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

From: Stephen Smalley <sds@tycho.nsa.gov>
To: Ondrej Mosnacek <omosnace@redhat.com>
Cc: selinux@vger.kernel.org, Casey Schaufler <casey@schaufler-ca.com>,
	Linux Security Module list 
	<linux-security-module@vger.kernel.org>,
	Tejun Heo <tj@kernel.org>,
	linux-fsdevel@vger.kernel.org, cgroups@vger.kernel.org
Subject: Re: [PATCH v2 0/3] Allow initializing the kernfs node's secctx based on its parent
Date: Tue, 22 Jan 2019 09:17:43 -0500	[thread overview]
Message-ID: <3f663024-98f0-98c9-6235-fa4ffafa6a03@tycho.nsa.gov> (raw)
In-Reply-To: <CAFqZXNsm2+79gMTqhxoZuPgibxfhSjJXu8742o7Ho7Yrf=2QCw@mail.gmail.com>

On 1/22/19 3:49 AM, Ondrej Mosnacek wrote:
> On Mon, Jan 14, 2019 at 10:01 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
>> On Thu, Jan 10, 2019 at 6:55 PM Casey Schaufler <casey@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.