linux-kernel - Re: [PATCH] LDT improvements

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Date:   Fri, 8 Dec 2017 09:42:52 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>, 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, Dec 8, 2017 at 9:37 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 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.

But if you unmap ldt and then map something else at the same VA, you're root.