[PATCH] overlayfs: Trigger file re-evaluation by IMA / EVM after writes

[Mimi Zohar]

On Fri, 2023-04-07 at 09:29 -0400, Jeff Layton wrote:
> > > > > 
> > > > > I would ditch the original proposal in favor of this 2-line patch shown here:
> > > > > 
> > > > > https://lore.kernel.org/linux-integrity/a95f62ed-8b8a-38e5-e468-ecbde3b221af@linux.ibm.com/T/#m3bd047c6e5c8200df1d273c0ad551c645dd43232
> > 
> > We should cool it with the quick hacks to fix things. :)
> > 
> 
> Yeah. It might fix this specific testcase, but I think the way it uses
> the i_version is "gameable" in other situations. Then again, I don't
> know a lot about IMA in this regard.
> 
> When is it expected to remeasure? If it's only expected to remeasure on
> a close(), then that's one thing. That would be a weird design though.

Historical background:

Prior to IMA being upstreamed there was a lot of discussion about how
much/how frequently to measure files.  Re-measuring files after each
write would impact performance.  Instead of re-measuring files after
each write, if a file already opened for write was opened for read
(open writers) or a file already opened for read was opened for write
(Time of Measure/Time of Use) the IMA meausrement list was invalidated
by including a violation record in the measurement list.

Only the BPRM hook prevents a file from being opened for write.

> 
> > > > > 
> > > > > 
> > > > 
> > > > Ok, I think I get it. IMA is trying to use the i_version from the
> > > > overlayfs inode.
> > > > 
> > > > I suspect that the real problem here is that IMA is just doing a bare
> > > > inode_query_iversion. Really, we ought to make IMA call
> > > > vfs_getattr_nosec (or something like it) to query the getattr routine in
> > > > the upper layer. Then overlayfs could just propagate the results from
> > > > the upper layer in its response.
> > > > 
> > > > That sort of design may also eventually help IMA work properly with more
> > > > exotic filesystems, like NFS or Ceph.
> > > > 
> > > > 
> > > > 
> > > 
> > > Maybe something like this? It builds for me but I haven't tested it. It
> > > looks like overlayfs already should report the upper layer's i_version
> > > in getattr, though I haven't tested that either:
> > > 
> > > -----------------------8<---------------------------
> > > 
> > > [PATCH] IMA: use vfs_getattr_nosec to get the i_version
> > > 
> > > IMA currently accesses the i_version out of the inode directly when it
> > > does a measurement. This is fine for most simple filesystems, but can be
> > > problematic with more complex setups (e.g. overlayfs).
> > > 
> > > Make IMA instead call vfs_getattr_nosec to get this info. This allows
> > > the filesystem to determine whether and how to report the i_version, and
> > > should allow IMA to work properly with a broader class of filesystems in
> > > the future.
> > > 
> > > Reported-by: Stefan Berger <stefanb at linux.ibm.com>
> > > Signed-off-by: Jeff Layton <jlayton at kernel.org>
> > > ---
> > 
> > So, I think we want both; we want the ovl_copyattr() and the
> > vfs_getattr_nosec() change:
> > 
> > (1) overlayfs should copy up the inode version in ovl_copyattr(). That
> >     is in line what we do with all other inode attributes. IOW, the
> >     overlayfs inode's i_version counter should aim to mirror the
> >     relevant layer's i_version counter. I wouldn't know why that
> >     shouldn't be the case. Asking the other way around there doesn't
> >     seem to be any use for overlayfs inodes to have an i_version that
> >     isn't just mirroring the relevant layer's i_version.
> 
> It's less than ideal to do this IMO, particularly with an IS_I_VERSION
> inode.
> 
> You can't just copy up the value from the upper. You'll need to call
> inode_query_iversion(upper_inode), which will flag the upper inode for a
> logged i_version update on the next write. IOW, this could create some
> (probably minor) metadata write amplification in the upper layer inode
> with IS_I_VERSION inodes.
> 
> 
> > (2) Jeff's changes for ima to make it rely on vfs_getattr_nosec().
> >     Currently, ima assumes that it will get the correct i_version from
> >     an inode but that just doesn't hold for stacking filesystem.
> > 
> > While (1) would likely just fix the immediate bug (2) is correct and
> > _robust_. If we change how attributes are handled vfs_*() helpers will
> > get updated and ima with it. Poking at raw inodes without using
> > appropriate helpers is much more likely to get ima into trouble.
> 
> This will fix it the right way, I think (assuming it actually works),
> and should open the door for IMA to work properly with networked
> filesystems that support i_version as well.

On a local filesystem, there are guarantees that the calculated file
hash is that of the file being used.  Reminder IMA reads a file, page
size chunk at a time into a single buffer, calculating the file hash. 
Once the file hash is calculated, the memory is freed.

There are no guarantees on a fuse filesystem, for example, that the
original file read and verified is the same as the one being executed. 
I'm not sure that the integrity guarantees of a file on a remote
filesystem will be the same as those on a local file system.

> 
> Note that there Stephen is correct that calling getattr is probably
> going to be less efficient here since we're going to end up calling
> generic_fillattr unnecessarily, but I still think it's the right thing
> to do.
> 
> If it turns out to cause measurable performance regressions though,
> maybe we can look at adding a something that still calls ->getattr if it
> exists but only returns the change_cookie value.

Sure.  For now,

Reviewed-by: Mimi Zohar <zohar at linux.ibm.com>