> On 18 Dec 2018, at 18:18, Cfir Cohen <cfir@xxxxxxxxxx> wrote: > > nested_get_vmcs12_pages() processes the posted_intr address in vmcs12. It > caches the kmap()ed page object and pointer, however, it doesn't handle > errors correctly: it's possible to cache a valid pointer, then release > the page and later dereference the dangling pointer. > > I was able to reproduce with the following steps: > > 1. Call vmlaunch with valid posted_intr_desc_addr but an invalid > MSR_EFER. This causes nested_get_vmcs12_pages() to cache the kmap()ed > pi_desc_page and pi_desc. Later the invalid EFER value fails > check_vmentry_postreqs() which fails the first vmlaunch. > > 2. Call vmlanuch with a valid EFER but an invalid posted_intr_desc_addr > (I set it to 2G - 0x80). The second time we call nested_get_vmcs12_pages > pi_desc_page is unmapped and released and pi_desc_page is set to NULL > (the "shouldn't happen" clause). Due to the invalid > posted_intr_desc_addr, kvm_vcpu_gpa_to_page() fails and > nested_get_vmcs12_pages() returns. It doesn't return an error value so > vmlaunch proceeds. Note that at this time we have a dangling pointer in > vmx->nested.pi_desc and POSTED_INTR_DESC_ADDR in L0's vmcs. > > 3. Issue an IPI in L2 guest code. This triggers a call to > vmx_complete_nested_posted_interrupt() and pi_test_and_clear_on() which > dereferences the dangling pointer. > > Vulnerable code requires nested and enable_apicv variables to be set to > true. The host CPU must also support posted interrupts. > > Fixes: 5e2f30b756a37 "KVM: nVMX: get rid of nested_get_page()" > Reviewed-by: Andy Honig <ahonig@xxxxxxxxxx> > Signed-off-by: Cfir Cohen <cfir@xxxxxxxxxx> > --- > arch/x86/kvm/vmx.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c > index 02edd9960e9d..8d5d984541be 100644 > --- a/arch/x86/kvm/vmx.c > +++ b/arch/x86/kvm/vmx.c > @@ -11985,6 +11985,8 @@ static void nested_get_vmcs12_pages(struct kvm_vcpu *vcpu) > kunmap(vmx->nested.pi_desc_page); > kvm_release_page_dirty(vmx->nested.pi_desc_page); > vmx->nested.pi_desc_page = NULL; > + vmx->nested.pi_desc = NULL; > + vmcs_write64(POSTED_INTR_DESC_ADDR, -1ull); > } > page = kvm_vcpu_gpa_to_page(vcpu, vmcs12->posted_intr_desc_addr); > if (is_error_page(page)) > -- > 2.20.0.405.gbc1bbc6f85-goog > Nice catch. Fix looks correct to me. Reviewed-by: Liran Alon <liran.alon@xxxxxxxxxx>
