Messages in this thread | | | Subject | Re: [PATCH v10 3/5] overlayfs: add __get xattr method | From | Stephen Smalley <> | Date | Tue, 30 Jul 2019 11:55:00 -0400 |
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On 7/26/19 2:30 PM, Mark Salyzyn wrote: > 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.
As you say, SELinux is not the only user of __vfs_getxattr; in addition to the other security modules, there is the integrity/evm subsystem and ecryptfs. Further, __vfs_getxattr does not merely skip LSM/SELinux-related processing; it also skips xattr_permission(). As such, I don't believe this is something that can be solved entirely within the security subsystem.
Not excited about a process flag to implicitly disable LSM/SELinux and other security-related processing on a code path; potential for abuse is high.
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