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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Tianyu Lan <ltykernel@gmail.com>, kys@microsoft.com,
        haiyangz@microsoft.com, wei.liu@kernel.org, decui@microsoft.com,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com,
        daniel.lezcano@linaro.org, arnd@arndb.de,
        michael.h.kelley@microsoft.com
Cc:     Tianyu Lan <tiala@microsoft.com>, linux-arch@vger.kernel.org,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/9] x86/hyperv: Mark Hyper-V vp assist page unencrypted
 in SEV-SNP enlightened guest
In-Reply-To: <20230601151624.1757616-4-ltykernel@gmail.com>
References: <20230601151624.1757616-1-ltykernel@gmail.com>
 <20230601151624.1757616-4-ltykernel@gmail.com>
Date:   Mon, 05 Jun 2023 14:13:29 +0200
Message-ID: <873536ksye.fsf@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
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Tianyu Lan <ltykernel@gmail.com> writes:

> From: Tianyu Lan <tiala@microsoft.com>
>
> hv vp assist page needs to be shared between SEV-SNP guest and Hyper-V.
> So mark the page unencrypted in the SEV-SNP guest.
>
> Signed-off-by: Tianyu Lan <tiala@microsoft.com>
> ---
>  arch/x86/hyperv/hv_init.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index b4a2327c823b..331b855314b7 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -18,6 +18,7 @@
>  #include <asm/hyperv-tlfs.h>
>  #include <asm/mshyperv.h>
>  #include <asm/idtentry.h>
> +#include <asm/set_memory.h>
>  #include <linux/kexec.h>
>  #include <linux/version.h>
>  #include <linux/vmalloc.h>
> @@ -113,6 +114,11 @@ static int hv_cpu_init(unsigned int cpu)
>  
>  	}
>  	if (!WARN_ON(!(*hvp))) {
> +		if (hv_isolation_type_en_snp()) {
> +			WARN_ON_ONCE(set_memory_decrypted((unsigned long)(*hvp), 1));
> +			memset(*hvp, 0, PAGE_SIZE);
> +		}

Why do we need to set the page as decrypted here and not when we
allocate the page (a few lines above)? And why do we need to clear it
_after_ we made it decrypted? In case we care about not leaking the
stale content to the hypervisor, we should've cleared it _before_, but
the bigger problem I see is that memset() is problemmatic e.g. for KVM
which uses enlightened VMCS. You put a CPU offline and then back online
and this path will be taken. Clearing VP assist page will likely brake
things. (AFAIU SEV-SNP Hyper-V guests don't expose SVM yet so the
problem is likely theoretical only, but still).

> +
>  		msr.enable = 1;
>  		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>  	}

-- 
Vitaly