Re: [PATCH v10 3/5] overlayfs: add __get xattr method

[Mark Salyzyn <salyzyn@android.com>]

On 7/25/19 10:04 PM, Amir Goldstein wrote:
> On Thu, Jul 25, 2019 at 7:22 PM Mark Salyzyn <salyzyn@android.com> wrote:
>> On 7/25/19 8:43 AM, Amir Goldstein wrote:
>>> On Thu, Jul 25, 2019 at 6:03 PM Mark Salyzyn <salyzyn@android.com> wrote:
>>>> On 7/24/19 10:48 PM, Amir Goldstein wrote:
>>>>> On Wed, Jul 24, 2019 at 10:57 PM Mark Salyzyn <salyzyn@android.com> wrote:
>>>>>> Because of the overlayfs getxattr recursion, the incoming inode fails
>>>>>> to update the selinux sid resulting in avc denials being reported
>>>>>> against a target context of u:object_r:unlabeled:s0.
>>>>> This description is too brief for me to understand the root problem.
>>>>> What's wring with the overlayfs getxattr recursion w.r.t the selinux
>>>>> security model?
>>>> __vfs_getxattr (the way the security layer acquires the target sid
>>>> without recursing back to security to check the access permissions)
>>>> calls get xattr method, which in overlayfs calls vfs_getxattr on the
>>>> lower layer (which then recurses back to security to check permissions)
>>>> and reports back -EACCES if there was a denial (which is OK) and _no_
>>>> sid copied to caller's inode security data, bubbles back to the security
>>>> layer caller, which reports an invalid avc: message for
>>>> u:object_r:unlabeled:s0 (the uninitialized sid instead of the sid for
>>>> the lower filesystem target). The blocked access is 100% valid, it is
>>>> supposed to be blocked. This does however result in a cosmetic issue
>>>> that makes it impossible to use audit2allow to construct a rule that
>>>> would be usable to fix the access problem.
>>>>
>>> Ahhh you are talking about getting the security.selinux.* xattrs?
>>> I was under the impression (Vivek please correct me if I wrong)
>>> that overlayfs objects cannot have individual security labels and
>> They can, and we _need_ them for Android's use cases, upper and lower
>> filesystems.
>>
>> Some (most?) union filesystems (like Android's sdcardfs) set sepolicy
>> from the mount options, we did not need this adjustment there of course.
>>
>>> the only way to label overlayfs objects is by mount options on the
>>> entire mount? Or is this just for lower layer objects?
>>>
>>> Anyway, the API I would go for is adding a @flags argument to
>>> get() which can take XATTR_NOSECURITY akin to
>>> FMODE_NONOTIFY, GFP_NOFS, meant to avoid recursions.
>> I do like it better (with the following 7 stages of grief below), best
>> for the future.
>>
>> The change in this handler's API will affect all filesystem drivers
>> (well, my change affects the ABI, so it is not as-if I saved the world
>> from a module recompile) touching all filesystem sources with an even
>> larger audience of stakeholders. Larger audience of stakeholders, the
>> harder to get the change in ;-/. This is also concerning since I would
>> like this change to go to stable 4.4, 4.9, 4.14 and 4.19 where this
>> regression got introduced. I can either craft specific stable patches or
>> just let it go and deal with them in the android-common distributions
>> rather than seeking stable merged down. ABI/API breaks are a problem for
>> stable anyway ...
>>
> Use the memalloc_nofs_save/restore design pattern will avoid all that
> grief.
> As a matter of fact, this issue could and should be handled inside security
> subsystem without bothering any other subsystem.
> LSM have per task context right? That context could carry the recursion
> flags to know that the getxattr call is made by the security subsystem itself.
> The problem is not limited to union filesystems.
> In general its a stacking issue. ecryptfs is also a stacking fs, out-of-tree
> shiftfs as well. But it doesn't end there.
> A filesystem on top of a loop device inside another filesystem could
> also maybe result in security hook recursion (not sure if in practice).
>
> Thanks,
> Amir.

Good point, back to Stephen Smalley?

There are four __vfs_getxattr calls inside security, not sure I see any 
natural way to determine the recursion in security/selinux I can 
beg/borrow/steal from; but I get the strange feeling that it is better 
to detect recursion in __vfs_getxattr in this manner, and switch out 
checking in vfs_getxattr since it is localized to just fs/xattr.c. 
selinux might not be the only user of __vfs_getxattr nature ...

I have implemented and tested the solution where we add a flag to the 
.get method, it works. I would be tempted to submit that instead in case 
someone in the future can imagine using that flag argument to solve 
other problem(s) (if you build it, they will come).

<flips coin>

Will add a new per-process flag that __vfs_getxattr and vfs_getxattr 
plays with and see how it works and what it looks like.

Sincerely -- Mark Salyzyn