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

This change “KVM: x86: Update cpuid properly when CR4.OSXAVE or CR4.PKE is changed” in Linux kernel is authored by Wei Huang <wei [at] redhat.com> on Tue May 1 09:49:54 2018 -0500.

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

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>
Cc: stable@vger.kernel.org
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit c4d2188.

There are 5 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 arch/x86/kvm/x86.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 59371de..e310327 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7985,6 +7985,7 @@ static int __set_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
 	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;
 	int ret = -EINVAL;
@@ -8023,8 +8024,10 @@ static int __set_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
 	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 | X86_CR4_PKE));
 	kvm_x86_ops->set_cr4(vcpu, sregs->cr4);
-	if (sregs->cr4 & (X86_CR4_OSXSAVE | X86_CR4_PKE))
+	if (cpuid_update_needed)
 	idx = srcu_read_lock(&vcpu->kvm->srcu);

The commit for this change in Linux stable tree is c4d2188 (patch).

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