2022-02-23 15:28:53

by Ondrej Mosnacek

[permalink] [raw]
Subject: Re: [PATCH V2] security/selinux: Always allow FIOCLEX and FIONCLEX

On Wed, Feb 23, 2022 at 12:58 PM Richard Haines
<[email protected]> wrote:
> On Tue, 2022-02-22 at 18:28 -0500, Paul Moore wrote:
> > On Mon, Feb 21, 2022 at 8:15 AM Richard Haines
> > <[email protected]> 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 <[email protected]>
> > > Signed-off-by: Richard Haines <[email protected]>
> > > ---
> > > V2 Change: Control via a policy capability. See this thread for
> > > discussion:
> > > https://lore.kernel.org/selinux/[email protected]om/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.

That one seems to be implemented only by some filesystems. Is there
any more generic one we could use?

>
> >
> > > 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.

I agree with Paul's suggestion. Better to duplicate the simple call
than to complicate the code flow.

--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.