely to have
> > > >>copied the pattern from old ones, but even then..
> > > >>
> > > >>Also, it's worth noting that "inode->i_rwlock" isn't even well-defined
> > > >>as a lock. You can have the question of *which* inode gets talked
> > > >>about when you have things like eoverlayfs etc. Normally it would be
> > > >>obvious, but sometimes you'd use "file->f_mapping->host" (which is the
> > > >>same thing in the simple cases), and sometimes it really wouldn't be
> > > >>obvious at all..
> > > >>
> > > >>So... I'm really not at all convinced that i_rwsem is sensible. It's
> > > >>one of those things that are "mostly right for the simple cases",
> > > >>but...
> > > >The thing pretty much common to all of them is that write() might need
> > > >to modify permissions (suid removal), which brings ->i_rwsem in one
> > > >way or another - notify_change() needs that held...


> > > For GFS2, if we are to hold the inode info constant while it is checked, we
> > > would need to take a glock (read lock in this case) across the relevant
> > > operations. The glock will be happy under i_rwlock, since we have a lock
> > > ordering that takes local locks ahead of cluster locks. I've not dug into
> > > this enough to figure out whether the current proposal will allow this to
> > > work with GFS2 though. Does IMA cache the results from the
> > > ->read_integrity() operation?
> 
> Up to now, the hash calculation was stored in the iint structure,
> which is then used to extend the TPM, verify the file's integrity
> compared to the value stored in the xattr, and included in an audit
> message.
> 
> A new patch set by Thiago Bauermann will add appended signature
> support, re-using the kernel module signature appended method, which
> might require re-calculating the file hash based on a different hash
> algorithm.
> 
> > So I have asked Mimi about clustered filesystems before. And for now the
> > answer was that IMA for clustered filesystems is not supported (it will
> > return some error since ->integrity_read is NULL). If we would ever want to
> > support those it would require larger overhaul of the IMA architecture to
> > give filesystem more control over the locking (which is essentially what
> > Linus wants).
> 
> For performance reasons, IMA is not on a write hook, but detects file
> change on the last __fput() opened for write.  At that point, the
> cached info is reset.  The file hash is re-calculated and written out
> as an xattr.  On the next file access (in policy), the file hash is
> re-calculated and stored in the iint.
> 
> In terms of remote/clustered/fuse filesystems, we wouldn't be on the
> __fput() path.  Support for remote/clustered/fuse filesystems, would
> be similar to filesystems that do not support i_version.  Meaning only
> the first file access (in policy) would be measured/appraised, but not
> subsequent ones.  Even if we could detect file change, we would be
> dependent on the remote/clustered/fuse filesystem to inform us of the
> change.  What type of integrity guarantees would that provide?

After thinking this over a bit, perhaps we shouldn't cache the file
integrity results for these filesystems, since we can't rely on them
to notify us of a change (eg. malicious fs), but simply re-measure/re-
validate files each time.

Mimi