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

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Message-ID: <57039ac1-efe2-2f97-386f-dab0b90f64a5@intel.com>
Date:   Mon, 8 Jan 2018 09:03:36 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Cc:     tglx@...utronix.de, 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 08:12 AM, 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 don't like this.

I think the prctl() should apply to an entire process, not to a thread.
If it applies to a process, you can unpoison the PGD.  I even had code
to do this in an earlier version of the (whole system) runtime PTI
on/off stuff.

Why are you even posting half-baked hacks like this now?  Is there
something super-pressing about this set that we need to lock in a new
ABI now?