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

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2023-04-17 at 08:45 -0400, Stefan Berger wrote:
> 
> On 4/17/23 06:05, Jeff Layton wrote:
> > On Sun, 2023-04-16 at 21:57 -0400, Stefan Berger wrote:
> > > 
> > > On 4/7/23 09:29, Jeff Layton wrote:
> 
> > > > 
> > > > 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?
> > > 
> > > I'll hopefully be able to test the proposed patch tomorrow.
> > > 
> > > 
> > 
> > No, you don't want to use inode_eq_iversion here because (as the comment
> > over it says):
> 
> In the ima_check_last_writer() case the usage of inode_eq_iversion() was correct since
> at this point no record of  its value was made and therefore no writer needed to change
> the i_value again due to IMA:
> 
> 		update = test_and_clear_bit(IMA_UPDATE_XATTR,
> 					    &iint->atomic_flags);
> 		if (!IS_I_VERSION(inode) ||
> 		    !inode_eq_iversion(inode, iint->version) ||
> 		    (iint->flags & IMA_NEW_FILE)) {
> 			iint->flags &= ~(IMA_DONE_MASK | IMA_NEW_FILE);
> 			iint->measured_pcrs = 0;
> 			if (update)
> 				ima_update_xattr(iint, file);
> 		}
> 
> The record of the value is only made when the actual measurement is done in
> ima_collect_measurement()
> 

True, but we don't have a generic mechanism to do a this. What you're
doing only works for IS_I_VERSION inodes.

> Compared to this the usage of vfs_getattr_nosec() is expensive since it resets the flag.
> 
>          if ((request_mask & STATX_CHANGE_COOKIE) && IS_I_VERSION(inode)) {
>                  stat->result_mask |= STATX_CHANGE_COOKIE;
>                  stat->change_cookie = inode_query_iversion(inode);
>          }
> 
> 	idmap = mnt_idmap(path->mnt);
> 	if (inode->i_op->getattr)
> 		return inode->i_op->getattr(idmap, path, stat,
> 					    request_mask, query_flags);
> 
> Also, many filesystems will have their getattr now called as well.
> 

...as they should!

> I understand Christian's argument about the maintenance headache to a certain degree...
> 

IMA is not equipped to understand the subtleties of how the i_version
counter is implemented on different filesystems. In the past it dealt
with this by limiting its usage to IS_I_VERSION inodes, but that is
already problematic today. For instance: xfs currently sets the
SB_I_VERSION flag, but its i_version counter also bumps the value on
atime updates. That means that IMA is doing more remeasurements on xfs
than are needed.

I'm trying to clean a lot of this up, but IMA's current usage isn't
really helping since it's poking around in areas it shouldn't be. Doing
a getattr is the canonical way to query this value since it leaves it up
to the filesystem how to report this value.

If this turns out to cause a performance regression we can look at
adding a getattr-like routine that _only_ reports the change attribute.
I wouldn't want to do that though unless the need were clear (and backed
up by performance numbers).
-- 
Jeff Layton <jlayton@kernel.org>