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Date: Tue, 16 May 2017 10:48:18 -0500
From: "Serge E. Hallyn" <serge@...lyn.com>
To: Kees Cook <keescook@...omium.org>
Cc: Matt Brown <matt@...tt.com>, Peter Dolding <oiaohm@...il.com>,
Alan Cox <gnomes@...rguk.ukuu.org.uk>,
"Serge E. Hallyn" <serge@...lyn.com>,
Greg KH <gregkh@...uxfoundation.org>, Jiri Slaby <jslaby@...e.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Jann Horn <jannh@...gle.com>, James Morris <jmorris@...ei.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
linux-security-module <linux-security-module@...r.kernel.org>,
linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Re: [PATCH v6 0/2] security: tty: make
TIOCSTI ioctl require CAP_SYS_ADMIN
Quoting Kees Cook (keescook@...omium.org):
> On Tue, May 16, 2017 at 5:22 AM, Matt Brown <matt@...tt.com> wrote:
> > On 05/16/2017 05:01 AM, Peter Dolding wrote:
> >>>
> >>>
> >>> I could see a case being make for CAP_SYS_TTY_CONFIG. However I still
> >>> choose to do with CAP_SYS_ADMIN because it is already in use in the
> >>> TIOCSTI ioctl.
> >>>
> >> Matt Brown don't give me existing behaviour. CAP_SYS_ADMIN is
> >> overload. The documentation tells you that you are not to expand it
> >> and you openly admit you have.
> >>
> >
> > This is not true that I'm openly going against what the documentation
> > instructs. The part of the email chain where I show this got removed
> > somehow. Again I will refer to the capabilities man page that you
> > quoted.
> >
> > From http://man7.org/linux/man-pages/man7/capabilities.7.html
> >
> > "Don't choose CAP_SYS_ADMIN if you can possibly avoid it!
> > ...
> > The only new features that should be associated with CAP_SYS_ADMIN are
> > ones that closely match existing uses in that silo."
> >
> > My feature affects the TIOCSTI ioctl. The TIOCSTI ioctl already falls
> > under CAP_SYS_ADMIN, therefore I actually *am* following the
> > documentation.
>
> CAP_SYS_ADMIN is the right choice here, I agree with Matt: it is
> already in use for TIOCSTI. We can't trivially add new capabilities
> flags (see the various giant threads debating this, the most recently
> that I remember from the kernel lock-down series related to Secure
> Boot).
Consideer that if we use CAP_SYS_TTY_CONFIG now, then any applications
which are currently being given CAP_SYS_ADMIN would need to be updated
with a second capability. Not acceptable. Even when we split up
CAP_SYSLOG, we took care to avoid that (by having the original capability
also suffice, so either capability worked).
> >> I fact this usage of TIOCSTI I personally think should require two
> >> capabilities flags set. CAP_SYS_ADMIN section left as it is at this
> >> stage. With TIOSCTI stuck behind another capability.
> >>
> >> If you had added a new capability flag you could set file capabilities
> >> on any of the old applications depending on the now secured behaviour.
>
> If we're adjusting applications, they should be made to avoid TIOSCTI
> completely. This looks to me a lot like the symlink restrictions: yes,
> userspace should be fixed to the do the right thing, but why not
> provide support to userspace to avoid the problem entirely?
>
> -Kees
>
> --
> Kees Cook
> Pixel Security
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