[Linux-kernel-mentees] [SYZBOT REPORT] divide error in usbtmc_generic_read

[Jaskaran Singh]

On Thu, 2019-10-03 at 21:56 +0200, Greg KH wrote:
> On Fri, Oct 04, 2019 at 01:00:16AM +0530, Jaskaran Singh wrote:
> > Hi,
> > 
> > Following is my analysis of syzbot report:
> > https://syzkaller.appspot.com/bug?id=688d05e810900459b03c82fb48dc609edbbf7df9
> > 
> > The bug pertains to a divide error. The report's call trace is as
> > follows:
> > 
> >  usbtmc_ioctl_generic_read drivers/usb/class/usbtmc.c:1029 [inline]
> >  usbtmc_ioctl+0x27d/0x2ab0 drivers/usb/class/usbtmc.c:2089
> >  vfs_ioctl fs/ioctl.c:46 [inline]
> >  file_ioctl fs/ioctl.c:509 [inline]
> >  do_vfs_ioctl+0xd2d/0x1330 fs/ioctl.c:696
> >  ksys_ioctl+0x9b/0xc0 fs/ioctl.c:713
> >  __do_sys_ioctl fs/ioctl.c:720 [inline]
> >  __se_sys_ioctl fs/ioctl.c:718 [inline]
> >  __x64_sys_ioctl+0x6f/0xb0 fs/ioctl.c:718
> >  do_syscall_64+0xb7/0x580 arch/x86/entry/common.c:296
> >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > 
> > The file:line where the error occurs is this:
> > 	drivers/usb/class/usbtmc.c:816
> > 
> > Line 816 of that file is as follows:
> > 		if ((max_transfer_size % data->wMaxPacketSize) == 0)
> > 
> > Running a 'git blame' on the file in question reveals that the last
> > modification to line 816 was commit
> > bb99794a4792068cb4bfd40e99e0f9d8fe7872fa.
> > The commit adds the USBTMC_IOCTL_READ call for the driver, and thus
> > the
> > usbtmc_generic_read function.
> > 
> > I was not able to reproduce the error using my native machine and
> > the
> > same gcc version used by syzkaller (gcc 9.0.0 20181231). However,
> > the
> > error probably occurs because data->wMaxPacketSize is zero.
> > 
> > An intuitive fix for this seems to be:
> > 
> > -		if ((max_transfer_size % data->wMaxPacketSize) == 0)
> > +		if (data->wMaxPacketSize &&
> > +		    (max_transfer_size % data->wMaxPacketSize) == 0)
> > 
> > Or perhaps introduce a guard condition above line 816 to raise an
> > error.
> 
> Yes, if wMaxPacketSize is 0, then the code should just return an
> error.
> Care to make up a patch for this and submit it to syzbot to see if it
> solves the problem or not?
> 
> thanks,
> 
> greg k-h

Hi Greg,

>From the following thread:

https://groups.google.com/forum/#!searchin/syzkaller-bugs/divide$20error%7Csort:date/syzkaller-bugs/RYfg7J46e4c/t2amCUU7DgAJ

It looks like a patch has already been submitted and a crash does not
trigger with it. However, it might make sense to add more sanity
checking. Should I submit the patch?

Cheers,
Jaskaran.