linux-kernel - [PATCH AUTOSEL for 4.9 117/281] KVM: nVMX: Fix handling of lmsw instruction

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Message-ID: <20180319155742.13731-117-alexander.levin@microsoft.com>
Date:   Mon, 19 Mar 2018 16:00:12 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     Jan H. Schönherr <jschoenh@...zon.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 4.9 117/281] KVM: nVMX: Fix handling of lmsw
 instruction

From: Jan H. Schönherr <jschoenh@...zon.de>

[ Upstream commit e1d39b17e044e8ae819827810d87d809ba5f58c0 ]

The decision whether or not to exit from L2 to L1 on an lmsw instruction is
based on bogus values: instead of using the information encoded within the
exit qualification, it uses the data also used for the mov-to-cr
instruction, which boils down to using whatever is in %eax at that point.

Use the correct values instead.

Without this fix, an L1 may not get notified when a 32-bit Linux L2
switches its secondary CPUs to protected mode; the L1 is only notified on
the next modification of CR0. This short time window poses a problem, when
there is some other reason to exit to L1 in between. Then, L2 will be
resumed in real mode and chaos ensues.

Signed-off-by: Jan H. Schönherr <jschoenh@...zon.de>
Reviewed-by: Wanpeng Li <wanpeng.li@...mail.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 arch/x86/kvm/vmx.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 0f3bb4632310..df7bf580c899 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -7917,11 +7917,13 @@ static bool nested_vmx_exit_handled_cr(struct kvm_vcpu *vcpu,
 {
 	unsigned long exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
 	int cr = exit_qualification & 15;
-	int reg = (exit_qualification >> 8) & 15;
-	unsigned long val = kvm_register_readl(vcpu, reg);
+	int reg;
+	unsigned long val;
 
 	switch ((exit_qualification >> 4) & 3) {
 	case 0: /* mov to cr */
+		reg = (exit_qualification >> 8) & 15;
+		val = kvm_register_readl(vcpu, reg);
 		switch (cr) {
 		case 0:
 			if (vmcs12->cr0_guest_host_mask &
@@ -7976,6 +7978,7 @@ static bool nested_vmx_exit_handled_cr(struct kvm_vcpu *vcpu,
 		 * lmsw can change bits 1..3 of cr0, and only set bit 0 of
 		 * cr0. Other attempted changes are ignored, with no exit.
 		 */
+		val = (exit_qualification >> LMSW_SOURCE_DATA_SHIFT) & 0x0f;
 		if (vmcs12->cr0_guest_host_mask & 0xe &
 		    (val ^ vmcs12->cr0_read_shadow))
 			return true;
-- 
2.14.1