[RFC PATCH v2] KVM: x86/mmu: replace BUG() with KVM_BUG() in shadow mmu

[Mingwei Zhang]

Replace BUG() in pte_list_remove() with KVM_BUG() to avoid crashing the
host. MMU bug is difficult to discover due to various racing conditions and
corner cases and thus it extremely hard to debug. The situation gets much
worse when it triggers the shutdown of a host. Host machine crash
eliminates everything including the potential clues for debugging.

>From cloud computing service perspective, BUG() or BUG_ON() is probably no
longer appropriate as the host reliability is top priority. Crashing the
physical machine is almost never a good option as it eliminates innocent
VMs and cause service outage in a larger scope. Even worse, if attacker can
reliably triggers this code by diverting the control flow or corrupting the
memory, then this becomes vm-of-death attack. This is a huge attack vector
to cloud providers, as the death of one single host machine is not the end
of the story. Without manual interferences, a failed cloud job may be
dispatched to other hosts and continue host crashes until all of them are
dead.

Because of the above reasons, shrink the scope of crash to the target VM
only. KVM_BUG() and KVM_BUG_ON() requires a valid struct kvm which requires
extra plumbing. Avoid it in this version by just using
kvm_get_running_vcpu()->kvm instead.

Compile tested only.

v1 -> v2:
 - compile test the code.
 - fill KVM_BUG() with kvm_get_running_vcpu()->kvm

v1:
https://lore.kernel.org/all/20221123231206.274392-1-mizhang@xxxxxxxxxx/

Cc: Nagareddy Reddy <nspreddy@xxxxxxxxxx>
Cc: Jim Mattson <jmattson@xxxxxxxxxx>
Cc: David Matlack <dmatlack@xxxxxxxxxx>
Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 4736d7849c60..f3b307c588ac 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -955,12 +955,12 @@ static void pte_list_remove(u64 *spte, struct kvm_rmap_head *rmap_head)
 
 	if (!rmap_head->val) {
 		pr_err("%s: %p 0->BUG\n", __func__, spte);
-		BUG();
+		KVM_BUG(true, kvm_get_running_vcpu()->kvm, "");
 	} else if (!(rmap_head->val & 1)) {
 		rmap_printk("%p 1->0\n", spte);
 		if ((u64 *)rmap_head->val != spte) {
 			pr_err("%s:  %p 1->BUG\n", __func__, spte);
-			BUG();
+			KVM_BUG(true, kvm_get_running_vcpu()->kvm, "");
 		}
 		rmap_head->val = 0;
 	} else {
@@ -979,7 +979,7 @@ static void pte_list_remove(u64 *spte, struct kvm_rmap_head *rmap_head)
 			desc = desc->more;
 		}
 		pr_err("%s: %p many->many\n", __func__, spte);
-		BUG();
+		KVM_BUG(true, kvm_get_running_vcpu()->kvm, "");
 	}
 }
 
-- 
2.38.1.584.g0f3c55d4c2-goog