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Date:   Mon, 29 Apr 2019 09:36:32 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "idosch@...sch.org" <idosch@...sch.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] packet: validate msg_namelen in send directly

On Mon, Apr 29, 2019 at 9:25 AM David Laight <David.Laight@...lab.com> wrote:
>
> From: Willem de Bruijn
> > Sent: 29 April 2019 13:53
> > On Mon, Apr 29, 2019 at 5:00 AM David Laight <David.Laight@...lab.com> wrote:
> > >
> > > From: Willem de Bruijn
> > > > Sent: 26 April 2019 20:28
> > > > Packet sockets in datagram mode take a destination address. Verify its
> > > > length before passing to dev_hard_header.
> > > >
> > > > Prior to 2.6.14-rc3, the send code ignored sll_halen. This is
> > > > established behavior. Directly compare msg_namelen to dev->addr_len.
> > > >
> > > > Fixes: 6b8d95f1795c4 ("packet: validate address length if non-zero")
> > > > Suggested-by: David Laight <David.Laight@...lab.com>
> > > > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> > > > ---
> > > >  net/packet/af_packet.c | 18 ++++++++++++------
> > > >  1 file changed, 12 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > > > index 9419c5cf4de5e..13301e36b4a28 100644
> > > > --- a/net/packet/af_packet.c
> > > > +++ b/net/packet/af_packet.c
> > > > @@ -2624,10 +2624,13 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
> > > >                                               sll_addr)))
> > > >                       goto out;
> > > >               proto   = saddr->sll_protocol;
> > > > -             addr    = saddr->sll_halen ? saddr->sll_addr : NULL;
> > > >               dev = dev_get_by_index(sock_net(&po->sk), saddr->sll_ifindex);
> > > > -             if (addr && dev && saddr->sll_halen < dev->addr_len)
> > > > -                     goto out_put;
> > > > +             if (po->sk.sk_socket->type == SOCK_DGRAM) {
> > > > +                     addr = saddr->sll_addr;
> > > > +                     if (dev && msg->msg_namelen < dev->addr_len +
> > > > +                                     offsetof(struct sockaddr_ll, sll_addr))
> > > > +                             goto out_put;
> > > > +             }
> > >
> > > IIRC you need to initialise 'addr - NULL' at the top of the functions.
> > > I'm surprised the compiler doesn't complain.
> >
> > It did complain when I moved it below the if (dev && ..) branch. But
> > inside a branch with exactly the same condition as the one where used,
> > the compiler did figure it out. Admittedly that is fragile.
>
> Even a function call should be enough since the called code is allowed
> to modify po->sk.sk_socket->type via a global pointer.
>
> > Then it might be simplest to restore the unconditional assignment
> >
> >                 proto   = saddr->sll_protocol;
> > +               addr    = saddr->sll_addr;
> >                 dev = dev_get_by_index(sock_net(sk), saddr->sll_ifindex);
>
> There is an 'addr = NULL' in the 'address absent' branch.
> Moving that higher up makes it even more clear that the address is
> only set in one place.

I can do that. Only, touching code in multiple locations can make
backporting to stable branches more difficult.

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