[net-next] tcp: Sanitize CMSG flags and reserved args in tcp_zerocopy_receive.

[net-next] tcp: Sanitize CMSG flags and reserved args in tcp_zerocopy_receive.

[Arjun Roy]

From: Arjun Roy <arjunroy@google.com>

Explicitly define reserved field and require it and any subsequent
fields to be zero-valued for now. Additionally, limit the valid CMSG
flags that tcp_zerocopy_receive accepts.

Fixes: 7eeba1706eba ("tcp: Add receive timestamp support for receive zerocopy.")
Signed-off-by: Arjun Roy <arjunroy@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Suggested-by: David Ahern <dsahern@gmail.com>
Suggested-by: Leon Romanovsky <leon@kernel.org>
Suggested-by: Jakub Kicinski <kuba@kernel.org>
---
 include/uapi/linux/tcp.h |  2 +-
 net/ipv4/tcp.c           | 11 ++++++++++-
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index 42fc5a640df4..8fc09e8638b3 100644
--- a/include/uapi/linux/tcp.h
+++ b/include/uapi/linux/tcp.h
@@ -357,6 +357,6 @@ struct tcp_zerocopy_receive {
 	__u64 msg_control; /* ancillary data */
 	__u64 msg_controllen;
 	__u32 msg_flags;
-	/* __u32 hole;  Next we must add >1 u32 otherwise length checks fail. */
+	__u32 reserved; /* set to 0 for now */
 };
 #endif /* _UAPI_LINUX_TCP_H */
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index e1a17c6b473c..9896ca10bb34 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2030,6 +2030,7 @@ static int tcp_zerocopy_vm_insert_batch(struct vm_area_struct *vma,
 		err);
 }
 
+#define TCP_VALID_ZC_MSG_FLAGS   (TCP_CMSG_TS)
 static void tcp_recv_timestamp(struct msghdr *msg, const struct sock *sk,
 			       struct scm_timestamping_internal *tss);
 static void tcp_zc_finalize_rx_tstamp(struct sock *sk,
@@ -4152,13 +4153,21 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
 			return -EFAULT;
 		if (len < offsetofend(struct tcp_zerocopy_receive, length))
 			return -EINVAL;
-		if (len > sizeof(zc)) {
+		if (unlikely(len > sizeof(zc))) {
+			err = check_zeroed_user(optval + sizeof(zc),
+						len - sizeof(zc));
+			if (err < 1)
+				return err == 0 ? -EINVAL : err;
 			len = sizeof(zc);
 			if (put_user(len, optlen))
 				return -EFAULT;
 		}
 		if (copy_from_user(&zc, optval, len))
 			return -EFAULT;
+		if (zc.reserved)
+			return -EINVAL;
+		if (zc.msg_flags &  ~(TCP_VALID_ZC_MSG_FLAGS))
+			return -EINVAL;
 		lock_sock(sk);
 		err = tcp_zerocopy_receive(sk, &zc, &tss);
 		release_sock(sk);
-- 
2.30.0.478.g8a0d178c01-goog

Re: [net-next] tcp: Sanitize CMSG flags and reserved args in tcp_zerocopy_receive.

[Jakub Kicinski]

On Thu, 11 Feb 2021 13:21:07 -0800 Arjun Roy wrote:
> +		if (unlikely(len > sizeof(zc))) {
> +			err = check_zeroed_user(optval + sizeof(zc),
> +						len - sizeof(zc));
> +			if (err < 1)
> +				return err == 0 ? -EINVAL : err;

nit: return err ? : -EINVAL;

>  			len = sizeof(zc);
>  			if (put_user(len, optlen))
>  				return -EFAULT;
>  		}
>  		if (copy_from_user(&zc, optval, len))
>  			return -EFAULT;
> +		if (zc.reserved)
> +			return -EINVAL;
> +		if (zc.msg_flags &  ~(TCP_VALID_ZC_MSG_FLAGS))

nit: parens unnecessary

But neither is a big deal:

Acked-by: Jakub Kicinski <kuba@kernel.org>

Re: [net-next] tcp: Sanitize CMSG flags and reserved args in tcp_zerocopy_receive.

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (refs/heads/master):

On Thu, 11 Feb 2021 13:21:07 -0800 you wrote:
> From: Arjun Roy <arjunroy@google.com>
> 
> Explicitly define reserved field and require it and any subsequent
> fields to be zero-valued for now. Additionally, limit the valid CMSG
> flags that tcp_zerocopy_receive accepts.
> 
> Fixes: 7eeba1706eba ("tcp: Add receive timestamp support for receive zerocopy.")
> Signed-off-by: Arjun Roy <arjunroy@google.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
> Suggested-by: David Ahern <dsahern@gmail.com>
> Suggested-by: Leon Romanovsky <leon@kernel.org>
> Suggested-by: Jakub Kicinski <kuba@kernel.org>
> 
> [...]

Here is the summary with links:
  - [net-next] tcp: Sanitize CMSG flags and reserved args in tcp_zerocopy_receive.
    https://git.kernel.org/netdev/net-next/c/3c5a2fd042d0

You are awesome, thank you!
--
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

Re: [net-next] tcp: Sanitize CMSG flags and reserved args in tcp_zerocopy_receive.

[Dan Carpenter]

[-- Attachment #1: Type: text/plain, Size: 3441 bytes --]

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@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.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".


110912bdf28392 Arjun Roy                2021-02-11  4159  			if (err < 1)
110912bdf28392 Arjun Roy                2021-02-11  4160  				return err == 0 ? -EINVAL : err;
c8856c05145490 Arjun Roy                2020-02-14  4161  			len = sizeof(zc);
0b7f41f68710cc Arjun Roy                2020-02-25  4162  			if (put_user(len, optlen))
0b7f41f68710cc Arjun Roy                2020-02-25  4163  				return -EFAULT;
0b7f41f68710cc Arjun Roy                2020-02-25  4164  		}

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org

[-- Attachment #2: .config.gz --]
[-- Type: application/gzip, Size: 29600 bytes --]

Re: [net-next] tcp: Sanitize CMSG flags and reserved args in tcp_zerocopy_receive.

[David Ahern]

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@intel.com>
> Reported-by: Dan Carpenter <dan.carpenter@oracle.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.

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;

Re: [net-next] tcp: Sanitize CMSG flags and reserved args in tcp_zerocopy_receive.

[Dan Carpenter]

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@intel.com>
> > Reported-by: Dan Carpenter <dan.carpenter@oracle.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

Re: [net-next] tcp: Sanitize CMSG flags and reserved args in tcp_zerocopy_receive.

[Arjun Roy]

On Mon, Feb 15, 2021 at 8:02 AM Dan Carpenter <dan.carpenter@oracle.com> wrote:
>
> 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@intel.com>
> > > Reported-by: Dan Carpenter <dan.carpenter@oracle.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
>
>
Thank you for the catch. I will send out a fix momentarily.

Actually, now I'm curious - why does do_tcp_getsockopt get called so
many times, per getsockopt target - rather than just using the
originally read value?

-Arjun