[PATCH v2 0/3] Allow initializing the kernfs node's secctx based on its parent

Tue Jan 22 15:26:58 UTC 2019

On 1/22/19 9:17 AM, Stephen Smalley wrote:
> On 1/22/19 3:49 AM, Ondrej Mosnacek wrote:
>> On Mon, Jan 14, 2019 at 10:01 AM Ondrej Mosnacek <omosnace at redhat.com> 
>> wrote:
>>> On Thu, Jan 10, 2019 at 6:55 PM Casey Schaufler 
>>> <casey at schaufler-ca.com> wrote:
>>>> Resending after email configuration repair.
>>>>
>>>> On 1/10/2019 6:15 AM, Stephen Smalley wrote:
>>>>> On 1/9/19 5:03 PM, Casey Schaufler wrote:
>>>>>> On 1/9/2019 12:37 PM, Stephen Smalley wrote:
>>>>>>> On 1/9/19 12:19 PM, Casey Schaufler wrote:
>>>>>>>> On 1/9/2019 8:28 AM, Ondrej Mosnacek wrote:
>>>>>>>>> Changes in v2:
>>>>>>>>> - add docstring for the new hook in union security_list_options
>>>>>>>>> - initialize *ctx to NULL and *ctxlen to 0 in case the hook is not
>>>>>>>>>      implemented
>>>>>>>>> v1: 
>>>>>>>>> https://lore.kernel.org/selinux/20190109091028.24485-1-omosnace@redhat.com/T/ 
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> This series adds a new security hook that allows to initialize 
>>>>>>>>> the security
>>>>>>>>> context of kernfs properly, taking into account the parent 
>>>>>>>>> context. Kernfs
>>>>>>>>> nodes require special handling here, since they are not bound 
>>>>>>>>> to specific
>>>>>>>>> inodes/superblocks, but instead represent the backing tree 
>>>>>>>>> structure that
>>>>>>>>> is used to build the VFS tree when the kernfs tree is mounted.
>>>>>>>>>
>>>>>>>>> The kernfs nodes initially do not store any security context 
>>>>>>>>> and rely on
>>>>>>>>> the LSM to assign some default context to inodes created over 
>>>>>>>>> them.
>>>>>>>>
>>>>>>>> This seems like a bug in kernfs. Why doesn't kernfs adhere to 
>>>>>>>> the usual
>>>>>>>> and expected filesystem behavior?
>>>>>>>
>>>>>>> sysfs / kernfs didn't support xattrs at all when we first added 
>>>>>>> support for setting security contexts to it, so originally all 
>>>>>>> sysfs / kernfs inodes had a single security context, and we only 
>>>>>>> required separate storage for the inodes that were explicitly 
>>>>>>> labeled by userspace.
>>>>>>>
>>>>>>> Later kernfs grew support for trusted.* xattrs using 
>>>>>>> simple_xattrs but the existing security.* support was left mostly 
>>>>>>> unchanged.
>>>>>>
>>>>>> OK, so as I said, this seems like a bug in kernfs.
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>>> Kernfs
>>>>>>>>> inodes, however, allow setting an explicit context via the 
>>>>>>>>> *setxattr(2)
>>>>>>>>> syscalls, in which case the context is stored inside the kernfs 
>>>>>>>>> node's
>>>>>>>>> metadata.
>>>>>>>>>
>>>>>>>>> SELinux (and possibly other LSMs) initialize the context of 
>>>>>>>>> newly created
>>>>>>>>> FS objects based on the parent object's context (usually the 
>>>>>>>>> child inherits
>>>>>>>>> the parent's context, unless the policy dictates otherwise).
>>>>>>>>
>>>>>>>> An LSM might use information about the parent other than the 
>>>>>>>> "context".
>>>>>>>> Smack, for example, uses an attribute SMACK64TRANSMUTE from the 
>>>>>>>> parent
>>>>>>>> to determine whether the Smack label of the new object should be 
>>>>>>>> taken
>>>>>>>> from the parent or the process. Passing the "context" of the 
>>>>>>>> parent is
>>>>>>>> insufficient for Smack.
>>>>>>>
>>>>>>> IIUC, this would involve switching the handling of security.* 
>>>>>>> xattrs in kernfs over to use simple_xattrs too (so that we can 
>>>>>>> store multiple such attributes), and then pass the entire 
>>>>>>> simple_xattrs list or at least anything with a security.* prefix 
>>>>>>> when initializing a new node or refreshing an existing inode.  
>>>>>>> Then the security module could extract any security.* attributes 
>>>>>>> of interest for use in determining the label of new inodes and in 
>>>>>>> refreshing the label of an inode.
>>>
>>> I actually had a patch to do just that at one point because I thought
>>> for a while that it would be required to call
>>> security_inode_init_security() (which I had tried to somehow force
>>> into the kernfs node creation at some point), but then I realized it
>>> is not actually needed (although would make thing a bit nicer) and put
>>> it away... I will try to dig it out and reuse here.
>>
>> Okay, now that I tried to do this with full xattr support I ran into a
>> problem. Along with converting kernfs to use simple_xattrs for
>> security attributes, I removed the call to
>> security_inode_notifysecctx() from kernfs_refresh_inode(), as it no
>> longer makes sense (kernfs doesn't know which attribute contains the
>> context; the LSM should now be able to pull it out via
>> vfs_getxattr()). However, SELinux now doesn't set the right security
>> context in the selinux_d_instantiate() hook, because the policy tells
>> it to use genfs, not xattr.
>>
>> So... I'm not sure how to fix this. Setting fs_use_xattr for cgroupfs
>> in the policy won't work, because then all nodes will be unlabeled_t
>> by default. Maybe we could patch the genfs case in
>> inode_doinit_with_dentry() to try fetching the xattr first? I'm not
>> very confident about touching that part of the code, so I would
>> welcome some advice here.
>>
>> This is the code I have so far, in case it helps:
>> https://gitlab.com/omos/linux-public/compare/selinux-next...selinux-fix-cgroupfs-v8 
>>
> 
> I would have left security_inode_notifysecctx() or an equivalent that 
> passes all of the xattrs to push the security attributes to the security 
> module.
> 
> Blindly calling __vfs_getxattr() on genfs could be a problem; IIRC, 
> doing so on fuse filesytems can create a deadlock during mount.  Or at 
> least that was the issue with switching fuse to fs_use_xattr in the past.

See commits 4d546f81717d253ab67643bf072c6d8821a9249c, 
102aefdda4d8275ce7d7100bc16c88c74272b260, 
089be43e403a78cd6889cde2fba164fefe9dfd89, 
811f3799279e567aa354c649ce22688d949ac7a9 and
https://bugzilla.redhat.com/show_bug.cgi?id=1256635#c34 for some prior 
work and discussions in this area.