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: <202307281537.AC1ED9CA@keescook>
Date:   Fri, 28 Jul 2023 15:43:26 -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, 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.

This all looks workable, but it's a unique solution in the kernel. That
is fine, of course, but would it be easier to maintain/test if it went
with the union style solutions?

struct foo {
	...
	union {
		type legacy_padding;
		DECLARE_FLEX_ARRAY(type, member);
	};
};

Then the size doesn't change and "member" can still be used. (i.e. no
collateral changes needed.)

-Kees

-- 
Kees Cook