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

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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>
This commit is contained in:
Sean Christopherson 2021-03-04 17:11:01 -08:00 committed by Paolo Bonzini
parent a91f387b4b
commit 73ad160693
1 changed files with 13 additions and 2 deletions
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15
arch/x86/kvm/mmu/mmu.c
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@ -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;
}