KVM: SVM: Disallow EFER.LMSLE when not supported by hardware

Modern AMD CPUs do not support segment limit checks in 64-bit mode
(i.e. EFER.LMSLE must be zero). Do not allow a guest to set EFER.LMSLE
on a CPU that requires the bit to be zero.

For backwards compatibility, allow EFER.LMSLE to be set on CPUs that
support segment limit checks in 64-bit mode, even though KVM's
implementation of the feature is incomplete (e.g. KVM's emulator does
not enforce segment limits in 64-bit mode).

Fixes: eec4b140c924 ("KVM: SVM: Allow EFER.LMSLE to be set with nested svm")
Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Nikunj A Dadhania <nikunj@amd.com>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://lore.kernel.org/r/20251001001529.1119031-3-jmattson@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 153c12dbf3eb15e521fd75468137573f7742f199..dadb562bd4b9d81ef11dbedd297092dfc114f334 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5320,7 +5320,9 @@ static __init int svm_hardware_setup(void)
 
        if (nested) {
                pr_info("Nested Virtualization enabled\n");
-               kvm_enable_efer_bits(EFER_SVME | EFER_LMSLE);
+               kvm_enable_efer_bits(EFER_SVME);
+               if (!boot_cpu_has(X86_FEATURE_EFER_LMSLE_MBZ))
+                       kvm_enable_efer_bits(EFER_LMSLE);
 
                r = nested_svm_init_msrpm_merge_offsets();
                if (r)