pktgen: Limit how much data we copy onto the stack.

A program that accidentally writes too much data to the pktgen file can overflow
the kernel stack and oops the machine. This is only triggerable by root, so
there's no security issue, but it's still an unfortunate bug.

printk() won't print more than 1024 bytes in a single call, anyways, so let's
just never copy more than that much data. We're on a fairly shallow stack, so
that should be safe even with CONFIG_4KSTACKS.
Signed-off-by: Nelson Elhage <nelhage@ksplice.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/pktgen.c

@@ -887,10 +887,11 @@ static ssize_t pktgen_if_write(struct file *file,
 	i += len;
 
 	if (debug) {
-		char tb[count + 1];
-		if (copy_from_user(tb, user_buffer, count))
+		size_t copy = min(count, 1023);
+		char tb[copy + 1];
+		if (copy_from_user(tb, user_buffer, copy))
 			return -EFAULT;
-		tb[count] = 0;
+		tb[copy] = 0;
 		printk(KERN_DEBUG "pktgen: %s,%lu  buffer -:%s:-\n", name,
 		       (unsigned long)count, tb);
 	}