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 17:42:19 +0200
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     Kees Cook <keescook@...omium.org>
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

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

Thanks,
Olek