linux-kernel - Re: [PATCH] LDT improvements

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Date:   Fri, 8 Dec 2017 18:37:59 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] LDT improvements

On Fri, 8 Dec 2017, Andy Lutomirski wrote:
> Can we take a step back here?  I think there are four vaguely sane
> ways to make the LDT work:
> 
> 1. The way it is right now -- in vmalloc space.  The only real
> downside is that it requires exposing that part of vmalloc space in
> the user tables, which is a bit gross.
> 
> 2. In some fixmap-like space, which is what my patch does, albeit
> buggily.  This requires a PGD that we treat as per-mm, not per-cpu,
> but that's not so bad.
> 
> 3. In one of the user PGDs but above TASK_SIZE_MAX.  This is
> functionally almost identical to #2, except that there's more concern
> about exploits that write past TASK_SIZE_MAX.
> 
> 4. In an actual vma.  I don't see the benefit of doing this at all --
> it's just like #2 except way more error prone.  Hell, you have to make
> sure that you can't munmap or mremap it, which isn't a consideration
> at all with the other choices.

Why? You can unmap vdso or uprobe or whatever VMAs and you will simply
die. You get what you asked for.

Thanks,

	tglx