netdev - RE: [PATCH net] packet: validate msg

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Date:   Mon, 29 Apr 2019 13:25:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Willem de Bruijn' <willemdebruijn.kernel@...il.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

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.

	David

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