[patch] s390: potential buffer overflow

[Dan Carpenter <error27@gmail.com>]

"len" hasn't been properly range checked so we shouldn't use it as an
array offset.  This can only be written to by root but it would still be
annoying to accidentally write more than 3 characters and corrupt your
memory.

Signed-off-by: Dan Carpenter <error27@gmail.com>
---
I don't have a cross compile environment to compile this.  Sorry.  :/

diff --git a/drivers/s390/char/sclp_async.c b/drivers/s390/char/sclp_async.c
index f449c69..72c2e90 100644
--- a/drivers/s390/char/sclp_async.c
+++ b/drivers/s390/char/sclp_async.c
@@ -84,7 +84,7 @@ static int proc_handler_callhome(struct ctl_table *ctl, int write,
 		rc = copy_from_user(buf, buffer, sizeof(buf));
 		if (rc != 0)
 			return -EFAULT;
-		buf[len - 1] = '\0';
+		buf[sizeof(buf) - 1] = '\0';
 		if (strict_strtoul(buf, 0, &val) != 0)
 			return -EINVAL;
 		if (val != 0 && val != 1)
@@ -99,6 +99,7 @@ static int proc_handler_callhome(struct ctl_table *ctl, int write,
 static struct ctl_table callhome_table[] = {
 	{
 		.procname	= "callhome",
+		.maxlen		= 3,
 		.mode		= 0644,
 		.proc_handler	= proc_handler_callhome,
 	},