Re: [syzbot] general protection fault in __device_attach

Alan Stern
2022-06-03T15:42:29Z

On Fri, Jun 03, 2022 at 02:04:04PM +0300, Andy Shevchenko wrote:
> On Fri, Jun 03, 2022 at 03:02:07AM -0700, syzbot wrote:
> > syzbot has bisected this issue to:
> >
> > commit a9c4cf299f5f79d5016c8a9646fa1fc49381a8c1
> > Author: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> > Date: Fri Jun 18 13:41:27 2021 +0000
> >
> > ACPI: sysfs: Use __ATTR_RO() and __ATTR_RW() macros
>
> Hmm... It's not obvious at all how this change can alter the behaviour so
> drastically. device_add() is called from USB core with intf->dev.name == NULL
> by some reason. A-ha, seems like fault injector, which looks like
>
> dev_set_name(&intf->dev, "%d-%s:%d.%d", dev->bus->busnum,
> dev->devpath, configuration, ifnum);
>
> missed the return code check.
>
> But I'm not familiar with that code at all, adding Linux USB ML and Alan.

I can't see any connection between this bug and acpi/sysfs.c. Is it a
bad bisection?

It looks like you're right about dev_set_name() failing. In fact, the
kernel appears to be littered with calls to that routine which do not
check the return code (the entire subtree below drivers/usb/ contains
only _one_ call that does check the return code!). The function doesn't
have any __must_check annotation, and its kerneldoc doesn't mention the
return code or the possibility of a failure.

Apparently the assumption is that if dev_set_name() fails then
device_add() later on will also fail, and the problem will be detected
then.

So now what should happen when device_add() for an interface fails in
usb_set_configuration()? I guess the interface should be deleted;
otherwise we have the possibility that people might still try to access
it via usbfs, as in the syzbot test run. Same goes for the
of_device_is_available() check.

Fixing that will be a little painful. Right now there are plenty of
places in the USB core that aren't prepared to cope with a non-existent
interface.

Alan Stern