linux-hardening - Re: [PATCH v3] batman-adv: Add flex array to struct batadv_tvlv_tt

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Date: Mon, 6 May 2024 09:27:46 -0700
From: Kees Cook <keescook@...omium.org>
To: Erick Archer <erick.archer@...look.com>
Cc: Sven Eckelmann <sven@...fation.org>,
	Marek Lindner <mareklindner@...mailbox.ch>,
	Simon Wunderlich <sw@...onwunderlich.de>,
	Antonio Quartulli <a@...table.cc>,
	"David S.  Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	b.a.t.m.a.n@...ts.open-mesh.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, linux-hardening@...r.kernel.org,
	llvm@...ts.linux.dev,
	Alexander Lobakin <aleksander.lobakin@...el.com>
Subject: Re: [PATCH v3] batman-adv: Add flex array to struct
 batadv_tvlv_tt_data

On Sat, May 04, 2024 at 07:08:39PM +0200, Erick Archer wrote:
> Hi Sven,
> 
> On Sat, May 04, 2024 at 11:35:38AM +0200, Sven Eckelmann wrote:
> > On Wednesday, 1 May 2024 17:02:42 CEST Erick Archer wrote:
> > > diff --git a/include/uapi/linux/batadv_packet.h b/include/uapi/linux/batadv_packet.h
> > > index 6e25753015df..dfbe30536995 100644
> > > --- a/include/uapi/linux/batadv_packet.h
> > > +++ b/include/uapi/linux/batadv_packet.h
> > [...]
> > > +/**
> > > + * struct batadv_tvlv_tt_data - tt data propagated through the tt tvlv container
> > > + * @flags: translation table flags (see batadv_tt_data_flags)
> > > + * @ttvn: translation table version number
> > > + * @num_vlan: number of announced VLANs. In the TVLV this struct is followed by
> > > + *  one batadv_tvlv_tt_vlan_data object per announced vlan
> > > + * @vlan_data: array of batadv_tvlv_tt_vlan_data objects
> > > + */
> > > +struct batadv_tvlv_tt_data {
> > > +       __u8   flags;
> > > +       __u8   ttvn;
> > > +       __be16 num_vlan;
> > > +       struct batadv_tvlv_tt_vlan_data vlan_data[] __counted_by_be(num_vlan);
> > > +};
> > 
> > Thanks for the updates. But I can't accept this at the moment because 
> > __counted_by_be is used in an uapi header without it being defined
> > include/uapi/linux/stddef.h (and this file is also not included in this 
> > header).
> > 
> > See commit c8248faf3ca2 ("Compiler Attributes: counted_by: Adjust name and 
> > identifier expansion") as an example for the similar __counted_by macro.
> 
> If I understand correctly, the following changes are also needed because
> the annotated struct is defined in a "uapi" header. Sorry if it's a stupid
> question, but I'm new to these topics.
> 
> diff --git a/include/uapi/linux/batadv_packet.h b/include/uapi/linux/batadv_packet.h
> index 6e25753015df..41f39d7661c9 100644
> --- a/include/uapi/linux/batadv_packet.h
> +++ b/include/uapi/linux/batadv_packet.h
> @@ -9,6 +9,7 @@
> 
>  #include <asm/byteorder.h>
>  #include <linux/if_ether.h>
> +#include <linux/stddef.h>
>  #include <linux/types.h>
> 
>  /**
> diff --git a/include/uapi/linux/stddef.h b/include/uapi/linux/stddef.h
> index 2ec6f35cda32..58154117d9b0 100644
> --- a/include/uapi/linux/stddef.h
> +++ b/include/uapi/linux/stddef.h
> @@ -55,4 +55,12 @@
>  #define __counted_by(m)
>  #endif
> 
> +#ifndef __counted_by_le
> +#define __counted_by_le(m)
> +#endif
> +
> +#ifndef __counted_by_be
> +#define __counted_by_be(m)
> +#endif
> +
>  #endif /* _UAPI_LINUX_STDDEF_H */

Yup, this is needed for UAPI as you have it. Thanks! I should have noticed
the need for this when I reviewed commit ca7e324e8ad3 ("compiler_types:
add Endianness-dependent __counted_by_{le,be}").

> If this is the right path, can these changes be merged into a
> single patch or is it better to add a previous patch to define
> __counted_by{le,be}?

We're almost on top of the merge window, so how about this: send me a
patch for just the UAPI addition, and I'll include it in this coming (next
week expected) merge window. Once -rc2 is out, re-send this batman-adv
patch since then netdev will be merged with -rc2 and the UAPI change
will be there.

-Kees

-- 
Kees Cook