netdev - Re: [PATCH] net: Fix potential out of bound write in skb_try

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Date:   Tue, 4 Aug 2020 07:34:07 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     linmiaohe <linmiaohe@...wei.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Pravin B Shelar <pshelar@....org>,
        Florian Westphal <fw@...len.de>, martin.varghese@...ia.com,
        Willem de Bruijn <willemb@...gle.com>,
        Davide Caratti <dcaratti@...hat.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Paolo Abeni <pabeni@...hat.com>, shmulik@...anetworks.com,
        kyk.segfault@...il.com, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: Fix potential out of bound write in skb_try_coalesce()

On Tue, Aug 4, 2020 at 4:46 AM linmiaohe <linmiaohe@...wei.com> wrote:
>
> From: Miaohe Lin <linmiaohe@...wei.com>
>
> The head_frag of skb would occupy one extra skb_frag_t. Take it into
> account or out of bound write to skb frags may happen.
>
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>

Please share a stack trace if this was a real bug spotted in the wild.

I do not believe this patch is correct.

if (A + B >= MAX)   is equivalent to  if (A + B + 1 > MAX)

Note how the other condition (when there is no bytes in skb header) is coded :

if (A + B > MAX) return false;

In anycase, please always provide a Fixes: tag for any bug fix.

Thanks.