linux-kernel - Re: Lazy FPU restoration / moving kernel_fpu

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]

Message-ID: <CAHmME9qhmSVNBq_GnkYrGU-PPEX9Fk0yiGktsCeEXo+LTB9KPg@mail.gmail.com>
Date:   Fri, 15 Jun 2018 22:33:47 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Andrew Lutomirski <luto@...nel.org>
Cc:     riel@...riel.com, dave.hansen@...ux.intel.com,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>
Subject: Re: Lazy FPU restoration / moving kernel_fpu_end() to context switch

On Fri, Jun 15, 2018 at 8:32 PM Andy Lutomirski <luto@...nel.org> wrote:
> quite in the form you imagined.  The idea that we've tossed around is
> to restore FPU state on return to user mode.  Roughly, we'd introduce
> a new thread flag TIF_FPU_UNLOADED (name TBD).
> prepare_exit_to_usermode() would notice this flag, copy the fpstate to
> fpregs, and clear the flag.  (Or maybe exit_to_usermode_loop() -- No
> one has quite thought it through, but I think it should be outside the
> loop.)  We'd update all the FPU accessors to understand the flag.

Yes! This is exactly what I was thinking. Then those calls to begin()
and end() could be placed as close to the actual FPU usage as
possible.