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    SubjectRe: [PATCH v2 3/6] Teach SELinux about a new userfaultfd class
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    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@tycho.nsa.gov>
    >> 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@google.com>
    >>>
    >>> (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,
    >>>> +
    >>>> +     /*
    >>>> +      * We shouldn't be creating secure anonymous inodes before LSM
    >>>> +      * initialization completes.
    >>>> +      */
    >>>> +     if (unlikely(!selinux_state.initialized))
    >>>> +             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.
    >
    >>>> +     /*
    >>>> +      * We only get here once per ephemeral inode.  The inode has
    >>>> +      * been initialized via inode_alloc_security but is otherwise
    >>>> +      * untouched, so check that the state is as
    >>>> +      * inode_alloc_security left it.
    >>>> +      */
    >>>> +     BUG_ON(isec->initialized != LABEL_INVALID);
    >>>> +     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
    >>>> +     if (fops == &userfaultfd_fops)
    >>>> +             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.
    >
    >>>> +     /*
    >>>> +      * Always give secure anonymous inodes the sid of the
    >>>> +      * creating task.
    >>>> +      */
    >>>> +
    >>>> +     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.

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