linux-kernel - Re: [PATCH RFC 2/4] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET

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Date:   Mon, 8 Jan 2018 21:49:23 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Willy Tarreau <w@....eu>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH RFC 2/4] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET_NOPTI
 to enable/disable PTI

On Mon, 8 Jan 2018, Willy Tarreau wrote:
> On Mon, Jan 08, 2018 at 09:54:05AM -0800, Linus Torvalds wrote:
> 
> > So make the thing per-mm, and then at task switch time as you switch
> > mms, you set the bit in a percpu variable for testing at kernel entry.
> 
> I'll see how to do that, this is not yet 100% clear to me, I'm still
> discovering (this code has immensely changed since last time I *really*
> dug into it). So I suspect I'll have to set this variable in
> __switch_to() based on this other MM flag.

The right thing is probably the cpu entry area because that's what you can
access before switching CR3 if PTI is enabled for the task

Thanks,

	tglx