netdev - Re: Data race on dev->mtu betwen __dev_set_mtu() and rawv6_send

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Message-ID: <CA+FuTSc0FRPJZubXrvojzNP9MvTp3rfJeH2zOQbETCNV80oOcA@mail.gmail.com>
Date:   Tue, 23 Feb 2021 10:25:36 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Gong, Sishuai" <sishuai@...due.edu>
Cc:     "davem@...emloft.net" <davem@...emloft.net>,
        "yoshfuji@...ux-ipv6.org" <yoshfuji@...ux-ipv6.org>,
        "dsahern@...nel.org" <dsahern@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Data race on dev->mtu betwen __dev_set_mtu() and rawv6_send_hdrinc()

On Mon, Feb 22, 2021 at 6:30 PM Gong, Sishuai <sishuai@...due.edu> wrote:
>
> Hello,
>
> We found a data race on dev->mtu between function __dev_set_mtu() and rawv6_send_hdrinc(). It happens with the following interleaving.
>
> writer: __dev_set_mtu()                                                                         reader: rawv6_send_hdrinc()
>                                                                                                                 if (length > rt->dst.dev->mtu) {
>         WRITE_ONCE(dev->mtu, new_mtu);
>                                                                                                                         ipv6_local_error(sk, EMSGSIZE, fl6, rt->dst.dev->mtu);
>
> If the writer happens to change dev->mtu to a value that is bigger than the variable ‘length’, then ipv6_local_error will read a value that doesn’t satisfy this conditional statement. While there is no need to use lock to protect the read, it is probably better to only read dev->mtu once in rawv6_send_hdrinc().

Makes sense. The same would then apply to raw_send_hdrinc().