From: Paolo Bonzini pbonzini@redhat.com
commit 3c346c0c60ab06a021d1c0884a0ef494bc4ee3a7 upstream.
Fixing nested_vmcb_check_save to avoid all TOC/TOU races is a bit harder in released kernels, so do the bare minimum by avoiding that EFER.SVME is cleared. This is problematic because svm_set_efer frees the data structures for nested virtualization if EFER.SVME is cleared.
Also check that EFER.SVME remains set after a nested vmexit; clearing it could happen if the bit is zero in the save area that is passed to KVM_SET_NESTED_STATE (the save area of the nested state corresponds to the nested hypervisor's state and is restored on the next nested vmexit).
Cc: stable@vger.kernel.org Fixes: 2fcf4876ada ("KVM: nSVM: implement on demand allocation of the nested state") Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kvm/svm/nested.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-)
--- a/arch/x86/kvm/svm/nested.c +++ b/arch/x86/kvm/svm/nested.c @@ -251,6 +251,13 @@ static bool nested_vmcb_check_save(struc struct kvm_vcpu *vcpu = &svm->vcpu; bool vmcb12_lma;
+ /* + * FIXME: these should be done after copying the fields, + * to avoid TOC/TOU races. For these save area checks + * the possible damage is limited since kvm_set_cr0 and + * kvm_set_cr4 handle failure; EFER_SVME is an exception + * so it is force-set later in nested_prepare_vmcb_save. + */ if ((vmcb12->save.efer & EFER_SVME) == 0) return false;
@@ -396,7 +403,14 @@ static void nested_prepare_vmcb_save(str svm->vmcb->save.gdtr = vmcb12->save.gdtr; svm->vmcb->save.idtr = vmcb12->save.idtr; kvm_set_rflags(&svm->vcpu, vmcb12->save.rflags | X86_EFLAGS_FIXED); - svm_set_efer(&svm->vcpu, vmcb12->save.efer); + + /* + * Force-set EFER_SVME even though it is checked earlier on the + * VMCB12, because the guest can flip the bit between the check + * and now. Clearing EFER_SVME would call svm_free_nested. + */ + svm_set_efer(&svm->vcpu, vmcb12->save.efer | EFER_SVME); + svm_set_cr0(&svm->vcpu, vmcb12->save.cr0); svm_set_cr4(&svm->vcpu, vmcb12->save.cr4); svm->vmcb->save.cr2 = svm->vcpu.arch.cr2 = vmcb12->save.cr2; @@ -1209,6 +1223,8 @@ static int svm_set_nested_state(struct k */ if (!(save->cr0 & X86_CR0_PG)) goto out_free; + if (!(save->efer & EFER_SVME)) + goto out_free;
/* * All checks done, we can enter guest mode. L1 control fields