pktgen: fix out-of-bounds access in pgctrl_write()

If a privileged user writes an empty string to /proc/net/pktgen/pgctrl
the code for stripping the (then non-existent) '\n' actually writes the
zero byte at index -1 of data[]. The then still uninitialized array will
very likely fail the command matching tests and the pr_warning() at the
end will therefore leak stack bytes to the kernel log.

Fix those issues by simply ensuring we're passed a non-empty string as
the user API apparently expects a trailing '\n' for all commands.

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/pktgen.c

@@ -485,6 +485,9 @@ static ssize_t pgctrl_write(struct file *file, const char __user *buf,
 		goto out;
 	}
 
+	if (count == 0)
+		return -EINVAL;
+
 	if (count > sizeof(data))
 		count = sizeof(data);
 
@@ -492,7 +495,7 @@ static ssize_t pgctrl_write(struct file *file, const char __user *buf,
 		err = -EFAULT;
 		goto out;
 	}
-	data[count - 1] = 0;	/* Make string */
+	data[count - 1] = 0;	/* Strip trailing '\n' and terminate string */
 
 	if (!strcmp(data, "stop"))
 		pktgen_stop_all_threads_ifs(pn);