KVM: x86: Update cpuid properly when CR4.OSXAVE or CR4.PKE is changed

commit c4d2188206bafa177ea58e9a25b952baa0bf7712 upstream.

The CPUID bits of OSXSAVE (function=0x1) and OSPKE (func=0x7, leaf=0x0)
allows user apps to detect if OS has set CR4.OSXSAVE or CR4.PKE. KVM is
supposed to update these CPUID bits when CR4 is updated. Current KVM
code doesn't handle some special cases when updates come from emulator.
Here is one example:

  Step 1: guest boots
  Step 2: guest OS enables XSAVE ==> CR4.OSXSAVE=1 and CPUID.OSXSAVE=1
  Step 3: guest hot reboot ==> QEMU reset CR4 to 0, but CPUID.OSXAVE==1
  Step 4: guest os checks CPUID.OSXAVE, detects 1, then executes xgetbv

Step 4 above will cause an #UD and guest crash because guest OS hasn't
turned on OSXAVE yet. This patch solves the problem by comparing the the
old_cr4 with cr4. If the related bits have been changed,
kvm_update_cpuid() needs to be called.

Signed-off-by: Wei Huang <wei@redhat.com>
Reviewed-by: Bandan Das <bsd@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
[bwh: Backported to 3.16: PKE is not supported]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 064c56cacac763bfd1a50a0d73c31bb8e1b71f8b..47e5aff3bf0451ab001967d200d46a26fb61e1b0 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6668,6 +6668,7 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
 {
        struct msr_data apic_base_msr;
        int mmu_reset_needed = 0;
+       int cpuid_update_needed = 0;
        int pending_vec, max_bits, idx;
        struct desc_ptr dt;
 
@@ -6702,8 +6703,10 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
        vcpu->arch.cr0 = sregs->cr0;
 
        mmu_reset_needed |= kvm_read_cr4(vcpu) != sregs->cr4;
+       cpuid_update_needed |= ((kvm_read_cr4(vcpu) ^ sregs->cr4) &
+                               X86_CR4_OSXSAVE);
        kvm_x86_ops->set_cr4(vcpu, sregs->cr4);
-       if (sregs->cr4 & X86_CR4_OSXSAVE)
+       if (cpuid_update_needed)
                kvm_update_cpuid(vcpu);
 
        idx = srcu_read_lock(&vcpu->kvm->srcu);