[PATCH 4/4] KVM: x86/mmu: Convert MMU walker's bounds check from BUG_ON() to KVM_BUG_ON()

[Sean Christopherson <seanjc@google.com>]

Bug the VM, not the host, if KVM's sanity check that walking guest PTEs
doesn't underflow the walker's level fires.  Bugging the host while holding
mmu_lock is all but guaranteed to panic the host, KVM hasn't _yet_ consumed
the out-of-bounds level (i.e. hasn't corrupted memory), and KVM is already
committed to bugging the VM and synthesizing a guest page fault if a fatal
MMU error occurs while walking guest PTEs.  I.e. there's no reason to keep
the BUG_ON() at this point.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/paging_tmpl.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 46a0f7796e55..2cf5e733aba4 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -392,7 +392,9 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
 		offset    = index * sizeof(pt_element_t);
 		pte_gpa   = gfn_to_gpa(table_gfn) + offset;
 
-		BUG_ON(walker->level < 1);
+		if (KVM_BUG_ON(walker->level < 1, vcpu->kvm))
+			goto error;
+
 		walker->table_gfn[walker->level - 1] = table_gfn;
 		walker->pte_gpa[walker->level - 1] = pte_gpa;
 
-- 
2.55.0.887.g758fc8c411-goog