Re: [PATCH v2 3/6] Teach SELinux about a new userfaultfd class

From: Stephen Smalley
Date: Wed Feb 12 2020 - 14:16:25 EST

On 2/12/20 2:11 PM, Stephen Smalley wrote:
On 2/12/20 2:04 PM, Daniel Colascione wrote:
On Wed, Feb 12, 2020 at 10:59 AM Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:

On 2/12/20 1:04 PM, Stephen Smalley wrote:
On 2/12/20 12:19 PM, Daniel Colascione wrote:
Thanks for taking a look.

On Wed, Feb 12, 2020 at 9:04 AM Stephen Smalley <sds@xxxxxxxxxxxxx>
wrote:

On 2/11/20 5:55 PM, Daniel Colascione wrote:
Use the secure anonymous inode LSM hook we just added to let SELinux
policy place restrictions on userfaultfd use. The create operation
applies to processes creating new instances of these file objects;
transfer between processes is covered by restrictions on read, write,
and ioctl access already checked inside selinux_file_receive.

Signed-off-by: Daniel Colascione <dancol@xxxxxxxxxx>

(please add linux-fsdevel and viro to the cc for future versions of this
patch since it changes the VFS)

---
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 1659b59fb5d7..e178f6f40e93 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2915,6 +2919,69 @@ static int selinux_inode_init_security(struct
inode *inode, struct inode *dir,
+
+ÂÂÂÂ /*
+ÂÂÂÂÂ * We shouldn't be creating secure anonymous inodes before LSM
+ÂÂÂÂÂ * initialization completes.
+ÂÂÂÂÂ */
+ÂÂÂÂ if (unlikely(!selinux_state.initialized))
+ÂÂÂÂÂÂÂÂÂÂÂÂ return -EBUSY;

I don't think this is viable; any arbitrary actions are possible before
policy is loaded, and a Linux distro can be brought up fully with
SELinux enabled and no policy loaded. You'll just need to have a
default behavior prior to initialization.

We'd have to fail open then, I think, and return an S_PRIVATE inode
(the regular anon inode).

Not sure why. You aren't doing anything in the hook that actually
relies on selinux_state.initialized being set (i.e. nothing requires a
policy). The avc_has_perm() call will just succeed until a policy is
loaded. So if these inodes are created prior to policy load, they will
get assigned the task SID (which would be the kernel SID prior to policy
load or first exec or write to /proc/self/attr/current afterward) and
UFFD class (in your current code), be permitted, and then once policy is
loaded any further access will get checked against the kernel SID.

+ÂÂÂÂ /*
+ * We only get here once per ephemeral inode. The inode has
+ÂÂÂÂÂ * been initialized via inode_alloc_security but is otherwise
+ÂÂÂÂÂ * untouched, so check that the state is as
+ÂÂÂÂÂ * inode_alloc_security left it.
+ÂÂÂÂÂ */
+ÂÂÂÂ BUG_ON(isec->initialized != LABEL_INVALID);
+ÂÂÂÂ BUG_ON(isec->sclass != SECCLASS_FILE);

I think the kernel discourages overuse of BUG_ON/BUG/...

I'm not sure what counts as overuse.

Me either (not my rule) but I'm pretty sure this counts or you'd see a
lot more of these kinds of BUG_ON() checks throughout. Try to reserve
them for really critical cases.

+
+#ifdef CONFIG_USERFAULTFD
+ÂÂÂÂ if (fops == &userfaultfd_fops)
+ÂÂÂÂÂÂÂÂÂÂÂÂ isec->sclass = SECCLASS_UFFD;
+#endif

Not sure we want or need to introduce a new security class for each user
of anonymous inodes since the permissions should be the same as for
file.

The purpose of this change is to apply special policy to userfaultfd
FDs in particular. Isn't having a UFFD security class the best way to
go about that? (There's no path.) Am I missing something?

It is probably the simplest approach; it just doesn't generalize to all
users of anonymous inodes. We can distinguish them in one of two ways:
use a different class like you did (requires a code change every time we
add a new one and yet another duplicate of the file class) or use a
different SID/context/type. The latter could be achieved by calling
security_transition_sid() with the provided name wrapped in a qstr and
specifying type_transition rules on the name. Then policy could define
derived types for each domain, ala
type_transition init self:file "[userfaultfd]" init_userfaultfd;
type_transition untrusted_app self:file "[userfaultfd]"
untrusted_app_userfaultfd;
...

Also not sure we want to be testing fops for each such case.

I was also thinking of just providing some kind of context string
(maybe the name), which might be friendlier to modules, but the loose
coupling kind of scares me, and for this particular application, since
UFFD is always in the core and never in a module, checking the fops
seems a bit more robust and doesn't hurt anything.

Yes, not sure how the vfs folks feel about either coupling (the
name-based one or the fops-based one). Neither seems great.

We
were looking at possibly leveraging the name as a key and using
security_transition_sid() to generate a distinct SID/context/type for
the inode via type_transition rules in policy. We have some WIP along
those lines.

Where? Any chance it would be ready soon? I'd rather not hold up this
work for a more general mechanism.

Hopefully will have a patch available soon. But not saying this
necessarily has to wait either.

+ÂÂÂÂ /*
+ÂÂÂÂÂ * Always give secure anonymous inodes the sid of the
+ÂÂÂÂÂ * creating task.
+ÂÂÂÂÂ */
+
+ÂÂÂÂ isec->sid = tsec->sid;

This doesn't generalize for other users of anonymous inodes, e.g. the
/dev/kvm case where we'd rather inherit the SID and class from the
original /dev/kvm inode itself.

I think someone mentioned on the first version of this patch that we
could make it more flexible if the need arose. If we do want to do it
now, we could have the anon_inode security hook accept a "parent" or
"context" inode that modules could inspect for the purposes of forming
the new inode's SID. Does that make sense to you?

Yes, that's the approach in our current WIP, except we call it a
"related" inode since it isn't necessarily connected to the anon inode
in any vfs sense.

The other key difference in our WIP approach is that we assumed that we
couldn't mandate allocating a separate anon inode for each of these fds
and we wanted to cover all anonymous inodes (not opt-in), so we are
storing the SID/class pair as additional fields in the
file_security_struct and have modified file_has_perm() and others to
look there for anonymous inodes.

A separate inode seems like the simpler approach for now, because it
means that we have fewer places to check for security information ---
and it's not as if an inode is particularly expensive. We can always
switch later.

We'd prefer having a separate inode if possible but didn't think that would fly with the vfs folks, especially if we try to apply this to all anonymous inodes. It might be ok for userfaultfd usage as a specific case but there is a reason why anonymous inodes were introduced and creating a separate inode each time defeats that purpose IIUC. It will be interesting to see how they respond.

I suppose an optimization of your approach could be to only allocate a new anon inode if there isn't already one that has the same security info (SID/class pair in the SELinux case).