linux-kernel - Re: [PATCH V2] security/selinux: Always allow FIOCLEX and FIONCLEX

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Message-ID: <20396305e71619dbee4fa3c612925b57f4bb0a4b.camel@btinternet.com>
Date:   Wed, 23 Feb 2022 11:58:19 +0000
From:   Richard Haines <richard_c_haines@...nternet.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     stephen.smalley.work@...il.com, eparis@...isplace.org,
        demiobenour@...il.com, selinux@...r.kernel.org,
        linux-kernel@...r.kernel.org, selinux-refpolicy@...r.kernel.org,
        jeffv@...gle.com
Subject: Re: [PATCH V2] security/selinux: Always allow FIOCLEX and FIONCLEX

On Tue, 2022-02-22 at 18:28 -0500, Paul Moore wrote:
> On Mon, Feb 21, 2022 at 8:15 AM Richard Haines
> <richard_c_haines@...nternet.com> wrote:
> > 
> > These ioctls are equivalent to fcntl(fd, F_SETFD, flags), which
> > SELinux
> > always allows too.  Furthermore, a failed FIOCLEX could result in a
> > file
> > descriptor being leaked to a process that should not have access to
> > it.
> > 
> > As this patch removes access controls, a policy capability needs to
> > be
> > enabled in policy to always allow these ioctls.
> > 
> > Based-on-patch-by: Demi Marie Obenour <demiobenour@...il.com>
> > Signed-off-by: Richard Haines <richard_c_haines@...nternet.com>
> > ---
> > V2 Change: Control via a policy capability. See this thread for
> > discussion:
> > https://lore.kernel.org/selinux/CAHC9VhQEPxYP_KU56gAGNHKQaxucY8gSsHiUB42PVgADBAccRQ@mail.gmail.com/T/#t
> > 
> > With this patch and the polcap enabled, the selinux-testsuite will
> > fail:
> > ioctl/test at line 47 - Will need a fix.
> > 
> >  security/selinux/hooks.c                   | 7 +++++++
> >  security/selinux/include/policycap.h       | 1 +
> >  security/selinux/include/policycap_names.h | 3 ++-
> >  security/selinux/include/security.h        | 7 +++++++
> >  4 files changed, 17 insertions(+), 1 deletion(-)
> 
> Thanks Richard for putting together the v2 of this patch.
> 
> As far as the test is concerned, it seems like the quick-n-dirty fix
> is to simply remove the ioctl(FIOCLEX) test in test_noioctl.c; is
> everyone okay with that?  At least that is what I'm going to do with
> my local copy that I use to validate the kernel-secnext builds unless
> someone has a better patch :)

To fix this I was planning to submit a patch that would change the
ioctl(FIOCLEX) tests to ioctl(FS_IOC_GETFSLABEL) as that would continue
to test the xperms.

> 
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index 5b6895e4f..030c41652 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -3745,6 +3745,13 @@ static int selinux_file_ioctl(struct file
> > *file, unsigned int cmd,
> >                                             CAP_OPT_NONE, true);
> >                 break;
> > 
> > +       case FIOCLEX:
> > +       case FIONCLEX:
> > +               /* Must always succeed if polcap set, else default:
> > */
> > +               if (selinux_policycap_ioctl_skip_cloexec())
> > +                       break;
> > +               fallthrough;
> > +
> 
> The break/fallthrough looks like it might be a little more fragile
> than necessary, how about something like this:
> 
>   case FIOCLEX:
>   case FIONCLEX:
>     if (!selinux_policycap_ioctl_skip_cloexec())
>       error = ioctl_has_perm(cred, file, FILE__IOCTL, (u16) cmd);
>       break;
> 
> Yes, it does duplicate the default ioctl_has_perm() call, but since
> we
> are effectively deprecating this and locking the FIOCLEX/FIONCLEX
> behavior with this policy capability it seems okay to me (and
> preferable to relying on the fallthrough).
> 
> Thoughts?

Yes I did ponder this and in my first attempt I had this before the
switch():

	/* Must always succeed if polcap set */
	if (selinux_policycap_ioctl_skip_cloexec() &&
	    (cmd == FIOCLEX || cmd == FIONCLEX))
		return 0;

 	switch (cmd) {
 	case FIONREAD:
 	case FIBMAP:

but changed to within the switch(), anyway I'm happy to resubmit a
patch either way.

>