[PATCH 3.16 086/157] USB: core: Fix unterminated string returned by usb_string()

[Ben Hutchings]

3.16.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

commit c01c348ecdc66085e44912c97368809612231520 upstream.

Some drivers (such as the vub300 MMC driver) expect usb_string() to
return a properly NUL-terminated string, even when an error occurs.
(In fact, vub300's probe routine doesn't bother to check the return
code from usb_string().)  When the driver goes on to use an
unterminated string, it leads to kernel errors such as
stack-out-of-bounds, as found by the syzkaller USB fuzzer.

An out-of-range string index argument is not at all unlikely, given
that some devices don't provide string descriptors and therefore list
0 as the value for their string indexes.  This patch makes
usb_string() return a properly terminated empty string along with the
-EINVAL error code when an out-of-range index is encountered.

And since a USB string index is a single-byte value, indexes >= 256
are just as invalid as values of 0 or below.

Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Reported-by: syzbot+b75b85111c10b8d680f1@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/usb/core/message.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/drivers/usb/core/message.c
+++ b/drivers/usb/core/message.c
@@ -822,9 +822,11 @@ int usb_string(struct usb_device *dev, i
 
 	if (dev->state == USB_STATE_SUSPENDED)
 		return -EHOSTUNREACH;
-	if (size <= 0 || !buf || !index)
+	if (size <= 0 || !buf)
 		return -EINVAL;
 	buf[0] = 0;
+	if (index <= 0 || index >= 256)
+		return -EINVAL;
 	tbuf = kmalloc(256, GFP_NOIO);
 	if (!tbuf)
 		return -ENOMEM;