linux-hardening - Re: [PATCH 2/2] selinux: add capability checks for TIOCSTI ioctl

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Message-ID: <20250624.3bb75890f0b0@gnoack.org>
Date: Tue, 24 Jun 2025 22:58:18 +0200
From: Günther Noack <gnoack3000@...il.com>
To: Paul Moore <paul@...l-moore.com>
Cc: Stephen Smalley <stephen.smalley.work@...il.com>, xandfury@...il.com,
	Shuah Khan <shuah@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Ondrej Mosnacek <omosnace@...hat.com>, linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org, llvm@...ts.linux.dev,
	selinux@...r.kernel.org, kees@...nel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH 2/2] selinux: add capability checks for TIOCSTI ioctl

On Mon, Jun 23, 2025 at 11:15:39AM -0400, Paul Moore wrote:
> On Mon, Jun 23, 2025 at 8:39 AM Stephen Smalley
> <stephen.smalley.work@...il.com> wrote:
> > On Sun, Jun 22, 2025 at 9:41 PM Abhinav Saxena via B4 Relay
> > <devnull+xandfury.gmail.com@...nel.org> wrote:
> > > --- a/security/selinux/hooks.c
> > > +++ b/security/selinux/hooks.c
> > > @@ -3847,6 +3847,12 @@ static int selinux_file_ioctl(struct file *file, unsigned int cmd,
> > >                                             CAP_OPT_NONE, true);
> > >                 break;
> > >
> > > +       case TIOCSTI:
> > > +               if (!file_ns_capable(file, &init_user_ns, CAP_SYS_ADMIN) ||
> > > +                   !capable(CAP_SYS_ADMIN))
> > > +                       error = -EPERM;
> > > +               break;
> > > +
> >
> > So, aside from what I said previously, this also will break any
> > existing policies currently controlling TIOCSTI
> > via the selinux ioctl checking in the default case, so at the very
> > least, this would need to be gated by a new
> > SELinux policy capability for compatibility purposes. But I'm still
> > unconvinced that this is the right approach.
> 
> I want to add my voice to the other comments that adding these
> capability checks to the SELinux code and not the main TIOCSTI kernel
> code is not an approach we want to support.  Beyond that, as others
> have already pointed out, I think some additional inspection and
> testing is needed to ensure that the additional capability checks do
> not break existing, valid use cases.

+1 from me as well.

If the perceived problem is in core TTY logic, but the proposed fix is
in SELinux, it only addresses a fraction of the install base, as not
all machines use SELinux.

Also, it's not clear to me why the perceived problem of FD-passsing
with SCM_RIGHTS is a problem at all.  If a CAP_SYS_ADMIN process
accepts FDs over SCM_RIGHTS, it is the responsibility of that process
not to do unjustified privileged operations with these FDs, on behalf
of other, less privileged, processes.

In the more classic attack scenarios (as described in a series of CVEs
[1]) the process who had the FD first is normally the more privileged
one, for for those ones, the existing CAP_SYS_ADMIN check seems fine.

—Günther

[1] https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=TIOCSTI

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