KVM: selftests: Use magic value to signal ucall_alloc() failure

Use a magic value to signal a ucall_alloc() failure instead of simply
doing GUEST_ASSERT().  GUEST_ASSERT() relies on ucall_alloc() and so a
failure puts the guest into an infinite loop.

Use -1 as the magic value, as a real ucall struct should never wrap.
Reported-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/lib/ucall_common.c

@@ -4,6 +4,8 @@
 #include "linux/bitmap.h"
 #include "linux/atomic.h"
 
+#define GUEST_UCALL_FAILED -1
+
 struct ucall_header {
 	DECLARE_BITMAP(in_use, KVM_MAX_VCPUS);
 	struct ucall ucalls[KVM_MAX_VCPUS];
@@ -41,7 +43,8 @@ static struct ucall *ucall_alloc(void)
 	struct ucall *uc;
 	int i;
 
-	GUEST_ASSERT(ucall_pool);
+	if (!ucall_pool)
+		goto ucall_failed;
 
 	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
 		if (!test_and_set_bit(i, ucall_pool->in_use)) {
@@ -51,7 +54,13 @@ static struct ucall *ucall_alloc(void)
 		}
 	}
 
-	GUEST_ASSERT(0);
+ucall_failed:
+	/*
+	 * If the vCPU cannot grab a ucall structure, make a bare ucall with a
+	 * magic value to signal to get_ucall() that things went sideways.
+	 * GUEST_ASSERT() depends on ucall_alloc() and so cannot be used here.
+	 */
+	ucall_arch_do_ucall(GUEST_UCALL_FAILED);
 	return NULL;
 }
 
@@ -93,6 +102,9 @@ uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc)
 
 	addr = ucall_arch_get_ucall(vcpu);
 	if (addr) {
+		TEST_ASSERT(addr != (void *)GUEST_UCALL_FAILED,
+			    "Guest failed to allocate ucall struct");
+
 		memcpy(uc, addr, sizeof(*uc));
 		vcpu_run_complete_io(vcpu);
 	} else {