KVM: x86/mmu: WARN on NULL pae_root or lm_root, or bad shadow root level

WARN if KVM is about to dereference a NULL pae_root or lm_root when
loading an MMU, and convert the BUG() on a bad shadow_root_level into a
WARN (now that errors are handled cleanly).  With nested NPT, botching
the level and sending KVM down the wrong path is all too easy, and the
on-demand allocation of pae_root and lm_root means bugs crash the host.
Obviously, KVM could unconditionally allocate the roots, but that's
arguably a worse failure mode as it would potentially corrupt the guest
instead of crashing it.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210305011101.3597423-18-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 45039e7..bb01435 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3255,6 +3255,9 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
                root = mmu_alloc_root(vcpu, 0, 0, shadow_root_level, true);
                mmu->root_hpa = root;
        } else if (shadow_root_level == PT32E_ROOT_LEVEL) {
+               if (WARN_ON_ONCE(!mmu->pae_root))
+                       return -EIO;
+
                for (i = 0; i < 4; ++i) {
                        WARN_ON_ONCE(mmu->pae_root[i] &&
                                     VALID_PAGE(mmu->pae_root[i]));
@@ -3265,8 +3268,10 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
                                           shadow_me_mask;
                }
                mmu->root_hpa = __pa(mmu->pae_root);
-       } else
-               BUG();
+       } else {
+               WARN_ONCE(1, "Bad TDP root level = %d\n", shadow_root_level);
+               return -EIO;
+       }
 
        /* root_pgd is ignored for direct MMUs. */
        mmu->root_pgd = 0;
@@ -3310,6 +3315,9 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
                goto set_root_pgd;
        }
 
+       if (WARN_ON_ONCE(!mmu->pae_root))
+               return -EIO;
+
        /*
         * We shadow a 32 bit page table. This may be a legacy 2-level
         * or a PAE 3-level page table. In either case we need to be aware that
@@ -3319,6 +3327,9 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
        if (mmu->shadow_root_level == PT64_ROOT_4LEVEL) {
                pm_mask |= PT_ACCESSED_MASK | PT_WRITABLE_MASK | PT_USER_MASK;
 
+               if (WARN_ON_ONCE(!mmu->lm_root))
+                       return -EIO;
+
                mmu->lm_root[0] = __pa(mmu->pae_root) | pm_mask;
        }