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

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Message-ID: <d89e76ab-4db8-9307-6c31-628df3b4fd94@intel.com>
Date:   Fri, 4 Aug 2023 19:33:33 +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 10:29:48 -0700

> 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/
> 

Nevermind, my mail rules did put it in the folder other than the one
where the main thread was, sorry :s
Much thanks, I'm now a fan of _Generic() too :D

Olek