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

[Stefan Berger]

On 4/6/23 10:36, 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.

Exactly.

In ima_check_last_writer() I want to trigger the code path with iint->flags &= ...



	if (atomic_read(&inode->i_writecount) == 1) {
		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);
		}
	}


This patch here resolves it for my use case and triggers the expected code paths when
ima_file_free() -> ima_check_last_writer() is called because then the i_version is seen
as having been modified.

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 6011f955436b..1dfe5e7bfe1c 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/iversion.h>
 #include "overlayfs.h"

 struct ovl_aio_req {
@@ -408,6 +409,8 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
                if (ret != -EIOCBQUEUED)
                        ovl_aio_cleanup_handler(aio_req);
        }
+       if (ret > 0)
+               inode_maybe_inc_iversion(inode, false);
 out:
        revert_creds(old_cred);
 out_fdput:



I have been testing this in a OpenBMC/Yocto environment where overlayfs is used as
root filesystem with the lower filesystem being a squashfs.

Regards,
   Stefan