Re: BUG: bad usercopy in ld_usb_read

[Kees Cook <keescook@chromium.org>]

On Thu, Aug 08, 2019 at 02:46:54PM +0200, Greg KH wrote:
> On Thu, Aug 08, 2019 at 05:38:06AM -0700, syzbot wrote:
> > Hello,
> > 
> > syzbot found the following crash on:
> > 
> > HEAD commit:    e96407b4 usb-fuzzer: main usb gadget fuzzer driver
> > git tree:       https://github.com/google/kasan.git usb-fuzzer
> > console output: https://syzkaller.appspot.com/x/log.txt?x=13aeaece600000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=cfa2c18fb6a8068e
> > dashboard link: https://syzkaller.appspot.com/bug?extid=45b2f40f0778cfa7634e
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > 
> > Unfortunately, I don't have any reproducer for this crash yet.
> > 
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+45b2f40f0778cfa7634e@syzkaller.appspotmail.com
> > 
> > ldusb 6-1:0.124: Read buffer overflow, -131383996186150 bytes dropped
> 
> That's a funny number :)
> 
> Nice overflow found, I see you are now starting to fuzz the char device
> nodes of usb drivers...
> 
> Michael, care to fix this up?

This looks like the length in the read-from-device buffer is unchecked:

        /* actual_buffer contains actual_length + interrupt_in_buffer */
        actual_buffer = (size_t *)(dev->ring_buffer + dev->ring_tail * (sizeof(size_t)+dev->interrupt_in_endpoint_size));
        bytes_to_read = min(count, *actual_buffer);
        if (bytes_to_read < *actual_buffer)
                dev_warn(&dev->intf->dev, "Read buffer overflow, %zd bytes dropped\n",
                         *actual_buffer-bytes_to_read);

        /* copy one interrupt_in_buffer from ring_buffer into userspace */
        if (copy_to_user(buffer, actual_buffer+1, bytes_to_read)) {
                retval = -EFAULT;
                goto unlock_exit;
        }

I assume what's stored at actual_buffer is bogus and needs validation
somewhere before it's actually used. (If not here, maybe where ever the
write into the buffer originally happens?)

-- 
Kees Cook