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

[Stefan Berger]

On 4/6/23 10:05, Paul Moore wrote:
> On Thu, Apr 6, 2023 at 6:26 AM Christian Brauner <brauner at kernel.org> 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 at linux.ibm.com>
>>> ---
>>>   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().