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

Stefan Berger
2023-04-06T16:10:24Z

On 4/6/23 10:36, Paul Moore wrote:
> On Thu, Apr 6, 2023 at 10:20 AM Stefan Berger <stefanb@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@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@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