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Date: Mon, 15 Feb 2021 19:02:22 +0300
From: Dan Carpenter <dan.carpenter@...cle.com>
To: David Ahern <dsahern@...il.com>
Cc: kbuild@...ts.01.org, Arjun Roy <arjunroy.kdev@...il.com>,
davem@...emloft.net, netdev@...r.kernel.org, lkp@...el.com,
kbuild-all@...ts.01.org, arjunroy@...gle.com, edumazet@...gle.com,
soheil@...gle.com, Leon Romanovsky <leon@...nel.org>,
Jakub Kicinski <kuba@...nel.org>
Subject: Re: [net-next] tcp: Sanitize CMSG flags and reserved args in
tcp_zerocopy_receive.
On Mon, Feb 15, 2021 at 08:04:11AM -0700, David Ahern wrote:
> On 2/15/21 5:03 AM, Dan Carpenter wrote:
> > Hi Arjun,
> >
> > url: https://github.com/0day-ci/linux/commits/Arjun-Roy/tcp-Sanitize-CMSG-flags-and-reserved-args-in-tcp_zerocopy_receive/20210212-052537
> > base: https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git e4b62cf7559f2ef9a022de235e5a09a8d7ded520
> > config: x86_64-randconfig-m001-20210209 (attached as .config)
> > compiler: gcc-9 (Debian 9.3.0-15) 9.3.0
> >
> > If you fix the issue, kindly add following tag as appropriate
> > Reported-by: kernel test robot <lkp@...el.com>
> > Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
> >
> > smatch warnings:
> > net/ipv4/tcp.c:4158 do_tcp_getsockopt() warn: check for integer overflow 'len'
> >
> > vim +/len +4158 net/ipv4/tcp.c
> >
> > 3fdadf7d27e3fb Dmitry Mishin 2006-03-20 3896 static int do_tcp_getsockopt(struct sock *sk, int level,
> > 3fdadf7d27e3fb Dmitry Mishin 2006-03-20 3897 int optname, char __user *optval, int __user *optlen)
> > ^1da177e4c3f41 Linus Torvalds 2005-04-16 3898 {
> > 295f7324ff8d9e Arnaldo Carvalho de Melo 2005-08-09 3899 struct inet_connection_sock *icsk = inet_csk(sk);
> > ^1da177e4c3f41 Linus Torvalds 2005-04-16 3900 struct tcp_sock *tp = tcp_sk(sk);
> > 6fa251663069e0 Nikolay Borisov 2016-02-03 3901 struct net *net = sock_net(sk);
> > ^1da177e4c3f41 Linus Torvalds 2005-04-16 3902 int val, len;
> >
> > "len" is int.
> >
> > [ snip ]
> > 05255b823a6173 Eric Dumazet 2018-04-27 4146 #ifdef CONFIG_MMU
> > 05255b823a6173 Eric Dumazet 2018-04-27 4147 case TCP_ZEROCOPY_RECEIVE: {
> > 7eeba1706eba6d Arjun Roy 2021-01-20 4148 struct scm_timestamping_internal tss;
> > e0fecb289ad3fd Arjun Roy 2020-12-10 4149 struct tcp_zerocopy_receive zc = {};
> > 05255b823a6173 Eric Dumazet 2018-04-27 4150 int err;
> > 05255b823a6173 Eric Dumazet 2018-04-27 4151
> > 05255b823a6173 Eric Dumazet 2018-04-27 4152 if (get_user(len, optlen))
> > 05255b823a6173 Eric Dumazet 2018-04-27 4153 return -EFAULT;
> > c8856c05145490 Arjun Roy 2020-02-14 4154 if (len < offsetofend(struct tcp_zerocopy_receive, length))
> > 05255b823a6173 Eric Dumazet 2018-04-27 4155 return -EINVAL;
> >
> >
> > The problem is that negative values of "len" are type promoted to high
> > positive values. So the fix is to write this as:
> >
> > if (len < 0 || len < offsetofend(struct tcp_zerocopy_receive, length))
> > return -EINVAL;
> >
> > 110912bdf28392 Arjun Roy 2021-02-11 4156 if (unlikely(len > sizeof(zc))) {
> > 110912bdf28392 Arjun Roy 2021-02-11 4157 err = check_zeroed_user(optval + sizeof(zc),
> > 110912bdf28392 Arjun Roy 2021-02-11 @4158 len - sizeof(zc));
> > ^^^^^^^^^^^^^^^^
> > Potentially "len - a negative value".
> >
> >
>
> get_user(len, optlen) is called multiple times in that function. len < 0
> was checked after the first one at the top.
>
What you're describing is a "Double Fetch" bug, where the attack is we
get some data from the user, and we verify it, then we get it from the
user a second time and trust it. The problem is that the user modifies
it between the first and second get_user() call so it ends up being a
security vulnerability.
But I'm glad you pointed out the first get_user() because it has an
ancient, harmless pre git bug in it.
net/ipv4/tcp.c
3888 static int do_tcp_getsockopt(struct sock *sk, int level,
3889 int optname, char __user *optval, int __user *optlen)
3890 {
3891 struct inet_connection_sock *icsk = inet_csk(sk);
3892 struct tcp_sock *tp = tcp_sk(sk);
3893 struct net *net = sock_net(sk);
3894 int val, len;
3895
3896 if (get_user(len, optlen))
3897 return -EFAULT;
3898
3899 len = min_t(unsigned int, len, sizeof(int));
3900
3901 if (len < 0)
^^^^^^^
This is impossible. "len" has to be in the 0-4 range because of the
min_t() assignment. It's harmless though and the condition should just
be removed.
3902 return -EINVAL;
3903
3904 switch (optname) {
3905 case TCP_MAXSEG:
Anyway, I will create a new Smatch warning for this situation.
> Also, maybe I am missing something here, but offsetofend can not return
> a negative value, so this checks catches len < 0 as well:
>
> if (len < offsetofend(struct tcp_zerocopy_receive, length))
> return -EINVAL;
>
offsetofend is (unsigned long)12. If we compare a negative integer with
(unsigned long)12 then negative number is type promoted to a high
positive value.
if (-1 < (usigned long)12)
printf("dan is wrong\n");
regards,
dan carpenter
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