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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