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

[Jeff Layton]

On Thu, 2023-04-06 at 16:22 -0400, Stefan Berger wrote:
> 
> On 4/6/23 15:37, Jeff Layton wrote:
> > On Thu, 2023-04-06 at 15:11 -0400, Stefan Berger wrote:
> > > 
> > > On 4/6/23 14:46, Jeff Layton wrote:
> > > > On Thu, 2023-04-06 at 17:01 +0200, Christian Brauner wrote:
> > > > > On Thu, Apr 06, 2023 at 10:36:41AM -0400, Paul Moore wrote:
> > > 
> > > > 
> > > > Correct. As long as IMA is also measuring the upper inode then it seems
> > > > like you shouldn't need to do anything special here.
> > > 
> > > Unfortunately IMA does not notice the changes. With the patch provided in the other email IMA works as expected.
> > > 
> > 
> > 
> > It looks like remeasurement is usually done in ima_check_last_writer.
> > That gets called from __fput which is called when we're releasing the
> > last reference to the struct file.
> > 
> > You've hooked into the ->release op, which gets called whenever
> > filp_close is called, which happens when we're disassociating the file
> > from the file descriptor table.
> > 
> > So...I don't get it. Is ima_file_free not getting called on your file
> > for some reason when you go to close it? It seems like that should be
> > handling this.
> 
> 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
> 
> 

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.

> The new proposed i_version increase occurs on the inode that IMA sees later on for
> the file that's being executed and on which it must do a re-evaluation.
> 
> Upon file changes ima_inode_free() seems to see two ima_file_free() calls,
> one for what seems to be the upper layer (used for vfs_* functions below)
> and once for the lower one.
> The important thing is that IMA will see the lower one when the file gets
> executed later on and this is the one that I instrumented now to have its
> i_version increased, which in turn triggers the re-evaluation of the file post
> modification.
> 
> static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
> [...]
> 	struct fd real;
> [...]
> 	ret = ovl_real_fdget(file, &real);
> 	if (ret)
> 		goto out_unlock;
> 
> [...]
> 	if (is_sync_kiocb(iocb)) {
> 		file_start_write(real.file);
> -->		ret = vfs_iter_write(real.file, iter, &iocb->ki_pos,
> 				     ovl_iocb_to_rwf(ifl));
> 		file_end_write(real.file);
> 		/* Update size */
> 		ovl_copyattr(inode);
> 	} else {
> 		struct ovl_aio_req *aio_req;
> 
> 		ret = -ENOMEM;
> 		aio_req = kmem_cache_zalloc(ovl_aio_request_cachep, GFP_KERNEL);
> 		if (!aio_req)
> 			goto out;
> 
> 		file_start_write(real.file);
> 		/* Pacify lockdep, same trick as done in aio_write() */
> 		__sb_writers_release(file_inode(real.file)->i_sb,
> 				     SB_FREEZE_WRITE);
> 		aio_req->fd = real;
> 		real.flags = 0;
> 		aio_req->orig_iocb = iocb;
> 		kiocb_clone(&aio_req->iocb, iocb, real.file);
> 		aio_req->iocb.ki_flags = ifl;
> 		aio_req->iocb.ki_complete = ovl_aio_rw_complete;
> 		refcount_set(&aio_req->ref, 2);
> -->		ret = vfs_iocb_iter_write(real.file, &aio_req->iocb, iter);
> 		ovl_aio_put(aio_req);
> 		if (ret != -EIOCBQUEUED)
> 			ovl_aio_cleanup_handler(aio_req);
> 	}
>          if (ret > 0)						<--- this get it to work
>                  inode_maybe_inc_iversion(inode, false);		<--- since this inode is known to IMA
> out:
>          revert_creds(old_cred);
> out_fdput:
>          fdput(real);
> 
> out_unlock:
>          inode_unlock(inode);
> 
> 
> 
> 
>     Stefan
> 
> > 
> > In any case, I think this could use a bit more root-cause analysis.
> 

-- 
Jeff Layton <jlayton at kernel.org>