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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 11 Jan 2018 10:31:58 -0800
Message-ID: <CA+55aFygqFbpgc6UaPqn9KiPVj0srd5wyPNmFZcef+gG_v+QBw@mail.gmail.com>
Subject: Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Willy Tarreau <w@1wt.eu>, Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kees Cook <keescook@chromium.org>
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Sender: linux-kernel-owner@vger.kernel.org

On Thu, Jan 11, 2018 at 10:15 AM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
>
> Well, on the bright side, we don't need IPIs when _removing_ NX.  We can
> just handle those like a spurious fault.

I think I agree.

> 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.

Actually, I really don't think we should even allow "re-enable PTI".

The only thing that re-enables PTI is a completely new page table,
notably "execve()".

And I think that is when the "NOW" vs "NEXT" *may* make sense. Not for
enabling PTI, but if we want to have a "disable PTI", I think it
should act on the next execve().

And one reason I think we want that behavior is that once you've
disabled PTI, I don't think the double page tables would necessarily
even exist, and I don't think we should try to re-populate them. A
noPTI process might simply *have* just the single page table.

That wouldn't be the first implementation, but I think the interface
should be designed for that kind of thing in mind, where nopti really
means "stop doing two page tables for this process". And that may make
it *impossible* to re-enable PTI for this process, simply because we
don't have the required double-page PGD allocation at all.

              Linus