linux-kernel - Re: [Linux-kernel-mentees] [PATCH v3] media/v4l2-core: Fix kernel-infoleak in video_put

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Message-ID: <20200730114842.GH3703480@smile.fi.intel.com>
Date:   Thu, 30 Jul 2020 14:48:42 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Bartosz Golaszewski <brgl@...ev.pl>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Peilin Ye <yepeilin.cs@...il.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Vandana BN <bnvandana@...il.com>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Niklas Söderlund 
        <niklas.soderlund+renesas@...natech.se>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Linux-kernel-mentees] [PATCH v3] media/v4l2-core: Fix
 kernel-infoleak in video_put_user()

On Thu, Jul 30, 2020 at 11:18:04AM +0200, Arnd Bergmann wrote:
> On Thu, Jul 30, 2020 at 10:38 AM Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:
> > 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@...ev.pl> wrote:

...

> > > 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.
> 
> Any reason why the workaround I suggested above would not work?

That is the question to somebody who has better understanding. If it works,
I vote up to get it fixed with little effort. I would be glad to test patches!

> The in_ia32_syscall() check should be completely reliable in telling whether
> we are called from read() by an ia32 task or not, and we use the same
> logic for input_event, which has a similar problem (on all compat architectures,
> not just x86).

By the way any reason why we have to have in_ia32_syscall() instead of
in_compat_syscall()?

> > However I'm not an expert in compat IOCTL code (you are :-) and perhaps you may
> > provide ideas better than mine.
> 
> What makes this interface tricky is that this is actually a read() call, not
> ioctl() which is usually easier because it encodes the data length in the
> command code. As far as I could tell from skimming the interface, the
> ioctls are actually fine here.

Right.

-- 
With Best Regards,
Andy Shevchenko