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Date:   Thu, 21 Apr 2022 09:58:23 +0200
From:   Johan Hovold <johan@kernel.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Jann Horn <jannh@google.com>, 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,
        Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH] USB: serial: Fix heap overflow in WHITEHEAT_GET_DTR_RTS
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[ +CC: Arnd ]

On Wed, Apr 20, 2022 at 11:11:26AM -0700, Kees Cook wrote:
> On Wed, Apr 20, 2022 at 02:33:06PM +0200, Jann Horn wrote:
> > 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.
> 
> I thought that was surprising too, and suspected it was something
> specific to the build (as Jann also suggested). I tracked it down[1] to
> "-mabi=apcs-gnu", which is from CONFIG_AEABI=n.
> 
> whiteheat_private {
>         __u8                       mcr;                  /*     0     1 */
> 
>         /* size: 4, cachelines: 1, members: 1 */
>         /* padding: 3 */
>         /* last cacheline: 4 bytes */
> };

I stand corrected, thanks.

Do we have other ABIs that can increase the alignment of structures like
this?

Skimming lore reveals a few subsystems that have started depending on
!OABI to not have to deal with this. Apparently the old ARM ABI is
deprecated in user space since gcc-4.6:

	https://lore.kernel.org/all/20190304193723.657089-1-arnd@arndb.de/

Perhaps time to drop support from the kernel too?

> Given nothing actually uses "struct whiteheat_dr_info", except for the
> sizing (har har), I suspect the better solution is just to do:
> 
> 	info->mcr = command_info->result_buffer[0];

Yeah, that works for now. Ideally, we'd cast the result buffer to struct
whiteheat_dr_info and access its single field. But that's not what
current code does and the above is no less confusing.

Patch applied, thanks.

Johan