netdev - Re: [PATCH net] ip: in cmsg IP(V6)_ORIGDSTADDR do not read beyond headlen

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Message-ID: <CAF=yD-KssWL_trOUGtz2qTxJR06mwe4QMJwu8OP-y6=aAkbcUA@mail.gmail.com>
Date:   Sun, 22 Jul 2018 22:01:22 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] ip: in cmsg IP(V6)_ORIGDSTADDR do not read beyond headlen

On Sun, Jul 22, 2018 at 9:43 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 07/22/2018 05:43 PM, Willem de Bruijn wrote:
> > From: Willem de Bruijn <willemb@...gle.com>
> >
> > Syzbot reported a read beyond the end of the skb head when returning
> > IPV6_ORIGDSTADDR:
> >
> >   BUG: KMSAN: kernel-infoleak in put_cmsg+0x5ef/0x860 net/core/scm.c:242
> >   CPU: 0 PID: 4501 Comm: syz-executor128 Not tainted 4.17.0+ #9
> >   Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> >   Google 01/01/2011
> >   Call Trace:
> >     __dump_stack lib/dump_stack.c:77 [inline]
> >     dump_stack+0x185/0x1d0 lib/dump_stack.c:113
> >     kmsan_report+0x188/0x2a0 mm/kmsan/kmsan.c:1125
> >     kmsan_internal_check_memory+0x138/0x1f0 mm/kmsan/kmsan.c:1219
> >     kmsan_copy_to_user+0x7a/0x160 mm/kmsan/kmsan.c:1261
> >     copy_to_user include/linux/uaccess.h:184 [inline]
> >     put_cmsg+0x5ef/0x860 net/core/scm.c:242
> >     ip6_datagram_recv_specific_ctl+0x1cf3/0x1eb0 net/ipv6/datagram.c:719
> >     ip6_datagram_recv_ctl+0x41c/0x450 net/ipv6/datagram.c:733
> >     rawv6_recvmsg+0x10fb/0x1460 net/ipv6/raw.c:521
> >     [..]
> >
> > This logic and its ipv4 counterpart read the destination port from
> > the packet at skb_transport_offset(skb) + 4.
> >
> > With MSG_MORE and a local SOCK_RAW sender, syzbot was able to cook a
> > packet that stores headers exactly up to skb_transport_offset(skb) in
> > the head and the remainder in a frag.
> >
> > Avoid reading beyond skb->tail by testing skb_headlen(skb) instead of
> > skb->len when trying to access this field.
> >
> > Link: http://lkml.kernel.org/r/CAF=yD-LEJwZj5a1-bAAj2Oy_hKmGygV6rsJ_WOrAYnv-fnayiQ@mail.gmail.com
> > Reported-by: syzbot+9adb4b567003cac781f0@...kaller.appspotmail.com
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> > ---
> >  net/ipv4/ip_sockglue.c | 2 +-
> >  net/ipv6/datagram.c    | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
> > index 64c76dcf7386..5f228d641d9b 100644
> > --- a/net/ipv4/ip_sockglue.c
> > +++ b/net/ipv4/ip_sockglue.c
> > @@ -152,7 +152,7 @@ static void ip_cmsg_recv_dstaddr(struct msghdr *msg, struct sk_buff *skb)
> >       const struct iphdr *iph = ip_hdr(skb);
> >       __be16 *ports = (__be16 *)skb_transport_header(skb);
> >
> > -     if (skb_transport_offset(skb) + 4 > (int)skb->len)
> > +     if (skb_transport_offset(skb) + 4 > (int)skb_headlen(skb))
> >               return;
> >
>
> This really should call pskb_may_pull() or skb_header_pointer().

Okay. Since in all common cases these bytes do lie in the skb head,
pskb_may_pull is pretty cheap. I'll use that. The linked discussion
already outlines a rough patch.