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

[Christian Brauner]

On Sun, Apr 16, 2023 at 09:57:10PM -0400, Stefan Berger wrote:
> 
> 
> On 4/7/23 09:29, 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@xxxxxxxxxxxxx/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.
> 
> IMA should remeasure the file when it has visibly changed for another thread or process.
> 
> 
> > > > -----------------------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@xxxxxxxxxxxxx>
> > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > > ---
> > > 
> > > 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.
> > 
> > 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.
> 
> I was wondering whether to use the existing inode_eq_iversion() for all
> other filesystems than overlayfs, nfs, and possibly other ones (which ones?)
> where we would use the vfs_getattr_nosec() via a case on inode->i_sb->s_magic?
> If so, would this function be generic enough to be a public function for libfs.c?

That's just an invitation for bugs and maintenance headaches. Just call
vfs_getattr_nosec() directly and measure the performance impact before
trying to optimize this. If you see performance impact that is worth
mentioning then we can explore other options such as allowing
->getattr() to only query for i_version and nothing else.