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 18:30:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org,
        x86@...nel.org, tglx@...utronix.de, gnomes@...rguk.ukuu.org.uk,
        torvalds@...ux-foundation.org, Ingo Molnar <mingo@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH RFC 3/4] x86/pti: don't mark the user PGD with _PAGE_NX.

On Mon, Jan 08, 2018 at 09:23:49AM -0800, Dave Hansen wrote:
> On 01/08/2018 09:17 AM, Willy Tarreau wrote:
> >> I think the prctl() should apply to an entire process, not to a thread.
> > 
> > As I mentionned in another mail, I didn't know how to do it, even less
> > how to do it fast enough so that we didn't add more cycles to the syscall
> > code.
> 
> You can _implement_ it with a task thread if you want.  Just spray it
> across all threads at the prctl()-time instead of a single thread.
> It'll take a wee bit of locking.
> 
> I just don't think the API should apply to a single thread.

It is surprisingly hard to find all tasks that share an mm. Finding all
threads in a threadgroup is easy, but we have CLONE_THREAD and CLONE_VM
as separate bits.

In any case, aside from that, setting this remotely is indeed
'intersting'.