netdev - Re: [PATCH net] ppp: ensure minimum packet size in ppp

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Message-ID: <20220105131929.GA17823@pc-1.home>
Date:   Wed, 5 Jan 2022 14:19:29 +0100
From:   Guillaume Nault <gnault@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Paul Mackerras <paulus@...ba.org>, linux-ppp@...r.kernel.org,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ppp: ensure minimum packet size in ppp_write()

On Wed, Jan 05, 2022 at 03:48:42AM -0800, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> It seems pretty clear ppp layer assumed user space
> would always be kind to provide enough data
> in their write() to a ppp device.
> 
> This patch makes sure user provides at least
> 2 bytes.
> 
> It adds PPP_PROTO_LEN macro that could replace
> in net-next many occurrences of hard-coded 2 value.

The PPP header can be compressed to only 1 byte, but since 2 bytes is
assumed in several parts of the code, rejecting such packets in
ppp_xmit() is probably the best we can do.

Acked-by: Guillaume Nault <gnault@...hat.com>