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Date:   Wed, 24 Apr 2019 15:34:35 -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 Wed, Apr 24, 2019 at 3:14 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Tue, Apr 23, 2019 at 2:21 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > On Tue, Apr 23, 2019 at 1:21 PM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> > >
> > > On Tue, Apr 23, 2019 at 1:04 PM Willem de Bruijn
> > > <willemdebruijn.kernel@...il.com> wrote:
> > > >
> > > > 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.
> > >
> > > Actually, this only matters if sll_addr may be read, which is only
> > > true for SOCK_DGRAM. It is fine to pass a sockaddr_ll without an
> > > address for SOCK_RAW.
> >
> > So something like
> >
> > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > index 5c4a118d6f969..64ab3c960f538 100644
> > --- a/net/packet/af_packet.c
> > +++ b/net/packet/af_packet.c
> > @@ -2819,12 +2819,10 @@ static int packet_snd(struct socket *sock,
> > struct msghdr *msg, size_t len)
> >                 err = -EINVAL;
> >                 if (msg->msg_namelen < sizeof(struct sockaddr_ll))
> >                         goto out;
> > -               if (msg->msg_namelen < (saddr->sll_halen +
> > offsetof(struct sockaddr_ll, sll_addr)))
> > -                       goto out;
> >                 proto   = saddr->sll_protocol;
> > -               addr    = saddr->sll_halen ? saddr->sll_addr : NULL;
> > +               addr    = sock->type == SOCK_DGRAM ? saddr->sll_addr : NULL;
> >                 dev = dev_get_by_index(sock_net(sk), saddr->sll_ifindex);
> > -               if (addr && dev && saddr->sll_halen < dev->addr_len)
> > +               if (addr && dev && msg->msg_namelen < (dev->addr_len +
> > offsetof(struct sockaddr_ll, sll_addr)))
> >                         goto out_unlock;
>
> Sent http://patchwork.ozlabs.org/patch/1090340/
>
> Though I probably misunderstood your issue.
>
> I initially thought that an sll_halen > 0 < dev->addr_len was
> triggering the immediately check.
>
> But I guess that the real issue was an sll_halen == 0 was causing addr
> to be NULL, but dev_hard_header still called. For Ethernet, eth_header
> then does not fail, but simply does not fill in eth->h_dest.

Luckily I misread. It *does* fail and so does sendto. But indeed in a
different way than I previously understood your report.

> Let me know if you'd prefer a revised commit message.

This, then, still stands.

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