Date:   Wed, 3 May 2023 13:51:12 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Evgeniy Baskov <baskov@...ras.ru>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Peter Jones <pjones@...hat.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        Mario Limonciello <mario.limonciello@....com>,
        Kees Cook <keescook@...omium.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/6] efi/x86: Avoid legacy decompressor during EFI boot

On 5/3/23 12:44, Ard Biesheuvel wrote:
> On Tue, 2 May 2023 at 18:08, Tom Lendacky <thomas.lendacky@....com> wrote:
>>
>> On 5/2/23 08:39, Ard Biesheuvel wrote:
>>> On Tue, 2 May 2023 at 15:37, Tom Lendacky <thomas.lendacky@....com> wrote:
>>>>
>>>> On 4/24/23 11:57, Ard Biesheuvel wrote:
>>>>> This series is conceptually a combination of Evgeny's series [0] and
>>>>> mine [1], both of which attempt to make the early decompressor code more
>>>>> amenable to executing in the EFI environment with stricter handling of
>>>>> memory permissions.
>>>>>
>>>>> My series [1] implemented zboot for x86, by getting rid of the entire
>>>>> x86 decompressor, and replacing it with existing EFI code that does the
>>>>> same but in a generic way. The downside of this is that only EFI boot is
>>>>> supported, making it unviable for distros, which need to support BIOS
>>>>> boot and hybrid EFI boot modes that omit the EFI stub.
>>>>>
>>>>> Evgeny's series [0] adapted the entire decompressor code flow to allow
>>>>> it to execute in the EFI context as well as the bare metal context, and
>>>>> this involves changes to the 1:1 mapping code and the page fault
>>>>> handlers etc, none of which are really needed when doing EFI boot in the
>>>>> first place.
>>>>>
>>>>> So this series attempts to occupy the middle ground here: it makes
>>>>> minimal changes to the existing decompressor so some of it can be called
>>>>> from the EFI stub. Then, it reimplements the EFI boot flow to decompress
>>>>> the kernel and boot it directly, without relying on the trampoline code,
>>>>> page table code or page fault handling code. This allows us to get rid
>>>>> of quite a bit of unsavory EFI stub code, and replace it with two clear
>>>>> invocations of the EFI firmware APIs to clear NX restrictions from
>>>>> allocations that have been populated with executable code.
>>>>>
>>>>> The only code that is being reused is the decompression library itself,
>>>>> along with the minimal ELF parsing that is required to copy the ELF
>>>>> segments in place, and the relocation processing that fixes up absolute
>>>>> symbol references to refer to the correct virtual addresses.
>>>>>
>>>>> Note that some of Evgeny's changes to clean up the PE/COFF header
>>>>> generation will still be needed, but I've omitted those here for
>>>>> brevity.
>>>>
>>>> I tried booting an SEV and an SEV-ES guest using this and both failed to boot:
>>>>
>>>> EFI stub: WARNING: Decompression failed: Out of memory while allocating
>>>> z_stream
>>>>
>>>> I'll have to take a closer look as to why, but it might be a couple of
>>>> days before I can get to it.
>>>>
>>>
>>> Thanks Tom.
>>>
>>> The internal malloc() seems to be failing, which is often caused by
>>> BSS clearing problems. Could you elaborate a little bit on the boot
>>> environment you are using here?
>>
>> I'm using Qemu v7.2.1 as my VMM, Linux 6.3 with your series applied for my
>> host/hypervisor and guest kernel and the current OVMF tree built using
>> OvmfPkgX64.dsc.
>>
>> I was originally using the current merge window Linux, but moved to the
>> release version just to . With the release version SEV and SEV-ES still fail to
>> boot, but SEV actually #GPs now. And some of the register contents look
>> like encrypted data:
>>
>> ConvertPages: range 1000000 - 4FA1FFF covers multiple entries
>> !!!! X64 Exception Type - 0D(#GP - General Protection)  CPU Apic ID - 00000000 !!!!
>> ExceptionData - 0000000000000000
>> RIP  - 00000000597E71C1, CS  - 0000000000000038, RFLAGS - 0000000000210206
>> RAX  - 1FBA02A45943B920, RCX - 0000000000AF7009, RDX - A9DAE761B64A1F1B
>> RBX  - 1FBA02A45943B8C0, RSP - 000000007FD97320, RBP - 0000000002000000
>> RSI  - 0000000001000000, RDI - 1FBA02A45943DE68
>> R8   - 0000000003EF3C94, R9  - 0000000000000000, R10 - 000000007D7C6018
>> R11  - 0000000000000000, R12 - 0000000001000000, R13 - 00000000597EDD98
>> R14  - 0000000001000000, R15 - 000000007E0A5198
>> DS   - 0000000000000030, ES  - 0000000000000030, FS  - 0000000000000030
>> GS   - 0000000000000030, SS  - 0000000000000030
>> CR0  - 0000000080010033, CR2 - 0000000000000000, CR3 - 000000007FA01000
>> CR4  - 0000000000000668, CR8 - 0000000000000000
>> DR0  - 0000000000000000, DR1 - 0000000000000000, DR2 - 0000000000000000
>> DR3  - 0000000000000000, DR6 - 00000000FFFF0FF0, DR7 - 0000000000000400
>> GDTR - 000000007F7DC000 0000000000000047, LDTR - 0000000000000000
>> IDTR - 000000007F34C018 0000000000000FFF,   TR - 0000000000000000
>> FXSAVE_STATE - 000000007FD96F80
>> !!!! Find image based on IP(0x597E71C1) /root/kernels/ovmf-build-X64/Build/OvmfX64/DEBUG_GCC5/X64/MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeDxe/DEBUG/Variable
>> RuntimeDxe.dll (ImageBase=0000000000D4792C, EntryPoint=0000000000D50CC3) !!!!
>>
>> So, yes, probably an area of memory that was zeroes when mapped
>> unencrypted, but wasn't cleared after changing the mapping to
>> encrypted.
>>
> 
> Thanks.
> 
> It seems I was a bit naive and underestimated the amount of SEV
> related processing that goes on in the decompressor after the EFI stub
> has handed over. I will have to take some time and go through this,
> and decide whether there is a way we can share this code with the EFI
> stub without introducing yet another permutation that requires testing
> and maintenance.
> 
> Any suggestions on how to test this stuff is appreciated - does QEMU
> emulate any of this? Does consumer-level AMD hardware implement the
> pieces I'd need to run a SEV host with SNP support etc?

Unfortunately Qemu doesn't emulate any of this and SEV support, 
specifically the SEV firmware, isn't available on consumer-level parts, so 
you would need an EPYC part to do SEV.

Thanks,
Tom

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