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Subject[PATCH 3.16 086/157] USB: core: Fix unterminated string returned by usb_string()
3.16.72-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Alan Stern <stern@rowland.harvard.edu>

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@rowland.harvard.edu>
Reported-by: syzbot+b75b85111c10b8d680f1@syzkaller.appspotmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
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;
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