linux-kernel - Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when pti

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Date:   Thu, 11 Jan 2018 10:15:03 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Willy Tarreau <w@....eu>, Andy Lutomirski <luto@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set

On 01/11/2018 10:05 AM, Willy Tarreau wrote:
>> That's exactly what it does for now, but Linus doesn't like it at all.
>> So I'll switch it back to per-mm + per-CPU variable. Well he has a valid
>> point regarding the pgd and _PAGE_NX setting. point Now at least we know
>> the change is minimal if we have a good reason for doing differently
>> later.
> Yuck, I hate this.  Are you really going to wire it up complete with
> all the IPIs needed to get the thing synced up right?

Well, on the bright side, we don't need IPIs when _removing_ NX.  We can
just handle those like a spurious fault.

But, when re-enabling it, we need all the TLB flushing for all the CPUs
that have run with the un-NX'd page tables.  I guess that's the one
benefit if you come up with an API that only allows "disable PTI" while
a task is running but leaves execve()/fork() as places that implicitly
reenable PTI.