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Date:   Tue, 23 Apr 2019 11:07:47 -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 address length if non-zero

On Tue, Apr 23, 2019 at 5:59 AM David Laight <David.Laight@...lab.com> wrote:
>
> From: Willem de Bruijn
> > Sent: 22 December 2018 21:54
> > Validate packet socket address length if a length is given. Zero
> > length is equivalent to not setting an address.
> >
> > Fixes: 99137b7888f4 ("packet: validate address length")
> > Reported-by: Ido Schimmel <idosch@...sch.org>
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> > ---
> >  net/packet/af_packet.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > index 5dda263b4a0a..eedacdebcd4c 100644
> > --- a/net/packet/af_packet.c
> > +++ b/net/packet/af_packet.c
> > @@ -2625,7 +2625,7 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
> >                                               sll_addr)))
> >                       goto out;
> >               proto   = saddr->sll_protocol;
> > -             addr    = saddr->sll_addr;
> > +             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;
> > @@ -2825,7 +2825,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
> >               if (msg->msg_namelen < (saddr->sll_halen + offsetof(struct sockaddr_ll, sll_addr)))
> >                       goto out;
> >               proto   = saddr->sll_protocol;
> > -             addr    = saddr->sll_addr;
> > +             addr    = saddr->sll_halen ? saddr->sll_addr : NULL;
> >               dev = dev_get_by_index(sock_net(sk), saddr->sll_ifindex);
> >               if (addr && dev && saddr->sll_halen < dev->addr_len)
> >                       goto out;
> > --
> > 2.20.1.415.g653613c723-goog
>
> We've just discovered the combination of this patch and the one it 'fixes'
> breaks some of our userspace code.
>
> Prior to these changes it didn't matter if code using AF_PACKET to
> send ethernet frames on a specific 'ethertype' failed to set sll_addr.
> Everything assumed it would be 6 - and the packets were sent.
>
> With both changes you get a -EINVAL return from somewhere.
> I can fix our code, but I doubt it is the only code affected.
> Other people are likely to have copied the same example.

Thanks for the report.

Usage trumps correctness. But this seems to be a case of damned if you
do, damned if you don't.

Syzbot found a real use case of reading beyond the end of
msg->msg_namelen, since that is checked against

                if (msg->msg_namelen < (saddr->sll_halen +
offsetof(struct sockaddr_ll, sll_addr)))

Just assuming that address length is dev->addr_len allows an
ns_capable root to build link layer packets with address set to
uninitialized data.

Ethernet is not the most problematic link layer. Indeed, since
ETH_ALEN < sizeof(sll_addr), the previous check

                if (msg->msg_namelen < sizeof(struct sockaddr_ll))

Will be sufficient in this case. The syzbot report was on a device of
type ip6gre, with addr_len sizeof(struct in6_addr).

So I can refine to only perform the check on protocols with addr_len
>= sizeof(sll_addr), excluding Ethernet.

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