Re: [Linux-kernel-mentees] [PATCH v3] media/v4l2-core: Fix kernel-infoleak in video_put_user()

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Thu, Jul 30, 2020 at 10:15:24AM +0200, Arnd Bergmann wrote:
> On Thu, Jul 30, 2020 at 10:07 AM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> >
> > On Tue, Jul 28, 2020 at 3:58 PM Arnd Bergmann <arnd@arndb.de> wrote:
> > >
> > > On Tue, Jul 28, 2020 at 3:06 PM Dan Carpenter <dan.carpenter@oracle.com> wrote:
> > > Something like
> > >
> > > static int lineevent_put_data(void __user *uptr, struct gpioevent_data *ge)
> > > {
> > > #ifdef __x86_64__
> > >         /* i386 has no padding after 'id' */
> > >         if (in_ia32_syscall()) {
> > >                 struct {
> > >                         compat_u64      timestamp __packed;
> > >                         u32             id;
> > >                 } compat_ge = { ge->timestamp, ge->id };
> > >
> > >                 if (copy_to_user(uptr, &compat_ge, sizeof(compat_ge)))
> > >                         return -EFAULT;
> > >
> > >                 return sizeof(compat_ge);
> > >         }
> > > #endif
> > >
> > >         if (copy_to_user(uptr, ge, sizeof(*ge))
> > >                 return -EFAULT;
> > >
> > >         return sizeof(*ge);
> > > }
> > >
> > >        Arnd
> >
> > Hi Arnd,
> >
> > Andy actually had a patch for that but since this isn't a regression
> > (it never worked), we decided to leave it as it is and get it right in
> > v2 API.
> 
> I would argue that it needs to be fixed anyway, unless you also want
> to remove the v1 interface for native mode. If this works on 32-bit
> kernels, on 64-bit kernels with 64-bit user space and on compat
> 32-bit user space on 64-bit non-x86 architectures, I see no reason
> to leave it broken specifically on x86 compat user space. There are
> still reasons to use 32-bit x86 user space for low-memory machines
> even though native i386 kernels are getting increasingly silly.

It was possible to "fix" (mitigate to some extent) before libgpiod got support
for several events in a request. Now it seems to be impossible to fix. AFAIU we
must discard any request to more than one event in it.

However I'm not an expert in compat IOCTL code (you are :-) and perhaps you may
provide ideas better than mine.

-- 
With Best Regards,
Andy Shevchenko