KVM: s390: Limit sthyi execution

Store hypervisor information is a valid instruction not only in
supervisor state but also in problem state, i.e. the guest's
userspace. Its execution is not only computational and memory
intensive, but also has to get hold of the ipte lock to write to the
guest's memory.

This lock is not intended to be held often and long, especially not
from the untrusted guest userspace. Therefore we apply rate limiting
of sthyi executions per VM.
Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Acked-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

arch/s390/include/asm/kvm_host.h

@@ -652,6 +652,7 @@ struct kvm_arch{
 	wait_queue_head_t ipte_wq;
 	int ipte_lock_count;
 	struct mutex ipte_mutex;
+	struct ratelimit_state sthyi_limit;
 	spinlock_t start_stop_lock;
 	struct sie_page2 *sie_page2;
 	struct kvm_s390_cpu_model model;

arch/s390/kvm/kvm-s390.c

@@ -1151,6 +1151,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 
 	rc = -ENOMEM;
 
+	ratelimit_state_init(&kvm->arch.sthyi_limit, 5 * HZ, 500);
+
 	kvm->arch.use_esca = 0; /* start with basic SCA */
 	rwlock_init(&kvm->arch.sca_lock);
 	kvm->arch.sca = (struct bsca_block *) get_zeroed_page(GFP_KERNEL);

arch/s390/kvm/sthyi.c

@@ -12,6 +12,7 @@
 #include <linux/errno.h>
 #include <linux/pagemap.h>
 #include <linux/vmalloc.h>
+#include <linux/ratelimit.h>
 
 #include <asm/kvm_host.h>
 #include <asm/asm-offsets.h>
@@ -403,6 +404,16 @@ int handle_sthyi(struct kvm_vcpu *vcpu)
 	u64 code, addr, cc = 0;
 	struct sthyi_sctns *sctns = NULL;
 
+	/*
+	 * STHYI requires extensive locking in the higher hypervisors
+	 * and is very computational/memory expensive. Therefore we
+	 * ratelimit the executions per VM.
+	 */
+	if (!__ratelimit(&vcpu->kvm->arch.sthyi_limit)) {
+		kvm_s390_retry_instr(vcpu);
+		return 0;
+	}
+
 	kvm_s390_get_regs_rre(vcpu, &reg1, &reg2);
 	code = vcpu->run->s.regs.gprs[reg1];
 	addr = vcpu->run->s.regs.gprs[reg2];