linux-kernel - RE: [RFC PATCH v2 2/6] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET

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Date:   Fri, 12 Jan 2018 15:03:23 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Willy Tarreau' <w@....eu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "Brian Gerst" <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Ingo Molnar" <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kees Cook" <keescook@...omium.org>
Subject: RE: [RFC PATCH v2 2/6] x86/arch_prctl: add ARCH_GET_NOPTI and
 ARCH_SET_NOPTI to enable/disable PTI

From: Willy Tarreau
> Sent: 09 January 2018 12:56
> 
> This allows to report the current state of the PTI protection and to
> enable or disable it for the current process. The state change is only
> allowed if the mm is not shared (no threads have been created yet).
> 
> Setting the flag to disable the protection is subject to CAP_SYS_RAWIO.
> However it is possible to re-enable the protection without this privilege.

While a process with CAP_SYS_RAWIO can easily access kernel memory so
get little extra protection from PTI, you probably want to be able
to disable PTI without giving a process CAP_SYS_RAWIO.
No point leaving the door wide open.

If you are trying to find the size of the performance hit you also
need to be able to disable (and re-enable) PTI for all the current and
future threads of a given process.

	David