netdev - Re: [PATCH] net:fix up skbs delta_truesize in UDP GRO frag

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Date:   Thu, 24 Feb 2022 09:08:35 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     "lena.wang" <lena.wang@...iatek.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     <davem@...emloft.net>, <matthias.bgg@...il.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <wsd_upstream@...iatek.com>, <hao.lin@...iatek.com>
Subject: Re: [PATCH] net:fix up skbs delta_truesize in UDP GRO frag_list

On Wed, 23 Feb 2022 18:08:47 +0800 lena.wang wrote:
> The truesize for a UDP GRO packet is added by main skb and skbs in main
> skb's frag_list:
> skb_gro_receive_list
>         p->truesize += skb->truesize;
> 
> When uncloning skb, it will call pskb_expand_head and trusesize for
> frag_list skbs may increase. This can occur when allocators uses
> __netdev_alloc_skb and not jump into __alloc_skb. This flow does not
> use ksize(len) to calculate truesize while pskb_expand_head uses.
> skb_segment_list
> err = skb_unclone(nskb, GFP_ATOMIC);
> pskb_expand_head
>         if (!skb->sk || skb->destructor == sock_edemux)
>                 skb->truesize += size - osize;
> 
> If we uses increased truesize adding as delta_truesize, it will be
> larger than before and even larger than previous total truesize value
> if skbs in frag_list are abundant. The main skb truesize will become
> smaller and even a minus value or a huge value for an unsigned int
> parameter. Then the following memory check will drop this abnormal skb.
> 
> To avoid this error we should use the original truesize to segment the
> main skb.
> 
> Signed-off-by: lena wang <lena.wang@...iatek.com>

CC: Eric