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

[Christian Brauner]

On Thu, Apr 06, 2023 at 10:36:41AM -0400, Paul Moore wrote:
> On Thu, Apr 6, 2023 at 10:20 AM Stefan Berger <stefanb@xxxxxxxxxxxxx> wrote:
> > On 4/6/23 10:05, Paul Moore wrote:
> > > On Thu, Apr 6, 2023 at 6:26 AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> > >> On Wed, Apr 05, 2023 at 01:14:49PM -0400, Stefan Berger wrote:
> > >>> Overlayfs fails to notify IMA / EVM about file content modifications
> > >>> and therefore IMA-appraised files may execute even though their file
> > >>> signature does not validate against the changed hash of the file
> > >>> anymore. To resolve this issue, add a call to integrity_notify_change()
> > >>> to the ovl_release() function to notify the integrity subsystem about
> > >>> file changes. The set flag triggers the re-evaluation of the file by
> > >>> IMA / EVM once the file is accessed again.
> > >>>
> > >>> Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> > >>> ---
> > >>>   fs/overlayfs/file.c       |  4 ++++
> > >>>   include/linux/integrity.h |  6 ++++++
> > >>>   security/integrity/iint.c | 13 +++++++++++++
> > >>>   3 files changed, 23 insertions(+)
> > >>>
> > >>> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> > >>> index 6011f955436b..19b8f4bcc18c 100644
> > >>> --- a/fs/overlayfs/file.c
> > >>> +++ b/fs/overlayfs/file.c
> > >>> @@ -13,6 +13,7 @@
> > >>>   #include <linux/security.h>
> > >>>   #include <linux/mm.h>
> > >>>   #include <linux/fs.h>
> > >>> +#include <linux/integrity.h>
> > >>>   #include "overlayfs.h"
> > >>>
> > >>>   struct ovl_aio_req {
> > >>> @@ -169,6 +170,9 @@ static int ovl_open(struct inode *inode, struct file *file)
> > >>>
> > >>>   static int ovl_release(struct inode *inode, struct file *file)
> > >>>   {
> > >>> +     if (file->f_flags & O_ACCMODE)
> > >>> +             integrity_notify_change(inode);
> > >>> +
> > >>>        fput(file->private_data);
> > >>>
> > >>>        return 0;
> > >>> diff --git a/include/linux/integrity.h b/include/linux/integrity.h
> > >>> index 2ea0f2f65ab6..cefdeccc1619 100644
> > >>> --- a/include/linux/integrity.h
> > >>> +++ b/include/linux/integrity.h
> > >>> @@ -23,6 +23,7 @@ enum integrity_status {
> > >>>   #ifdef CONFIG_INTEGRITY
> > >>>   extern struct integrity_iint_cache *integrity_inode_get(struct inode *inode);
> > >>>   extern void integrity_inode_free(struct inode *inode);
> > >>> +extern void integrity_notify_change(struct inode *inode);
> > >>
> > >> I thought we concluded that ima is going to move into the security hook
> > >> infrastructure so it seems this should be a proper LSM hook?
> > >
> > > We are working towards migrating IMA/EVM to the LSM layer, but there
> > > are a few things we need to fix/update/remove first; if anyone is
> > > curious, you can join the LSM list as we've been discussing some of
> > > these changes this week.  Bug fixes like this should probably remain
> > > as IMA/EVM calls for the time being, with the understanding that they
> > > will migrate over with the rest of IMA/EVM.
> > >
> > > That said, we should give Mimi a chance to review this patch as it is
> > > possible there is a different/better approach.  A bit of patience may
> > > be required as I know Mimi is very busy at the moment.
> >
> > There may be a better approach actually by increasing the inode's i_version,
> > which then should trigger the appropriate path in ima_check_last_writer().
> 
> I'm not the VFS/inode expert here, but I thought the inode's i_version
> field was only supposed to be bumped when the inode metadata changed,
> not necessarily the file contents, right?
> 
> That said, overlayfs is a bit different so maybe that's okay, but I
> think we would need to hear from the VFS folks if this is acceptable.

Ccing Jeff for awareness since he did the i_version rework a short time ago.

The documentation in include/linux/iversion.h states:

 * [...] The i_version must
 * appear larger to observers if there was an explicit change to the inode's
 * data or metadata since it was last queried.

what I'm less sure in all of this is why this is called in ovl_release() and
whether it's correct to increment the overlayfs inode's i_version.

The change is done to the inode of the copied up/modified file's inode in the
upper layer. So the i_version should already be incremented when we call into
the upper layer usually via vfs_*() methods.