[Patch net] kcm: fix a socket double free

[Cong Wang]

Dmitry reported a double free on kcm socket, which could
be easily reproduced by:

        #include <unistd.h>
        #include <sys/syscall.h>

        int main()
        {
          int fd = syscall(SYS_socket, 0x29ul, 0x5ul, 0x0ul, 0, 0, 0);
          syscall(SYS_ioctl, fd, 0x89e2ul, 0x20a98000ul, 0, 0, 0);
          return 0;
        }

This is because on the error path, after we install
the new socket file, we call sock_release() to clean
up the socket, which leaves the fd pointing to a freed
socket. Fix this by calling sys_close() on that fd
directly.

Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
Reported-by: Dmitry Vyukov <dvyu...@google.com>
Cc: Tom Herbert <t...@herbertland.com>
Signed-off-by: Cong Wang <xiyou.wangc...@gmail.com>
---
 net/kcm/kcmsock.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index cb39e05..4116932 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -13,6 +13,7 @@
 #include <linux/socket.h>
 #include <linux/uaccess.h>
 #include <linux/workqueue.h>
+#include <linux/syscalls.h>
 #include <net/kcm.h>
 #include <net/netns/generic.h>
 #include <net/sock.h>
@@ -2029,7 +2030,7 @@ static int kcm_ioctl(struct socket *sock, unsigned int 
cmd, unsigned long arg)
                        if (copy_to_user((void __user *)arg, &info,
                                         sizeof(info))) {
                                err = -EFAULT;
-                               sock_release(newsock);
+                               sys_close(info.fd);
                        }
                }
 
-- 
1.8.4.5