linux-kernel - RE: [PATCH v2 2/2] x86/hyperv: fix invalid writes to MSRs during root partition kexec

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Date:   Thu, 27 Oct 2022 13:44:40 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
CC:     "stanislav.kinsburskiy@...il.com" <stanislav.kinsburskiy@...il.com>,
        "kumarpraveen@...ux.microsoft.com" <kumarpraveen@...ux.microsoft.com>,
        "mail@...rudhrb.com" <mail@...rudhrb.com>
Subject: RE: [PATCH v2 2/2] x86/hyperv: fix invalid writes to MSRs during root
 partition kexec

From: Anirudh Rayabharam <anrayabh@...ux.microsoft.com> Sent: Thursday, October 27, 2022 2:57 AM
> 
> hv_cleanup resets the hypercall page by setting the MSR to 0. However,

The function name is hyperv_cleanup(), not hv_cleanup().

> the root partition is not allowed to write to the GPA bits of the MSR.
> Instead, it uses the hypercall page provided by the MSR. Similar is the
> case with the reference TSC MSR.
> 
> Clear only the enable bit instead of zeroing the entire MSR to make
> the code valid for root partition too.
> 
> Signed-off-by: Anirudh Rayabharam <anrayabh@...ux.microsoft.com>
> ---
>  arch/x86/hyperv/hv_init.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 29774126e931..80fdfff9266c 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -537,6 +537,7 @@ void __init hyperv_init(void)
>  void hyperv_cleanup(void)
>  {
>  	union hv_x64_msr_hypercall_contents hypercall_msr;
> +	union hv_reference_tsc_msr tsc_msr;
> 
>  	unregister_syscore_ops(&hv_syscore_ops);
> 
> @@ -552,12 +553,14 @@ void hyperv_cleanup(void)
>  	hv_hypercall_pg = NULL;
> 
>  	/* Reset the hypercall page */
> -	hypercall_msr.as_uint64 = 0;
> -	wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> +	hypercall_msr.as_uint64 = hv_get_register(HV_X64_MSR_HYPERCALL);
> +	hypercall_msr.enable = 0;
> +	hv_set_register(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> 
>  	/* Reset the TSC page */
> -	hypercall_msr.as_uint64 = 0;
> -	wrmsrl(HV_X64_MSR_REFERENCE_TSC, hypercall_msr.as_uint64);
> +	tsc_msr.as_uint64 = hv_get_register(HV_X64_MSR_REFERENCE_TSC);
> +	tsc_msr.enable = 0;
> +	hv_set_register(HV_X64_MSR_REFERENCE_TSC, tsc_msr.as_uint64);
>  }
> 
>  void hyperv_report_panic(struct pt_regs *regs, long err, bool in_die)
> --
> 2.34.1

Modulo the nit in the commit message,

Reviewed-by: Michael Kelley <mikelley@...rosoft.com>