Re: [PATCH v12 3/4] selinux: teach SELinux about anonymous inodes

[Lokesh Gidra]

On Thu, Nov 12, 2020 at 4:13 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>
> On Tue, Nov 10, 2020 at 10:30 PM Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote:
> > On Tue, Nov 10, 2020 at 6:13 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
> > > On Tue, Nov 10, 2020 at 1:24 PM Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote:
> > > > On Mon, Nov 9, 2020 at 7:12 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
> > > > > On Fri, Nov 6, 2020 at 10:56 AM Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > From: Daniel Colascione <dancol@xxxxxxxxxx>
> > > > > >
> > > > > > This change uses the anon_inodes and LSM infrastructure introduced in
> > > > > > the previous patches to give SELinux the ability to control
> > > > > > anonymous-inode files that are created using the new
> > > > > > anon_inode_getfd_secure() function.
> > > > > >
> > > > > > A SELinux policy author detects and controls these anonymous inodes by
> > > > > > adding a name-based type_transition rule that assigns a new security
> > > > > > type to anonymous-inode files created in some domain. The name used
> > > > > > for the name-based transition is the name associated with the
> > > > > > anonymous inode for file listings --- e.g., "[userfaultfd]" or
> > > > > > "[perf_event]".
> > > > > >
> > > > > > Example:
> > > > > >
> > > > > > type uffd_t;
> > > > > > type_transition sysadm_t sysadm_t : anon_inode uffd_t "[userfaultfd]";
> > > > > > allow sysadm_t uffd_t:anon_inode { create };
> > > > > >
> > > > > > (The next patch in this series is necessary for making userfaultfd
> > > > > > support this new interface.  The example above is just
> > > > > > for exposition.)
> > > > > >
> > > > > > Signed-off-by: Daniel Colascione <dancol@xxxxxxxxxx>
> > > > > > Signed-off-by: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
> > > > > > ---
> > > > > >  security/selinux/hooks.c            | 53 +++++++++++++++++++++++++++++
> > > > > >  security/selinux/include/classmap.h |  2 ++
> > > > > >  2 files changed, 55 insertions(+)
> > > > > >
> > > > > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > > > > > index 6b1826fc3658..1c0adcdce7a8 100644
> > > > > > --- a/security/selinux/hooks.c
> > > > > > +++ b/security/selinux/hooks.c
> > > > > > @@ -2927,6 +2927,58 @@ static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
> > > > > >         return 0;
> > > > > >  }
> > > > > >
> > > > > > +static int selinux_inode_init_security_anon(struct inode *inode,
> > > > > > +                                           const struct qstr *name,
> > > > > > +                                           const struct inode *context_inode)
> > > > > > +{
> > > > > > +       const struct task_security_struct *tsec = selinux_cred(current_cred());
> > > > > > +       struct common_audit_data ad;
> > > > > > +       struct inode_security_struct *isec;
> > > > > > +       int rc;
> > > > > > +
> > > > > > +       if (unlikely(!selinux_initialized(&selinux_state)))
> > > > > > +               return 0;
> > > > > > +
> > > > > > +       isec = selinux_inode(inode);
> > > > > > +
> > > > > > +       /*
> > > > > > +        * We only get here once per ephemeral inode.  The inode has
> > > > > > +        * been initialized via inode_alloc_security but is otherwise
> > > > > > +        * untouched.
> > > > > > +        */
> > > > > > +
> > > > > > +       if (context_inode) {
> > > > > > +               struct inode_security_struct *context_isec =
> > > > > > +                       selinux_inode(context_inode);
> > > > > > +               isec->sclass = context_isec->sclass;
> > > > > > +               isec->sid = context_isec->sid;
> > > > >
> > > > > I suppose this isn't a major concern given the limited usage at the
> > > > > moment, but I wonder if it would be a good idea to make sure the
> > > > > context_inode's SELinux label is valid before we assign it to the
> > > > > anonymous inode?  If it is invalid, what should we do?  Do we attempt
> > > > > to (re)validate it?  Do we simply fallback to the transition approach?
> > > >
> > > > Frankly, I'm not too familiar with SELinux. Originally this patch
> > > > series was developed by Daniel, in consultation with Stephen Smalley.
> > > > In my (probably naive) opinion we should fallback to transition
> > > > approach. But I'd request you to tell me if this needs to be addressed
> > > > now, and if so then what's the right approach.
> > > >
> > > > If the decision is to address this now, then what's the best way to
> > > > check the SELinux label validity?
> > >
> > > You can check to see if an inode's label is valid by looking at the
> > > isec->initialized field; if it is LABEL_INITIALIZED then it is all
> > > set, if it is any other value then the label isn't entirely correct.
> > > It may have not have ever been fully initialized (and has a default
> > > value) or it may have live on a remote filesystem where the host has
> > > signaled that the label has changed (and the label is now outdated).
> > >
> > > This patchset includes support for userfaultfd, which means we don't
> > > really have to worry about the remote fs problem, but the
> > > never-fully-initialized problem could be real in this case.  Normally
> > > we would revalidate an inode in SELinux by calling
> > > __inode_security_revalidate() which requires either a valid dentry or
> > > one that can be found via the inode; does d_find_alias() work on
> > > userfaultfd inodes?
> > >
> > > If all else fails, it seems like the safest approach would be to
> > > simply fail the selinux_inode_init_security_anon() call if a
> > > context_inode was supplied and the label wasn't valid.  If we later
> > > decide to change it to falling back to the transition approach we can
> > > do that, we can't go the other way (from transition to error).
> >
> > I'm not sure about d_find_alias() on userfaultfd inodes. But it seems
> > ok to fail selinux_inode_init_security_anon() to begin with.
>
> I'm okay with simply failing here, but I'm growing a bit concerned
> that this patchset hasn't been well tested.  That is a problem.
>
> > > > > This brings up another question, and requirement - what testing are
> > > > > you doing for this patchset?  We require that new SELinux kernel
> > > > > functionality includes additions to the SELinux test suite to help
> > > > > verify the functionality.  I'm also *strongly* encouraging that new
> > > > > contributions come with updates to The SELinux Notebook.  If you are
> > > > > unsure about what to do for either, let us know and we can help get
> > > > > you started.
> > > > >
> > > > > * https://github.com/SELinuxProject/selinux-testsuite
> > > > > * https://github.com/SELinuxProject/selinux-notebook
> > > > >
> > > > I'd definitely need help with both of these. Kindly guide how to proceed.
> > >
> > > Well, perhaps the best way to start is to explain how you have been
> > > testing this so far and then using that information to draft a test
> > > for the testsuite.
> >
> > As I said in my previous reply, Daniel worked on this patch along with
> > Stephan Smalley. Here's the conversation regarding testing from back
> > then:
> > https://lore.kernel.org/lkml/CAEjxPJ4iquFSBfEj+UEFLUFHPsezuQ-Bzv09n+WgOWk38Nyw3w@xxxxxxxxxxxxxx/
> >
> > There have been only minor changes (fixing comments/coding-style),
> > except for addressing a double free issue with userfaultfd_ctx since
> > last time it was tested as per the link above.
>
> I should probably be more clear.  I honestly don't care who originally
> wrote the patch, the simple fact is that you are the one who is
> posting it *now* for inclusion in the kernel; at the very least I
> expect you to be able to demonstrate that you are able to reliably
> test this functionality and prove it is working.  While being able to
> test this submission initially is important, it is far more important
> to have the tests and docs necessary to maintain this functionality
> long term.  Perhaps you and/or Google will continue to contribute and
> support this functionality long term, but it would be irresponsible of
> me to assume that to be true; both people and companies come and go
> but code has a tendency to live forever.
>
> Let's start again; how have you been testing this code?
>
I have created a cuttlefish build and have tested with the attached
userfaultfd program:

