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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 4 Aug 2020 07:34:07 -0700
Message-ID: <CANn89iK_NVBWwT4QNrb3EpahG3zOQS-Dh68Qdhrm2_xAs7Yu=Q@mail.gmail.com>
Subject: Re: [PATCH] net: Fix potential out of bound write in skb_try_coalesce()
To:     linmiaohe <linmiaohe@huawei.com>
Cc:     David Miller <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Pravin B Shelar <pshelar@ovn.org>,
        Florian Westphal <fw@strlen.de>, martin.varghese@nokia.com,
        Willem de Bruijn <willemb@google.com>,
        Davide Caratti <dcaratti@redhat.com>,
        Steffen Klassert <steffen.klassert@secunet.com>,
        Paolo Abeni <pabeni@redhat.com>, shmulik@metanetworks.com,
        kyk.segfault@gmail.com, netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Aug 4, 2020 at 4:46 AM linmiaohe <linmiaohe@huawei.com> wrote:
>
> From: Miaohe Lin <linmiaohe@huawei.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@huawei.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.