Messages in this thread | | | Subject | Re: [PATCH v2 3/6] Teach SELinux about a new userfaultfd class | From | Stephen Smalley <> | Date | Wed, 12 Feb 2020 13:59:49 -0500 |
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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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