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Date:   Tue, 23 Apr 2019 13:04:13 -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 11:52 AM David Laight <David.Laight@...lab.com> wrote:
>
> From: Willem de Bruijn
> > Sent: 23 April 2019 16:08
> > 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.
>
> Maybe something like:
>                 addr    = saddr->sll_addr;
>                 dev = dev_get_by_index(sock_net(sk), saddr->sll_ifindex);
>                 if (dev && msg->msg_namelen < (dev->addr_len
>                                         + offsetof(struct sockaddr_ll,
>                                                 sll_addr)))
>                         /* Don't read address from beyond the end of the buffer */
>                         goto out;
>
> So it just checks that all of the address is in the buffer passed
> from the user.

Yes. sll_halen is never used outside this block. Testing that against
dev->addr_len just adds needless a level of indirection. We only care
that code that assumes the address is dev->addr_len won't read beyond
the end of msg->namelen. So this looks great to me (aside from goto
out_unlock). Thanks.

> Although I'd write those tests in a different order:
>                 if (dev && offsetof(struct sockaddr_ll, sll_addr) + dev->addr_len > msg->msg_namelen)
>                         goto out;

Since there already is a test for msg->msg_namelen < sizeof(struct
sockaddr_ll) just before, I'd keep the same order.

Do you want to send a fix, or shall I?

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