Subject: Re: [PATCH net] gro: ensure frag0 meets IP header alignment

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 13 Apr 2021 08:58:14 -0400

On Tue, Apr 13, 2021 at 05:41:35AM -0700, Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
> 
> After commit 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head")
> Guenter Roeck reported one failure in his tests using sh architecture.
> 
> After much debugging, we have been able to spot silent unaligned accesses
> in inet_gro_receive()
> 
> The issue at hand is that upper networking stacks assume their header
> is word-aligned. Low level drivers are supposed to reserve NET_IP_ALIGN
> bytes before the Ethernet header to make that happen.
> 
> This patch hardens skb_gro_reset_offset() to not allow frag0 fast-path
> if the fragment is not properly aligned.
> 
> Some arches like x86, arm64 and powerpc do not care and define NET_IP_ALIGN
> as 0, this extra check will be a NOP for them.
> 
> Note that if frag0 is not used, GRO will call pskb_may_pull()
> as many times as needed to pull network and transport headers.
> 
> Fixes: 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head")
> Fixes: 78a478d0efd9 ("gro: Inline skb_gro_header and cache frag0 virtual address")
> Signed-off-by: Eric Dumazet <[email-masked]>
> Reported-by: Guenter Roeck <[email-masked]>
> Cc: Xuan Zhuo <[email-masked]>
> Cc: "Michael S. Tsirkin" <[email-masked]>
> Cc: Jason Wang <[email-masked]>

Seems to make sense.

Acked-by: Michael S. Tsirkin <[email-masked]>

> ---
>  net/core/dev.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index af8c1ea040b9364b076e2d72f04dc3de2d7e2f11..1f79b9aa9a3f2392fddd1401f95ad098b5e03204 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5924,7 +5924,8 @@ static void skb_gro_reset_offset(struct sk_buff *skb)
>  	NAPI_GRO_CB(skb)->frag0_len = 0;
>  
>  	if (!skb_headlen(skb) && pinfo->nr_frags &&
> -	    !PageHighMem(skb_frag_page(frag0))) {
> +	    !PageHighMem(skb_frag_page(frag0)) &&
> +	    (!NET_IP_ALIGN || !(skb_frag_off(frag0) & 3))) {
>  		NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);
>  		NAPI_GRO_CB(skb)->frag0_len = min_t(unsigned int,
>  						    skb_frag_size(frag0),
> -- 
> 2.31.1.295.g9ea45b61b8-goog