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

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Date:   Wed, 1 Nov 2017 09:06:42 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "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,
        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


* Andy Lutomirski <luto@...nel.org> wrote:

> 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.

Yes, let's try that.

Thanks,

	Ingo