KVM: x86: use kvm_set_cr3/cr4 in ioctl_set_sregs

Marcelo Tosatti
2009-04-15T22:11:05Z

Matt T. Yourst notes that kvm_arch_vcpu_ioctl_set_sregs lacks validity
checking for the new cr3 value:

"Userspace callers of KVM_SET_SREGS can pass a bogus value of cr3 to
the kernel. This will trigger a NULL pointer access in gfn_to_rmap()
when userspace next tries to call KVM_RUN on the affected VCPU and kvm
attempts to activate the new non-existent page table root.

This happens since kvm only validates that cr3 points to a valid guest
physical memory page when code *inside* the guest sets cr3. However, kvm
currently trusts the userspace caller (e.g. QEMU) on the host machine to
always supply a valid page table root, rather than properly validating
it along with the rest of the reloaded guest state."

http://sourceforge.net/tracker/?func=detail&atid=893831&aid=2687641&group_id=180599

Follow Avi's suggestion to use kvm_set_cr3, and do the same for
assigment of cr4. Note kvm_set_cr4 unconditionally resets the mmu
context, as long as cr4 is valid.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 148cde2..89fb3c7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3985,25 +3985,19 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
kvm_x86_ops->set_gdt(vcpu, &dt);

vcpu->arch.cr2 = sregs->cr2;
- mmu_reset_needed |= vcpu->arch.cr3 != sregs->cr3;
- vcpu->arch.cr3 = sregs->cr3;

+ kvm_set_cr3(vcpu, sregs->cr3);
kvm_set_cr8(vcpu, sregs->cr8);

mmu_reset_needed |= vcpu->arch.shadow_efer != sregs->efer;
kvm_x86_ops->set_efer(vcpu, sregs->efer);
kvm_set_apic_base(vcpu, sregs->apic_base);

- kvm_x86_ops->decache_cr4_guest_bits(vcpu);
-
mmu_reset_needed |= vcpu->arch.cr0 != sregs->cr0;
kvm_x86_ops->set_cr0(vcpu, sregs->cr0);
vcpu->arch.cr0 = sregs->cr0;

- mmu_reset_needed |= vcpu->arch.cr4 != sregs->cr4;
- kvm_x86_ops->set_cr4(vcpu, sregs->cr4);
- if (!is_long_mode(vcpu) && is_pae(vcpu))
- load_pdptrs(vcpu, vcpu->arch.cr3);
+ kvm_set_cr4(vcpu, sregs->cr4);

if (mmu_reset_needed)
kvm_mmu_reset_context(vcpu);