linux-kernel - Re: [PATCH 23/62] x86/idt: Move IDT to data segment

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Message-Id: <EEAC8672-C98F-45D0-9F2D-0802516C3908@amacapital.net>
Date:   Wed, 12 Feb 2020 08:23:54 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        Juergen Gross <JGross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 23/62] x86/idt: Move IDT to data segment



> On Feb 12, 2020, at 3:55 AM, Joerg Roedel <joro@...tes.org> wrote:
> 
> On Tue, Feb 11, 2020 at 02:41:25PM -0800, Andy Lutomirski wrote:
>>> On Tue, Feb 11, 2020 at 5:53 AM Joerg Roedel <joro@...tes.org> wrote:
>>> 
>>> From: Joerg Roedel <jroedel@...e.de>
>>> 
>>> With SEV-ES, exception handling is needed very early, even before the
>>> kernel has cleared the bss segment. In order to prevent clearing the
>>> currently used IDT, move the IDT to the data segment.
>> 
>> Ugh.  At the very least this needs a comment in the code.
> 
> Yes, right, added a comment for that.
> 
>> I had a patch to fix the kernel ELF loader to clear BSS, which would
>> fix this problem once and for all, but it didn't work due to the messy
>> way that the decompressor handles memory.  I never got around to
>> fixing this, sadly.
> 
> Aren't there other ways of booting (Xen-PV?) which don't use the kernel
> ELF loader?

Dunno. I would hope the any sane loader would clear BSS before executing anything. This isn’t currently the case, though. Oh well.

> 
> Regards,
> 
>    Joerg