linux-kernel - Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

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Date:   Thu, 5 Jan 2017 10:01:21 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Garnier <thgarnie@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>, Dave Hansen <dave@...1.net>,
        Chen Yucong <slaoub@...il.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Juergen Gross <jgross@...e.com>,
        Richard Weinberger <richard@....at>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

On Thu, Jan 5, 2017 at 9:54 AM, Thomas Garnier <thgarnie@...gle.com> wrote:
> On Thu, Jan 5, 2017 at 9:51 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Wed, Jan 4, 2017 at 2:16 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
>>> Each processor holds a GDT in its per-cpu structure. The sgdt
>>> instruction gives the base address of the current GDT. This address can
>>> be used to bypass KASLR memory randomization. With another bug, an
>>> attacker could target other per-cpu structures or deduce the base of the
>>> main memory section (PAGE_OFFSET).
>>>
>>> In this change, a space is reserved at the end of the memory range
>>> available for KASLR memory randomization. The space is big enough to hold
>>> the maximum number of CPUs (as defined by setup_max_cpus). Each GDT is
>>> mapped at specific offset based on the target CPU. Note that if there is
>>> not enough space available, the GDTs are not remapped.
>>
>> Can we remap it read-only?  I.e. use PAGE_KERNEL_RO instead of
>> PAGE_KERNEL.  After all, the ability to modify the GDT is instant
>> root.
>
> That's my goal too. I started by doing a RO remap and got couple
> problems with hibernation. I can try again for the next iteration or
> delay it for another patch. I also need to look at KVM GDT usage, I am
> not familiar with it yet.

If you want a small adventure, I think a significant KVM-related
performance improvement is available.  Specifically, on VMX exits, the
GDT limit is hardwired to 0xffff (IIRC -- I could be remembering the
actual vaue wrong).  KVM does LGDT to fix it.

If we actually made the GDT have limit 0xffff (presumably by mapping
the zero page a few times to pad it out without wasting memory), then
we would avoid the LGDT.  LGDT is incredibly slow, so this would be a
big win. Want to see if you can make this work with your patch set?