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

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Date:   Wed, 5 Jan 2022 17:29:54 +0100
From:   Guillaume Nault <gnault@...hat.com>
To:     James Carlson <carlsonj@...kingcode.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "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 10:30:09AM -0500, James Carlson wrote:
> On 1/5/22 08:19, Guillaume Nault wrote:
> > 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.
> 
> The only ones that can be compressed are those less than 0x0100, which
> are (intentionally) all network layer protocols.  We should be getting
> only control protocol messages though the user-space interface, not
> network layer, so I'd say it's not just the best we can do, but indeed
> the right thing to do by design.

Well, I know of at least one implementation that used to transmit data
by writing on ppp unit file descriptors. That was a hack to work around
some other problems. Not a beautiful one, but it worked.