Re: [PATCH 0/6] efi/x86: Avoid legacy decompressor during EFI boot

[Tom Lendacky <thomas.lendacky@amd.com>]

On 5/2/23 08:39, Ard Biesheuvel wrote:
> On Tue, 2 May 2023 at 15:37, Tom Lendacky <thomas.lendacky@amd.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,
Tom