On 7/25/19 10:04 PM, Amir Goldstein wrote:
On Thu, Jul 25, 2019 at 7:22 PM Mark Salyzyn <salyzyn@xxxxxxxxxxx> wrote:
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 ...
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
