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Date:   Tue, 23 Apr 2019 14:21:25 -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 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;

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