linux-kernel - Re: [PATCH 02/23] x86, kaiser: do not set _PAGE_USER for init

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Date:   Thu, 2 Nov 2017 11:24:59 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at, Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 02/23] x86, kaiser: do not set _PAGE_USER for init_mm page tables

On Thu, Nov 2, 2017 at 11:19 AM, Andy Lutomirski <luto@...capital.net> wrote:
>
> We'd have to force NONE, and Linus won't like it.

Oh, I think it's fine for the kaiser case.

I am not convinced anybody will actually use it, but if you do use it,
I suspect "the legacy vsyscall page no longer works" is the least of
your worries.

That said, I think you can keep emulation, and just make it
unreadable. That will keep legacy binaries still working, and will
break a much smaller subset. So we have four cases:

 - native
 - read-only emulation
 - unreadable emulation
 - none

and kaiser triggering that unreadable case sounds like the option
least likely to cause trouble. vsyscalls still work, anybody who tries
to trace them and look at the code will not.

              Linus