linux-kernel - Re: [PATCHv1, RFC 0/8] Boot-time switching between 4- and 5-level paging

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 25 May 2017 17:40:16 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv1, RFC 0/8] Boot-time switching between 4- and 5-level paging

On Thu, May 25, 2017 at 4:24 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, May 25, 2017 at 1:33 PM, Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
>> Here' my first attempt to bring boot-time between 4- and 5-level paging.
>> It looks not too terrible to me. I've expected it to be worse.
>
> If I read this right, you just made it a global on/off thing.
>
> May I suggest possibly a different model entirely? Can you make it a
> per-mm flag instead?
>
> And then we
>
>  (a) make all kthreads use the 4-level page tables
>
>  (b) which means that all the init code uses the 4-level page tables
>
>  (c) which means that all those checks for "start_secondary" etc can
> just go away, because those all run with 4-level page tables.
>
> Or is it just much too expensive to switch between 4-level and 5-level
> paging at run-time?
>

Even ignoring expensiveness, I'm not convinced it's practical.  AFAICT
you can't atomically switch the paging mode and CR3, so either you
need some magic page table with trampoline that works in both modes
(which is presumably doable with some trickery) or you need to flip
paging off.  Good luck if an NMI hits in the mean time.  There was
code like that once upon a time for EFI mixed mode, but it got deleted
due to triple-faults.

Doing this in switch_mm() sounds painful.