KVM: x86/mmu: Bug the VM if a vCPU ends up in long mode without PAE enabled

Promote the ASSERT(), which is quite dead code in KVM, into a KVM_BUG_ON()
for KVM's sanity check that CR4.PAE=1 if the vCPU is in long mode when
performing a walk of guest page tables.  The sanity is quite cheap since
neither EFER nor CR4.PAE requires a VMREAD, especially relative to the
cost of walking the guest page tables.

More importantly, the sanity check would have prevented the true badness
fixed by commit 112e6601 ("KVM: nVMX: add missing consistency checks
for CR0 and CR4").  The missed consistency check resulted in some versions
of KVM corrupting the on-stack guest_walker structure due to KVM thinking
there are 4/5 levels of page tables, but wiring up the MMU hooks to point
at the paging32 implementation, which only allocates space for two levels
of page tables in "struct guest_walker32".

Queue a page fault for injection if the assertion fails, as both callers,
FNAME(gva_to_gpa) and FNAME(walk_addr_generic), assume that walker.fault
contains sane info on a walk failure.  E.g. not populating the fault info
could result in KVM consuming and/or exposing uninitialized stack data
before the vCPU is kicked out to userspace, which doesn't happen until
KVM checks for KVM_REQ_VM_DEAD on the next enter.

Move the check below the initialization of "pte_access" so that the
aforementioned to-be-injected page fault doesn't consume uninitialized
stack data.  The information _shouldn't_ reach the guest or userspace,
but there's zero downside to being paranoid in this case.

Link: https://lore.kernel.org/r/20230729004722.1056172-9-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/paging_tmpl.h

@@ -338,7 +338,6 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
 	}
 #endif
 	walker->max_level = walker->level;
-	ASSERT(!(is_long_mode(vcpu) && !is_pae(vcpu)));
 
 	/*
 	 * FIXME: on Intel processors, loads of the PDPTE registers for PAE paging
@@ -348,6 +347,17 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
 	nested_access = (have_ad ? PFERR_WRITE_MASK : 0) | PFERR_USER_MASK;
 
 	pte_access = ~0;
+
+	/*
+	 * Queue a page fault for injection if this assertion fails, as callers
+	 * assume that walker.fault contains sane info on a walk failure.  I.e.
+	 * avoid making the situation worse by inducing even worse badness
+	 * between when the assertion fails and when KVM kicks the vCPU out to
+	 * userspace (because the VM is bugged).
+	 */
+	if (KVM_BUG_ON(is_long_mode(vcpu) && !is_pae(vcpu), vcpu->kvm))
+		goto error;
+
 	++walker->level;
 
 	do {