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Subject: Re: [PATCH v2 04/16] skbuff: Phase out ksize() fallback for
 frag_size
From:   Paolo Abeni <pabeni@redhat.com>
To:     Kees Cook <keescook@chromium.org>, Vlastimil Babka <vbabka@suse.cz>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
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        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
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Date:   Sun, 25 Sep 2022 09:17:40 +0200
In-Reply-To: <20220923202822.2667581-5-keescook@chromium.org>
References: <20220923202822.2667581-1-keescook@chromium.org>
         <20220923202822.2667581-5-keescook@chromium.org>
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On Fri, 2022-09-23 at 13:28 -0700, Kees Cook wrote:
> All callers of APIs that allowed a 0-sized frag_size appear to be
> passing actual size information already

AFAICS, not yet:

drivers/net/ethernet/qlogic/qed/qed_ll2.c:
	skb = build_skb(buffer->data, 0); // -> __build_skb(..., 0) 
		// ->  __build_skb_around()

drivers/net/ethernet/broadcom/bnx2.c:
	skb = build_skb(data, 0);

I guess some more drivers have calls leading to 

	__build_skb_around(...,  0)

there are several call path to checks...


> , so this use of ksize() can
> be removed. However, just in case there is something still depending
> on this behavior, issue a WARN and fall back to as before to ksize()
> which means we'll also potentially get KASAN warnings.
> 
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: netdev@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  net/core/skbuff.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 0b30fbdbd0d0..84ca89c781cd 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -195,7 +195,11 @@ static void __build_skb_around(struct sk_buff *skb, void *data,
>  			       unsigned int frag_size)
>  {
>  	struct skb_shared_info *shinfo;
> -	unsigned int size = frag_size ? : ksize(data);
> +	unsigned int size = frag_size;
> +
> +	/* All callers should be setting frag size now? */
> +	if (WARN_ON_ONCE(size == 0))
> +		size = ksize(data);

At some point in the future, I guess we could even drop this check,
right?

Thanks!

Paolo