netdev - Re: [RFC PATCH net] net-gro: restore check for NULL skb in napi_gro

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Date: Fri, 4 Aug 2023 10:36:29 -0400
From: Tyler Stachecki <stachecki.tyler@...il.com>
To: edward.cree@....com
Cc: davem@...emloft.net, kuba@...nel.org, edumazet@...gle.com,
	pabeni@...hat.com, Edward Cree <ecree.xilinx@...il.com>,
	netdev@...r.kernel.org, Martin Habets <habetsm.xilinx@...il.com>
Subject: Re: [RFC PATCH net] net-gro: restore check for NULL skb in
 napi_gro_frags

On Wed, Aug 02, 2023 at 10:23:40AM +0100, edward.cree@....com wrote:
> From: Edward Cree <ecree.xilinx@...il.com>
> 
> Cited commit removed the check on the grounds that napi_gro_frags must
>  not be called by drivers if napi_get_frags failed.  But skb can also
>  be NULL if napi_frags_skb fails to pull the ethernet header ("dropping
>  impossible skb" message).  In this case return GRO_CONSUMED, as
>  otherwise continuing on would cause a NULL dereference panic in
>  dev_gro_receive().
> 
> Fixes: 1d11fa696733 ("net-gro: remove GRO_DROP")
> Reviewed-by: Martin Habets <habetsm.xilinx@...il.com>
> Signed-off-by: Edward Cree <ecree.xilinx@...il.com>
> ---
> An sfc customer has encountered this panic in the wild; we're still
>  investigating exactly how it happened (we have a reproducer) but it
>  seems wise to have the core handle this check rather than requiring
>  it in every driver.
> ---
>  net/core/gro.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/core/gro.c b/net/core/gro.c
> index 0759277dc14e..0159972038da 100644
> --- a/net/core/gro.c
> +++ b/net/core/gro.c
> @@ -731,6 +731,9 @@ gro_result_t napi_gro_frags(struct napi_struct *napi)
>  	gro_result_t ret;
>  	struct sk_buff *skb = napi_frags_skb(napi);
>  
> +	if (!skb)
> +		return GRO_CONSUMED;
> +
>  	trace_napi_gro_frags_entry(skb);
>  
>  	ret = napi_frags_finish(napi, skb, dev_gro_receive(napi, skb));

Given that this case is rarely hit, and given that there are some performance
concerns raised wrt this change, it may be beneficial to hint that this
branch is unlikely.