Date: Tue, 16 May 2017 17:54:38 -0400 From: Matt Brown <matt@...tt.com> To: Peter Dolding <oiaohm@...il.com>, Kees Cook <keescook@...omium.org> Cc: 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: [PATCH v6 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN On 05/16/2017 05:43 PM, Peter Dolding wrote: > On Wed, May 17, 2017 at 12:28 AM, Kees Cook <keescook@...omium.org> wrote: >> 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). > > We cannot just keep on expanding CAP_SYS_ADMIN either. >> >>>> 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 I like but you have forgot the all important rule. The Linus > Rule. Existing applications must have a method work. > So modify applications binary is not way out of problem. > > Please note making CAP_SYS_ADMIN the only way to use TIOCSTI also > means setting CAP_SYS_ADMIN on all the existing applications to obey > the Linus Rule of not break userspace. So this is why the patch is > strictly no as this means elevating privilege of existing applications > and possibly opening up more security flaws. This feature is not required so it is not "making CAP_SYS_ADMIN the only way to use TIOCSTI". It defaults to no as to not break some existing programs that use it. > > Reality any patch like the one we are talking about due to the Linus > Rule and the security risk it will open up obey this it just be > rejected. There is another kind of way I will cover with Serge. > > Peter Dolding. > Matt
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