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

[Stefan Berger <stefanb@linux.ibm.com>]

On 4/7/23 09:29, Jeff Layton wrote:
>>>>>
>>>>> I would ditch the original proposal in favor of this 2-line patch shown here:
>>>>>
>>>>> https://lore.kernel.org/linux-integrity/a95f62ed-8b8a-38e5-e468-ecbde3b221af@linux.ibm.com/T/#m3bd047c6e5c8200df1d273c0ad551c645dd43232
>>
>> We should cool it with the quick hacks to fix things. :)
>>
> 
> Yeah. It might fix this specific testcase, but I think the way it uses
> the i_version is "gameable" in other situations. Then again, I don't
> know a lot about IMA in this regard.
> 
> When is it expected to remeasure? If it's only expected to remeasure on
> a close(), then that's one thing. That would be a weird design though.

IMA should remeasure the file when it has visibly changed for another thread or process.


>>> -----------------------8<---------------------------
>>>
>>> [PATCH] IMA: use vfs_getattr_nosec to get the i_version
>>>
>>> IMA currently accesses the i_version out of the inode directly when it
>>> does a measurement. This is fine for most simple filesystems, but can be
>>> problematic with more complex setups (e.g. overlayfs).
>>>
>>> Make IMA instead call vfs_getattr_nosec to get this info. This allows
>>> the filesystem to determine whether and how to report the i_version, and
>>> should allow IMA to work properly with a broader class of filesystems in
>>> the future.
>>>
>>> Reported-by: Stefan Berger <stefanb@linux.ibm.com>
>>> Signed-off-by: Jeff Layton <jlayton@kernel.org>
>>> ---
>>
>> So, I think we want both; we want the ovl_copyattr() and the
>> vfs_getattr_nosec() change:
>>
>> (1) overlayfs should copy up the inode version in ovl_copyattr(). That
>>      is in line what we do with all other inode attributes. IOW, the
>>      overlayfs inode's i_version counter should aim to mirror the
>>      relevant layer's i_version counter. I wouldn't know why that
>>      shouldn't be the case. Asking the other way around there doesn't
>>      seem to be any use for overlayfs inodes to have an i_version that
>>      isn't just mirroring the relevant layer's i_version.
> 
> It's less than ideal to do this IMO, particularly with an IS_I_VERSION
> inode.
> 
> You can't just copy up the value from the upper. You'll need to call
> inode_query_iversion(upper_inode), which will flag the upper inode for a
> logged i_version update on the next write. IOW, this could create some
> (probably minor) metadata write amplification in the upper layer inode
> with IS_I_VERSION inodes.
> 
> 
>> (2) Jeff's changes for ima to make it rely on vfs_getattr_nosec().
>>      Currently, ima assumes that it will get the correct i_version from
>>      an inode but that just doesn't hold for stacking filesystem.
>>
>> While (1) would likely just fix the immediate bug (2) is correct and
>> _robust_. If we change how attributes are handled vfs_*() helpers will
>> get updated and ima with it. Poking at raw inodes without using
>> appropriate helpers is much more likely to get ima into trouble.
> 
> This will fix it the right way, I think (assuming it actually works),
> and should open the door for IMA to work properly with networked
> filesystems that support i_version as well.
> 
> Note that there Stephen is correct that calling getattr is probably
> going to be less efficient here since we're going to end up calling
> generic_fillattr unnecessarily, but I still think it's the right thing
> to do.

I was wondering whether to use the existing inode_eq_iversion() for all
other filesystems than overlayfs, nfs, and possibly other ones (which ones?)
where we would use the vfs_getattr_nosec() via a case on inode->i_sb->s_magic?
If so, would this function be generic enough to be a public function for libfs.c?

I'll hopefully be able to test the proposed patch tomorrow.

> 
> If it turns out to cause measurable performance regressions though,
> maybe we can look at adding a something that still calls ->getattr if it
> exists but only returns the change_cookie value.