From: Daniel Colascione <dancol@google.com>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Tim Murray <timmurray@google.com>,
Nosh Minwalla <nosh@google.com>, Nick Kralevich <nnk@google.com>,
Lokesh Gidra <lokeshgidra@google.com>,
linux-kernel <linux-kernel@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>,
selinux@vger.kernel.org
Subject: Re: [PATCH v2 3/6] Teach SELinux about a new userfaultfd class
Date: Wed, 12 Feb 2020 11:04:54 -0800 [thread overview]
Message-ID: <CAKOZuevoKDYGVSooWAhi7Jr6Ww-+NEd-sStaPcN5Q6g+NKKRPQ@mail.gmail.com> (raw)
In-Reply-To: <69f4ccce-18b2-42c1-71ac-3fe9caf2dfb6@tycho.nsa.gov>
On Wed, Feb 12, 2020 at 10:59 AM Stephen Smalley <sds@tycho.nsa.gov> 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@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.
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.
next prev parent reply other threads:[~2020-02-12 19:05 UTC|newest]
Thread overview: 80+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-11 22:55 [PATCH v2 0/6] Harden userfaultfd Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 1/6] Add a new flags-accepting interface for anonymous inodes Daniel Colascione
2020-02-12 16:37 ` Stephen Smalley
2020-02-12 17:23 ` Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 2/6] Add a concept of a "secure" anonymous file Daniel Colascione
2020-02-12 16:49 ` Stephen Smalley
2020-02-14 22:13 ` kbuild test robot
2020-02-11 22:55 ` [PATCH v2 3/6] Teach SELinux about a new userfaultfd class Daniel Colascione
2020-02-12 17:05 ` Stephen Smalley
2020-02-12 17:19 ` Daniel Colascione
2020-02-12 18:04 ` Stephen Smalley
2020-02-12 18:59 ` Stephen Smalley
2020-02-12 19:04 ` Daniel Colascione [this message]
2020-02-12 19:11 ` Stephen Smalley
2020-02-12 19:13 ` Daniel Colascione
2020-02-12 19:17 ` Stephen Smalley
2020-02-11 22:55 ` [PATCH v2 4/6] Wire UFFD up to SELinux Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 5/6] Let userfaultfd opt out of handling kernel-mode faults Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 6/6] Add a new sysctl for limiting userfaultfd to user mode faults Daniel Colascione
2020-02-11 23:13 ` [PATCH v2 0/6] Harden userfaultfd Casey Schaufler
2020-02-11 23:27 ` Daniel Colascione
2020-02-12 16:09 ` Stephen Smalley
2020-02-21 17:56 ` James Morris
2020-02-12 7:50 ` Kees Cook
2020-02-12 16:54 ` Jann Horn
2020-02-12 17:14 ` Peter Xu
2020-02-12 19:41 ` Andrea Arcangeli
2020-02-12 20:04 ` Daniel Colascione
2020-02-12 23:41 ` Andrea Arcangeli
2020-02-12 17:12 ` Daniel Colascione
2020-02-14 3:26 ` [PATCH 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-02-14 3:26 ` [PATCH 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-02-14 3:26 ` [PATCH 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-02-14 16:39 ` Stephen Smalley
2020-02-14 17:21 ` Daniel Colascione
2020-02-14 18:02 ` Stephen Smalley
2020-02-14 18:08 ` Stephen Smalley
2020-02-14 20:24 ` Stephen Smalley
2020-02-14 3:26 ` [PATCH 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-03-25 23:02 ` [PATCH v2 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-03-25 23:02 ` [PATCH v2 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-03-26 13:53 ` Stephen Smalley
2020-03-25 23:02 ` [PATCH v2 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-03-26 13:58 ` Stephen Smalley
2020-03-26 17:59 ` Daniel Colascione
2020-03-26 17:37 ` Stephen Smalley
2020-03-25 23:02 ` [PATCH v2 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-03-25 23:49 ` Casey Schaufler
2020-03-26 18:14 ` [PATCH v3 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-03-26 18:14 ` [PATCH v3 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-03-26 19:00 ` Stephen Smalley
2020-03-26 18:14 ` [PATCH v3 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-03-26 19:02 ` Stephen Smalley
2020-03-26 18:14 ` [PATCH v3 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-03-26 20:06 ` [PATCH v4 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-03-26 20:06 ` [PATCH v4 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-03-27 13:40 ` Stephen Smalley
2020-03-26 20:06 ` [PATCH v4 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-03-27 13:41 ` Stephen Smalley
2020-03-26 20:06 ` [PATCH v4 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-04-01 21:39 ` [PATCH v5 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-04-01 21:39 ` [PATCH v5 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-05-07 16:02 ` James Morris
2020-08-04 21:22 ` Eric Biggers
2020-04-01 21:39 ` [PATCH v5 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-04-01 21:39 ` [PATCH v5 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-08-04 21:16 ` Eric Biggers
2020-04-13 13:29 ` [PATCH v5 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-04-22 16:55 ` James Morris
2020-04-22 17:12 ` Casey Schaufler
2020-04-23 22:24 ` Casey Schaufler
2020-04-27 16:18 ` Casey Schaufler
2020-04-27 16:48 ` Stephen Smalley
2020-04-27 17:12 ` Casey Schaufler
2020-04-29 17:02 ` Stephen Smalley
2020-04-27 17:15 ` Casey Schaufler
2020-04-27 19:40 ` Stephen Smalley
2020-06-04 3:56 ` James Morris
2020-06-04 18:51 ` Stephen Smalley
2020-06-04 19:24 ` Lokesh Gidra
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