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

[Amir Goldstein]

On Thu, Apr 6, 2023 at 11:23 PM Stefan Berger <stefanb@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx/T/#m3bd047c6e5c8200df1d273c0ad551c645dd43232
>
>
> 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

If the aio is queued, then I think increasing i_version here may be premature.

Note that in this code flow, the ovl ctime is updated in
ovl_aio_cleanup_handler() => ovl_copyattr()
after file_end_write(), similar to the is_sync_kiocb() code patch.

It probably makes most sense to include i_version in ovl_copyattr().
Note that this could cause ovl i_version to go backwards on copy up
(i.e. after first open for write) when switching from the lower inode
i_version to the upper inode i_version.

Jeff's proposal to use vfs_getattr_nosec() in IMA code is fine too.
It will result in the same i_version jump.

IMO it wouldn't hurt to have a valid i_version value in the ovl inode
as well. If the ovl inode values would not matter, we would not have
needed  ovl_copyattr() at all, but it's not good to keep vfs in the dark...

Thanks,
Amir.