[PATCH 7.2.y 7/7] KVM: x86: Always initialize EFER reserved bits on vendor initialization

[Sasha Levin <sashal@kernel.org>]

From: Yosry Ahmed <yosry@kernel.org>

[ Upstream commit a4b8acd428ba3f03952ef309d7f438dbc357557b ]

EFER reserved bits are statically initialized, and do not reset if a
vendor module is re-loaded. For example, loading kvm_amd with nested=1
removes EFER.SVME (and potentially EFER.LMSLE) from the reserved bits.
Reloading kvm_amd with nested=0 does not add them back, allowing
userspace to set EFER.SVME with nested=0.

Re-initializing EFER reserved bits before configuring them on vendor
initialization.

Cc: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260713181020.2735367-4-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/kvm/msrs.c | 10 ++++++++--
 arch/x86/kvm/msrs.h |  1 +
 arch/x86/kvm/x86.c  |  2 ++
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/msrs.c b/arch/x86/kvm/msrs.c
index 87372113dc0f9..eb6791744303d 100644
--- a/arch/x86/kvm/msrs.c
+++ b/arch/x86/kvm/msrs.c
@@ -19,8 +19,7 @@ bool __read_mostly report_ignored_msrs = true;
 module_param(report_ignored_msrs, bool, 0644);
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(report_ignored_msrs);
 
-/* Enable syscall by default because its emulated by KVM */
-static u64 __read_mostly efer_reserved_bits = ~((u64)EFER_SCE);
+static u64 __read_mostly efer_reserved_bits;
 
 #define MAX_IO_MSRS 256
 
@@ -637,6 +636,13 @@ static int set_efer(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	return 0;
 }
 
+void kvm_init_efer_bits(void)
+{
+	/* Enable syscall by default because its emulated by KVM */
+	efer_reserved_bits = ~((u64)EFER_SCE);
+}
+EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_init_efer_bits);
+
 void kvm_enable_efer_bits(u64 mask)
 {
        efer_reserved_bits &= ~mask;
diff --git a/arch/x86/kvm/msrs.h b/arch/x86/kvm/msrs.h
index 09f1098b67846..adb2c03af0c0e 100644
--- a/arch/x86/kvm/msrs.h
+++ b/arch/x86/kvm/msrs.h
@@ -56,6 +56,7 @@ int kvm_get_set_one_reg(struct kvm_vcpu *vcpu, unsigned int ioctl,
 int kvm_get_reg_list(struct kvm_vcpu *vcpu,
 		     struct kvm_reg_list __user *user_list);
 
+void kvm_init_efer_bits(void);
 void kvm_enable_efer_bits(u64);
 bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer);
 int kvm_emulate_msr_read(struct kvm_vcpu *vcpu, u32 index, u64 *data);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 796fd7fafdb85..ee249ac15e200 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7304,6 +7304,8 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_setup_xss_caps);
 
 static void kvm_setup_efer_caps(void)
 {
+	kvm_init_efer_bits();
+
 	if (kvm_cpu_cap_has(X86_FEATURE_LM))
 		kvm_enable_efer_bits(EFER_LME | EFER_LMA);
 
-- 
2.53.0