On Wed, May 20, 2020 at 04:37:50PM +0000, Changming Liu wrote: > > > > -----Original Message----- > > From: Greg KH <greg@xxxxxxxxx> > > Sent: Wednesday, May 20, 2020 1:02 AM > > To: Changming Liu <liu.changm@xxxxxxxxxxxxxxxx> > > Cc: thomas@xxxxxxxxxxxxxxxx; linux-usb@xxxxxxxxxxxxxxx; Lu, Long > > <l.lu@xxxxxxxxxxxxxxxx>; yaohway@xxxxxxxxx > > Subject: Re: [Bug Report] drivers/usb/misc/sisusbvga: undefined result when > > left shift a possible negative value in sisusb_write_mem_bulk > > > > On Wed, May 20, 2020 at 03:51:04AM +0000, Changming Liu wrote: > > > Hi Greg and Thomas, > > > Greetings, I'm a first-year PhD student who is interested in the usage of > > UBSan for linux. And after some experiments, I've found that in > > drivers/usb/misc/sisusbvga/sisusb.c > > > function sisusb_write_mem_bulk, there is an undefined behavior caused by > > left shifting a possible negative number. > > > > > > More specifically, in the switch statement for case 3, after executing > > copy_from_user, the the lower 3 bytes of char buf[4] are filled with data from > > user space. > > > And these 3 bytes are left shifted accordingly to form a 32bit unsigned > > integer, swap32. > > > > > > The potential problem is, since the buf is declared as signed char buffer so > > each byte might be a negative number while being left shifted. According to the > > C standard, when the left-hand operand of the left shift operator is a negative > > value, the result is undefined. So I guess change the buf declaration to unsigned > > will help? Given that it's only used here. > > > > Sounds like a good idea, patches are welcome to fix this. > > Hi greg, > Thank you for this recognition! This means a lot to me. > Here's the patch as we agreed. Please resend this in a normal format where we can properly review it. But: > > Best, > Changming > > > >From 14ae7c67ea3fb96ed6bea0bc9919f3c597308813 Mon Sep 17 00:00:00 2001 > From: Changming Liu <liu.changm@xxxxxxxxxxxxxxxx> > Date: Wed, 20 May 2020 12:19:37 -0400 > Subject: [PATCH] USB: sisusbvga: Fix left shifting a possible negative value > > the char buffer buf, accepts user data which might be negative value and the content is left shifted to form an unsigned integer. > Since left shifting a negative value is undefined behavior, thus change the char to u8 to fix this Properly line-wrap your changelog when you resend this. thanks, greg k-h
