linux-hardening - Re: [PATCH net-next] netdevice: define and allocate &net_device _properly

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Date: Tue, 7 May 2024 20:21:58 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Kees Cook <keescook@...omium.org>, 
	"Gustavo A. R. Silva" <gustavoars@...nel.org>, Simon Horman <horms@...nel.org>, 
	nex.sw.ncis.osdt.itp.upstreaming@...el.com, linux-hardening@...r.kernel.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] netdevice: define and allocate &net_device _properly_

On Tue, May 7, 2024 at 2:40 PM Alexander Lobakin
<aleksander.lobakin@...el.com> wrote:
>
> In fact, this structure contains a flexible array at the end, but
> historically its size, alignment etc., is calculated manually.
> There are several instances of the structure embedded into other
> structures, but also there's ongoing effort to remove them and we
> could in the meantime declare &net_device properly.
> Declare the array explicitly, use struct_size() and store the array
> size inside the structure, so that __counted_by() can be applied.
> Don't use PTR_ALIGN(), as SLUB itself tries its best to ensure the
> allocated buffer is aligned to what the user expects.
> Also, change its alignment from %NETDEV_ALIGN to the cacheline size
> as per several suggestions on the netdev ML.
>
> bloat-o-meter for vmlinux:
>
> free_netdev                                  445     440      -5
> netdev_freemem                                24       -     -24
> alloc_netdev_mqs                            1481    1450     -31
>
> On x86_64 with several NICs of different vendors, I was never able to
> get a &net_device pointer not aligned to the cacheline size after the
> change.
>
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
> ---

...

> -       p = kvzalloc(alloc_size, GFP_KERNEL_ACCOUNT | __GFP_RETRY_MAYFAIL);
> -       if (!p)
> +       sizeof_priv = ALIGN(sizeof_priv, SMP_CACHE_BYTES);

If we have a __counted_by(priv_len), why do you ALIGN(sizeof_priv,
SMP_CACHE_BYTES) ?

If a driver pretends its private part is 4 bytes, we should get a
warning if 20 bytes are used instead.

You added two ____cacheline_aligned already in net_device already.