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

From: Mark Salyzyn
Date: Thu Jul 25 2019 - 14:08:19 EST

On 7/25/19 8:43 AM, Amir Goldstein wrote:
On Thu, Jul 25, 2019 at 6:03 PM Mark Salyzyn <salyzyn@xxxxxxxxxxx> wrote:
On 7/24/19 10:48 PM, Amir Goldstein wrote:
On Wed, Jul 24, 2019 at 10:57 PM Mark Salyzyn <salyzyn@xxxxxxxxxxx> 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 ...

-- Mark