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 09:23:49 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Willy Tarreau <w@....eu>
Cc:     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>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        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 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.