[PATCH 1/1][SRU][B/C] KVM: Fix UAF in nested posted interrupt processing

[Tyler Hicks]

From: Cfir Cohen <cfir at google.com>

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()"
Cc: stable at vger.kernel.org
Reviewed-by: Andy Honig <ahonig at google.com>
Signed-off-by: Cfir Cohen <cfir at google.com>
Reviewed-by: Liran Alon <liran.alon at oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>

CVE-2018-16882

(cherry picked from commit c2dd5146e9fe1f22c77c1b011adf84eea0245806)
Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
---
 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 9fcc3ec3ab78..cb8c74ab0586 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -11028,6 +11028,8 @@ 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))
-- 
2.7.4