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Subject: Re: [PATCH 23/62] x86/idt: Move IDT to data segment
To:     Andy Lutomirski <luto@amacapital.net>,
        Joerg Roedel <joro@8bytes.org>
Cc:     Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Jiri Slaby <jslaby@suse.cz>,
        Dan Williams <dan.j.williams@intel.com>,
        Tom Lendacky <Thomas.Lendacky@amd.com>,
        Kees Cook <keescook@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Joerg Roedel <jroedel@suse.de>
References: <20200212115516.GE20066@8bytes.org>
 <EEAC8672-C98F-45D0-9F2D-0802516C3908@amacapital.net>
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Date:   Wed, 12 Feb 2020 17:28:21 +0100
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On 12.02.20 17:23, Andy Lutomirski wrote:
> 
> 
>> On Feb 12, 2020, at 3:55 AM, Joerg Roedel <joro@8bytes.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@8bytes.org> wrote:
>>>>
>>>> From: Joerg Roedel <jroedel@suse.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.

Xen-PV is clearing BSS as the very first action.


Juergen