[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 at linux.ibm.com> wrote:
>> 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().
> 
> 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