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
