linux-kernel - Re: [PATCH 12/23] x86, kaiser: map dynamically-allocated LDTs

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Message-ID: <CALCETrVbGHJUeZP2X36s-gUcEywpv_uuAwZRVAJWL5U8DijPkQ@mail.gmail.com>
Date:   Wed, 1 Nov 2017 01:00:54 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     "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@...k.tugraz.at,
        michael.schwarz@...k.tugraz.at,
        Andrew Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 12/23] x86, kaiser: map dynamically-allocated LDTs

On Tue, Oct 31, 2017 at 3:32 PM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
>
> Normally, a process just has a NULL mm->context.ldt.  But, we
> have a syscall for a process to set a new one.  If a process does
> that, we need to map the new LDT.
>
> The original KAISER patch missed this case.

Tglx suggested that we instead increase the padding at the top of the
user address space from 4k to 64k and put the LDT there.  This is a
slight ABI break, but I'd be rather surprised if anything noticed,
especially because the randomized vdso currently regularly lands there
(IIRC), so any user code that explicitly uses those 60k already
collides with the vdso.

I can make this happen.