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

[Ondrej Mosnacek]

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

Thanks,

-- 
Ondrej Mosnacek <omosnace at redhat dot com>
Associate Software Engineer, Security Technologies
Red Hat, Inc.