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

[Paul Moore]

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.

-- 
paul-moore.com