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From:   Kees Cook <keescook@google.com>
Date:   Thu, 19 Apr 2018 09:55:16 -0700
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Subject: Re: [PATCH 11/11] x86/pti: leave kernel text global for !PCID
To:     Dave Hansen <dave.hansen@linux.intel.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Juergen Gross <jgross@suse.com>, X86 ML <x86@kernel.org>,
        namit@vmware.com
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On Thu, Apr 19, 2018 at 9:02 AM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
> On 04/18/2018 05:11 PM, Kees Cook wrote:
>> On Fri, Apr 6, 2018 at 1:55 PM, Dave Hansen <dave.hansen@linux.intel.com> wrote:
>>> +/*
>>> + * For some configurations, map all of kernel text into the user page
>>> + * tables.  This reduces TLB misses, especially on non-PCID systems.
>>> + */
>>> +void pti_clone_kernel_text(void)
>>> +{
>>> +       unsigned long start = PFN_ALIGN(_text);
>>> +       unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);
>> I think this is too much set global: _end is after data, bss, and brk,
>> and all kinds of other stuff that could hold secrets. I think this
>> should match what mark_rodata_ro() is doing and use
>> __end_rodata_hpage_align. (And on i386, this should be maybe _etext.)
>
> Sounds reasonable to me.  This does assume that there are no secrets
> built into the kernel image, right?

It's hard to say, but I was trying to consider the basic threat model
of having your kernel image available to an attacker (i.e. a distro
kernel can be examined from packages, etc). In that case, the text and
rodata are readable through much more direct mechanisms. Everything
after rodata is run-time state, and should be excluded in the general
case.

I would expect more paranoid system builders to boot with "pti=on",
but perhaps we should disable Global under other specific CONFIGs, or
make a specific CONFIG for it that other options can select, probably.

-Kees

-- 
Kees Cook
Pixel Security