linux-kernel - Re: [RFC kvmtool 18/31] arm64: Populate initial realm contents

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Date:   Thu, 2 Mar 2023 14:06:37 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Piotr Sawicki <piotr.sawickas@...il.com>
Cc:     Alexandru Elisei <alexandru.elisei@....com>,
        Andrew Jones <andrew.jones@...ux.dev>,
        Christoffer Dall <christoffer.dall@....com>,
        Fuad Tabba <tabba@...gle.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Joey Gouly <Joey.Gouly@....com>, Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Steven Price <steven.price@....com>,
        Thomas Huth <thuth@...hat.com>, Will Deacon <will@...nel.org>,
        Zenghui Yu <yuzenghui@...wei.com>, linux-coco@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC kvmtool 18/31] arm64: Populate initial realm contents

Hi Piotr

On 02/03/2023 14:03, Piotr Sawicki wrote:
> Hi,
> 
>> From: Alexandru Elisei <alexandru.elisei@....com>
>>
>> Populate the realm memory with the initial contents, which include
>> the device tree blob, the kernel image, and initrd, if specified,
>> or the firmware image.
>>
>> Populating an image in the realm involves two steps:
>>   a) Mark the IPA area as RAM - INIT_IPA_REALM
>>   b) Load the contents into the IPA - POPULATE_REALM
>>
>> Wherever we know the actual size of an image in memory, we make
>> sure the "memory area" is initialised to RAM.
>> e.g., Linux kernel image size from the header which includes the bss etc.
>> The "file size" on disk for the Linux image is much smaller.
>> We mark the region of size Image.header.size as RAM (a), from the kernel
>> load address. And load the Image file into the memory (b) above.
>> At the moment we only detect the Arm64 Linux Image header format.
>>
>> Since we're already touching the code that copies the
>> initrd in guest memory, let's do a bit of cleaning and remove a
>> useless local variable.
>>
>> Signed-off-by: Alexandru Elisei <alexandru.elisei@....com>
>> [ Make sure the Linux kernel image area is marked as RAM ]
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>


>> diff --git a/arm/kvm.c b/arm/kvm.c
>> index acb627b2..57c5b5f7 100644
>> --- a/arm/kvm.c
>> +++ b/arm/kvm.c
>> @@ -6,6 +6,7 @@
>>   #include "kvm/fdt.h"
>>   #include "arm-common/gic.h"
>> +#include <asm/realm.h>
>>   #include <sys/resource.h>
>> @@ -167,6 +168,9 @@ bool kvm__arch_load_kernel_image(struct kvm *kvm, 
>> int fd_kernel, int fd_initrd,
>>       pr_debug("Loaded kernel to 0x%llx (%llu bytes)",
>>            kvm->arch.kern_guest_start, kvm->arch.kern_size);
> 
> 
> I've noticed that multiple calling of the measurement test from the 
> kvm-unit-tests suite results in different Realm Initial Measurements, 
> although the kernel image is always the same.
> 
> After short investigation, I've found that the RIM starts being 
> different while populating the last 4kB chunk of the kernel image.
> The issue occurs when the image size is not aligned to the page size (4kB).
> 
> After zeroing the unused area of the last chunk, the measurements become 
> repeatable.
> 

That is a good point. We could memset() the remaining bits of the 4K 
page to 0. I will make this change.

Suzuki