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From:   Jann Horn <jannh@google.com>
Date:   Wed, 20 Apr 2022 14:33:06 +0200
Message-ID: <CAG48ez2Pikm5g2RfJxae=jz1C7KSCWc99sCa7fXFBKvDOPJubA@mail.gmail.com>
Subject: Re: [PATCH] USB: serial: Fix heap overflow in WHITEHEAT_GET_DTR_RTS
To:     Kees Cook <keescook@chromium.org>
Cc:     Johan Hovold <johan@kernel.org>, kernel test robot <lkp@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-usb@vger.kernel.org, stable@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
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On Wed, Apr 20, 2022 at 10:14 AM Johan Hovold <johan@kernel.org> wrote:
> On Mon, Apr 18, 2022 at 09:17:42PM -0700, Kees Cook wrote:
> > This looks like it's harmless, as both the source and the destinations are
> > currently the same allocation size (4 bytes) and don't use their padding,
> > but if anything were to ever be added after the "mcr" member in "struct
> > whiteheat_private", it would be overwritten. The structs both have a
> > single u8 "mcr" member, but are 4 bytes in padded size. The memcpy()
> > destination was explicitly targeting the u8 member (size 1) with the
> > length of the whole structure (size 4), triggering the memcpy buffer
> > overflow warning:
>
> Ehh... No. The size of a structure with a single u8 is 1, not 4. There's
> nothing wrong with the current code even if the use of memcpy for this
> is a bit odd.
>
> > In file included from include/linux/string.h:253,
> >                  from include/linux/bitmap.h:11,
> >                  from include/linux/cpumask.h:12,
> >                  from include/linux/smp.h:13,
> >                  from include/linux/lockdep.h:14,
> >                  from include/linux/spinlock.h:62,
> >                  from include/linux/mmzone.h:8,
> >                  from include/linux/gfp.h:6,
> >                  from include/linux/slab.h:15,
> >                  from drivers/usb/serial/whiteheat.c:17:
> > In function 'fortify_memcpy_chk',
> >     inlined from 'firm_send_command' at drivers/usb/serial/whiteheat.c:587:4:
> > include/linux/fortify-string.h:328:25: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
> >   328 |                         __write_overflow_field(p_size_field, size);
> >       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> So something is confused here.

So something's going wrong in fortify_memcpy_chk()? It looks like it
is called with constant "size" equal to 1, and the condition
"p_size_field < size" (with an unsigned comparison) is either true
(meaning p_size_field would have to be 0) or not known at compile
time?

The original report says it happened when compiling with
CONFIG_CC_OPTIMIZE_FOR_SIZE=y, maybe that matters?