Re: [PATCH] USB: core: limit access to rawdescriptors which were not allocated

[Xu, Yanfei]

On 8/26/20 2:00 AM, Alan Stern wrote:
> On Wed, Aug 26, 2020 at 12:16:59AM +0800, yanfei.xu@xxxxxxxxxxxxx wrote:
> > From: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
> >
> > When using systemcall to read the rawdescriptors, make sure we won't
> > access to the rawdescriptors never allocated, which are number
> > exceed the USB_MAXCONFIG.
> >
> > Reported-by: syzbot+256e56ddde8b8957eabd@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
> > ---
> >   drivers/usb/core/sysfs.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/usb/core/sysfs.c b/drivers/usb/core/sysfs.c
> > index a2ca38e25e0c..1a7a625e5f55 100644
> > --- a/drivers/usb/core/sysfs.c
> > +++ b/drivers/usb/core/sysfs.c
> > @@ -895,7 +895,8 @@ read_descriptors(struct file *filp, struct kobject *kobj,
> >   	 * configurations (config plus subsidiary descriptors).
> >   	 */
> >   	for (cfgno = -1; cfgno < udev->descriptor.bNumConfigurations &&
> > -			nleft > 0; ++cfgno) {
> > +			nleft > 0 &&
> > +			cfgno < USB_MAXCONFIG; ++cfgno) {
> >   		if (cfgno < 0) {
> >   			src = &udev->descriptor;
> >   			srclen = sizeof(struct usb_device_descriptor);
>
> This is not the right way to fix the problem.
>
> Instead, we should make sure that udev->descriptor.bNumConfigurations is
> always <= USB_MAXCONFIG.  That's what this code in
> usb_get_configuration() is supposed to do:
>
> 	int ncfg = dev->descriptor.bNumConfigurations;
> 	...
>
> 	if (ncfg > USB_MAXCONFIG) {
> 		dev_warn(ddev, "too many configurations: %d, "
> 		    "using maximum allowed: %d\n", ncfg, USB_MAXCONFIG);
> 		dev->descriptor.bNumConfigurations = ncfg = USB_MAXCONFIG;
> 	}
>
> If you want to fix the bug, you need to figure out why this isn't
> working.
Thanks for you suggestion. The patch is not right. I'll try to look
into the root cause.

Regard,
Yanfei.
> Alan Stern