diff --git a/drivers/isdn/capi/capi.c b/drivers/isdn/capi/capi.c
index 3c3ad42f22bf..a016d8c3c410 100644
--- a/drivers/isdn/capi/capi.c
+++ b/drivers/isdn/capi/capi.c
@@ -688,6 +688,9 @@ capi_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos
 	if (!cdev->ap.applid)
 		return -ENODEV;
 
+	if (count < CAPIMSG_BASELEN)
+		return -EINVAL;
+
 	skb = alloc_skb(count, GFP_USER);
 	if (!skb)
 		return -ENOMEM;
@@ -698,7 +701,8 @@ capi_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos
 	}
 	mlen = CAPIMSG_LEN(skb->data);
 	if (CAPIMSG_CMD(skb->data) == CAPI_DATA_B3_REQ) {
-		if ((size_t)(mlen + CAPIMSG_DATALEN(skb->data)) != count) {
+		if (count < 18 ||
+		    (size_t)(mlen + CAPIMSG_DATALEN(skb->data)) != count) {
 			kfree_skb(skb);
 			return -EINVAL;
 		}
@@ -711,6 +715,10 @@ capi_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos
 	CAPIMSG_SETAPPID(skb->data, cdev->ap.applid);
 
 	if (CAPIMSG_CMD(skb->data) == CAPI_DISCONNECT_B3_RESP) {
+		if (count < 12) {
+			kfree_skb(skb);
+			return -EINVAL;
+		}
 		mutex_lock(&cdev->lock);
 		capincci_free(cdev, CAPIMSG_NCCI(skb->data));
 		mutex_unlock(&cdev->lock);