Re: [syzbot] general protection fault in __device_attach

[Alan Stern]

On Fri, Jun 03, 2022 at 05:52:38PM +0200, Greg KH wrote:
> On Fri, Jun 03, 2022 at 11:42:19AM -0400, Alan Stern wrote:
> > 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@xxxxxxxxxxxxxxx>
> > > > 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()?
> 
> But how can that really fail on a real system?
> 
> Is this just due to error-injection stuff?  If so, I'm really loath to
> rework the world for something that can never happen in real life.
> 
> Or is this a real syzbot-found-with-reproducer issue?

Aren't there quite a few reasons why device_add() might fail?  (Although 
most of them probably are memory allocation errors...)

Basically, you have to make up your mind.  If a function can fail, you 
should be prepared to handle the failure.  If it can't fail, there's no 
point in even checking the return code.

Alan Stern