linux-kernel - Re: [PATCH RFC 3/4] x86/pti: don't mark the user PGD with _PAGE

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Date:   Mon, 8 Jan 2018 09:28:28 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>, Willy Tarreau <w@....eu>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        gnomes@...rguk.ukuu.org.uk, torvalds@...ux-foundation.org
Subject: Re: [PATCH RFC 3/4] x86/pti: don't mark the user PGD with _PAGE_NX.

On 01/08/2018 09:05 AM, Thomas Gleixner wrote:
> On Mon, 8 Jan 2018, Willy Tarreau wrote:
>> Since we're going to keep running on the same PGD when returning to
>> userspace for certain performance-critical tasks, we'll need the user
>> pages to be executable. So this code disables the extra protection
>> that was added consisting in marking user pages _PAGE_NX so that this
>> pgd remains usable for userspace.
>>
>> Note: it isn't necessarily the best approach, but one way or another
>>       if we want to be able to return to userspace from the kernel,
>>       we'll have to have this executable anyway. Another approach
>>       might consist in using another pgd for userland+kernel but
>>       the current core really looks like an extra careful measure
>>       to catch early bugs if any.
> 
> I surely want to keep that as a safety measure. The entry code is simple to
> get wrong and running with the wrong pagetables by a silly mistake and
> thereby undoing the protection is surely not what we want.
> 
> Need to find a free time slot to think about that.

This does get immensely easier if we choose a mode at exec() (or fork()
even) and never change it.  The prctl() _could_ just be a flag to tell
what your children should do.