1) Without these kernel patches the program executes without any restrictions

vsoc_x86_64:/ $ ./system/bin/userfaultfdSimple
api: 170
features: 511
ioctls: 9223372036854775811

read: Try again


2) With these patches applied but without any policy the 'permission
denied' is thrown

vsoc_x86_64:/ $ ./system/bin/userfaultfdSimple
syscall(userfaultfd): Permission denied

with the following logcat message:
11-18 14:21:44.041  3130  3130 W userfaultfdSimp: type=1400
audit(0.0:107): avc: denied { create } for dev="anon_inodefs"
ino=45031 scontext=u:r:shell:s0 tcontext=u:object_r:shell:s0
tclass=anon_inode permissive=0


3) With the attached .te policy file in place the following output is
observed, confirming that the patch is working as intended.
vsoc_x86_64:/ $ ./vendor/bin/userfaultfdSimple
UFFDIO_API: Permission denied

with the following logcat message:
11-18 14:33:29.142  2028  2028 W userfaultfdSimp: type=1400
audit(0.0:104): avc: denied { ioctl } for
path="anon_inode:[userfaultfd]" dev="anon_inodefs" ino=41169
ioctlcmd=0xaa3f scontext=u:r:userfaultfdSimple:s0
tcontext=u:object_r:uffd_t:s0 tclass=anon_inode permissive=0


> --
> paul moore
> www.paul-moore.com
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <cstring>

#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>

#include <linux/userfaultfd.h>

void print_api(const struct uffdio_api *api)
{
	printf("api: %llu\n", api->api);
	printf("features: %llu\n", api->features);
	printf("ioctls: %llu\n", api->ioctls);

	printf("\n");
}

int main(void)
{
	long uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
	if (uffd < 0) {
		perror("syscall(userfaultfd)");
		return -1;
	}

	struct uffdio_api api;
	std::memset(&api, 0x0, sizeof api);
	api.api = UFFD_API;
	if (ioctl(uffd, UFFDIO_API, &api) < 0) {
		perror("UFFDIO_API");
		return -1;
	}

	print_api(&api);

	struct uffd_msg msg;
	std::memset(&msg, 0x0, sizeof msg);
	ssize_t count = read(uffd, &msg, sizeof(msg));
	if (count < 0) {
		perror("read");
		return -1;
	} else if (count == 0) {
		printf("read EOF\n\n");
	}

	printf("read uffd\n\n");

	return 0;
}

Attachment: userfaultfdSimple.te
Description: Binary data