linux-kernel - [PATCH 1/2] x86/hyperv: fix invalid writes to MSRs during root partition kexec

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Message-Id: <20221026134715.1438789-2-anrayabh@linux.microsoft.com>
Date:   Wed, 26 Oct 2022 19:17:14 +0530
From:   Anirudh Rayabharam <anrayabh@...ux.microsoft.com>
To:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, daniel.lezcano@...aro.org,
        Arnd Bergmann <arnd@...db.de>, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Cc:     Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        kumarpraveen@...ux.microsoft.com, mail@...rudhrb.com
Subject: [PATCH 1/2] x86/hyperv: fix invalid writes to MSRs during root partition kexec

hv_cleanup resets the hypercall page by setting the MSR to 0. However,
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 | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 29774126e931..76ff63d69461 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;
+	u64 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;
+	rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
+	hypercall_msr.enable = 0;
 	wrmsrl(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);
+	rdmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr);
+	tsc_msr &= ~BIT_ULL(0);
+	wrmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr);
 }
 
 void hyperv_report_panic(struct pt_regs *regs, long err, bool in_die)
-- 
2.34.1