drivers/gpu/vga: allocate vga_arb_write() buffer on stack

Size of kmalloc() in vga_arb_write() is controlled by user.
Too large kmalloc() size triggers WARNING message on console.
Allocate the buffer on stack to avoid the WARNING.
The string must be small (e.g "target PCI:domain:bus:dev.fn").

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: syzkaller@googlegroups.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1476451342-146510-1-git-send-email-dvyukov@google.com

drivers/gpu/vga/vgaarb.c

@@ -1022,21 +1022,16 @@ static ssize_t vga_arb_write(struct file *file, const char __user *buf,
 
 	unsigned int io_state;
 
-	char *kbuf, *curr_pos;
+	char kbuf[64], *curr_pos;
 	size_t remaining = count;
 
 	int ret_val;
 	int i;
 
-
-	kbuf = kmalloc(count + 1, GFP_KERNEL);
-	if (!kbuf)
-		return -ENOMEM;
-
-	if (copy_from_user(kbuf, buf, count)) {
-		kfree(kbuf);
+	if (count >= sizeof(kbuf))
+		return -EINVAL;
+	if (copy_from_user(kbuf, buf, count))
 		return -EFAULT;
-	}
 	curr_pos = kbuf;
 	kbuf[count] = '\0';	/* Just to make sure... */
 
@@ -1259,11 +1254,9 @@ static ssize_t vga_arb_write(struct file *file, const char __user *buf,
 		goto done;
 	}
 	/* If we got here, the message written is not part of the protocol! */
-	kfree(kbuf);
 	return -EPROTO;
 
 done:
-	kfree(kbuf);
 	return ret_val;
 }