linux-hardening - Re: [PATCH net-next 1/3] virtchnl: fix fake 1-elem arrays in structs allocated as `nents + 1`

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Date:   Fri, 4 Aug 2023 10:29:48 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Alexander Lobakin <aleksander.lobakin@...el.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Larysa Zaremba <larysa.zaremba@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        netdev@...r.kernel.org, linux-hardening@...r.kernel.org,
        intel-wired-lan@...ts.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] virtchnl: fix fake 1-elem arrays in structs
 allocated as `nents + 1` - 1

On Fri, Aug 04, 2023 at 05:42:19PM +0200, Alexander Lobakin wrote:
> From: Kees Cook <keescook@...omium.org>
> Date: Fri, 4 Aug 2023 01:27:02 -0700
> 
> > On Fri, Jul 28, 2023 at 05:52:05PM +0200, Alexander Lobakin wrote:
> >> The two most problematic virtchnl structures are virtchnl_rss_key and
> >> virtchnl_rss_lut. Their "flex" arrays have the type of u8, thus, when
> >> allocating / checking, the actual size is calculated as `sizeof +
> >> nents - 1 byte`. But their sizeof() is not 1 byte larger than the size
> >> of such structure with proper flex array, it's two bytes larger due to
> >> the padding. That said, their size is always 1 byte larger unless
> >> there are no tail elements -- then it's +2 bytes.
> >> Add virtchnl_struct_size() macro which will handle this case (and later
> >> other cases as well). Make its calling conv the same as we call
> >> struct_size() to allow it to be drop-in, even though it's unlikely to
> >> become possible to switch to generic API. The macro will calculate a
> >> proper size of a structure with a flex array at the end, so that it
> >> becomes transparent for the compilers, but add the difference from the
> >> old values, so that the real size of sorta-ABI-messages doesn't change.
> >> Use it on the allocation side in IAVF and the receiving side (defined
> >> as static inline in virtchnl.h) for the mentioned two structures.
> >>
> >> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
> > 
> > This is a novel approach to solving the ABI issues for a 1-elem
> > conversion, but I have been convinced it's a workable approach here. :)
> > Thanks for doing this conversion!
> > 
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> > 
> 
> Thanks a lot!
> You gave Reviewed-by for patches #1 and #3, does it mean the whole
> series or something is wrong with the patch #2? :D

Hm, maybe delivery was delayed? I see it on lore:
https://lore.kernel.org/all/202308040128.667940394B@keescook/

-- 
Kees Cook