linux-kernel - [PATCH 3.16 348/366] KVM: x86: Update cpuid properly when CR4.OSXAVE or CR4.PKE is changed

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Message-ID: <lsq.1539530741.831180161@decadent.org.uk>
Date:   Sun, 14 Oct 2018 16:25:41 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Wei Huang" <wei@...hat.com>,
        "Radim Krčmář" <rkrcmar@...hat.com>,
        "Bandan Das" <bsd@...hat.com>
Subject: [PATCH 3.16 348/366] KVM: x86: Update cpuid properly when
 CR4.OSXAVE or CR4.PKE is changed

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Wei Huang <wei@...hat.com>

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@...hat.com>
Reviewed-by: Bandan Das <bsd@...hat.com>
Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
[bwh: Backported to 3.16: PKE is not supported]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kvm/x86.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6668,6 +6668,7 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct
 {
 	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
 	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);