From: Daniel Colascione <firstname.lastname@example.org> To: Stephen Smalley <email@example.com> Cc: Tim Murray <firstname.lastname@example.org>, Nosh Minwalla <email@example.com>, Nick Kralevich <firstname.lastname@example.org>, Lokesh Gidra <email@example.com>, linux-kernel <firstname.lastname@example.org>, Linux API <email@example.com>, firstname.lastname@example.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: <email@example.com> On Wed, Feb 12, 2020 at 10:59 AM Stephen Smalley <firstname.lastname@example.org> 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 <email@example.com> > >> 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 <firstname.lastname@example.org> > >>> > >>> (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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