Add j2 template for enable pam_limit and limit SSH session (#10177)

#### Why I did it
    When too many user login concurrently and run commands, SONiC may kernel panic on some device which has very limited memory.

#### How I did it
    Add j2 template for setup pam_limit plugin for limit SSH session per-user.

#### How to verify it
    Manually validate the j2 template can generate correct config file.

#### Which release branch to backport (provide reason below if selected)

- [x] 201811
- [ ] 201911
- [ ] 202006
- [x] 202012
- [x] 202106
- [x] 202111

#### Description for the changelog
    Add j2 template for setup pam_limit plugin for limit SSH session per-user.

#### A picture of a cute animal (not mandatory but encouraged)

src/sonic-host-services-data/templates/limits.conf.j2

@@ -0,0 +1,69 @@
+# /etc/security/limits.conf
+#
+# This file generate by j2 template file: src/sonic-host-services-data/templates/limits.conf.j2
+#
+# Each line describes a limit for a user in the form:
+#
+# <domain>        <type>  <item>  <value>
+#
+# Where:
+# <domain> can be:
+#        - a user name
+#        - a group name, with @group syntax
+#        - the wildcard *, for default entry
+#        - the wildcard %, can be also used with %group syntax,
+#                 for maxlogin limit
+#        - NOTE: group and wildcard limits are not applied to root.
+#          To apply a limit to the root user, <domain> must be
+#          the literal username root.
+#
+# <type> can have the two values:
+#        - "soft" for enforcing the soft limits
+#        - "hard" for enforcing hard limits
+#
+# <item> can be one of the following:
+#        - core - limits the core file size (KB)
+#        - data - max data size (KB)
+#        - fsize - maximum filesize (KB)
+#        - memlock - max locked-in-memory address space (KB)
+#        - nofile - max number of open file descriptors
+#        - rss - max resident set size (KB)
+#        - stack - max stack size (KB)
+#        - cpu - max CPU time (MIN)
+#        - nproc - max number of processes
+#        - as - address space limit (KB)
+#        - maxlogins - max number of logins for this user
+#        - maxsyslogins - max number of logins on the system
+#        - priority - the priority to run user process with
+#        - locks - max number of file locks the user can hold
+#        - sigpending - max number of pending signals
+#        - msgqueue - max memory used by POSIX message queues (bytes)
+#        - nice - max nice priority allowed to raise to values: [-20, 19]
+#        - rtprio - max realtime priority
+#        - chroot - change root to directory (Debian-specific)
+#
+#
+# <value> is related with <item>:
+#          All items support the values -1, unlimited or infinity indicating
+#          no limit, except for priority and nice.
+#
+#          If a hard limit or soft limit of a resource is set to a valid value,
+#          but outside of the supported range of the local system, the system
+#          may reject the new limit or unexpected behavior may occur. If the
+#          control value required is used, the module will reject the login if
+#          a limit could not be set.
+#
+# <domain>      <type>  <item>         <value>
+#
+
+# *               soft    core            0
+# root            hard    core            100000
+# *               hard    rss             10000
+# @student        hard    nproc           20
+# @faculty        soft    nproc           20
+# @faculty        hard    nproc           50
+# ftp             hard    nproc           0
+# ftp             -       chroot          /ftp
+# @student        -       maxlogins       4
+
+# End of file

src/sonic-host-services-data/templates/pam_limits.j2

@@ -0,0 +1,12 @@
+#THIS IS AN AUTO-GENERATED FILE
+#
+# This file generate by j2 template file: src/sonic-host-services-data/templates/pam_limits.j2
+#
+# /etc/pam.d/pam-limits settings common to all services
+# This file is included from other service-specific PAM config files,
+# and should contain a list of the authentication modules that define
+# the central authentication scheme for use on the system
+# (e.g., /etc/shadow, LDAP, Kerberos, etc.). The default is to use the
+# traditional Unix authentication mechanisms.
+#
+# here are the per-package modules (the "Primary" block)

src/sonic-host-services/scripts/hostcfgd

@@ -25,6 +25,10 @@ PAM_RADIUS_AUTH_CONF_TEMPLATE = "/usr/share/sonic/templates/pam_radius_auth.conf
 NSS_CONF = "/etc/nsswitch.conf"
 ETC_PAMD_SSHD = "/etc/pam.d/sshd"
 ETC_PAMD_LOGIN = "/etc/pam.d/login"
+PAM_LIMITS_CONF_TEMPLATE = "/usr/share/sonic/templates/pam_limits.j2"
+LIMITS_CONF_TEMPLATE = "/usr/share/sonic/templates/limits.conf.j2"
+PAM_LIMITS_CONF = "/etc/pam.d/pam-limits-conf"
+LIMITS_CONF = "/etc/security/limits.conf"
 
 # TACACS+
 TACPLUS_SERVER_PASSKEY_DEFAULT = ""
@@ -966,6 +970,64 @@ class NtpCfg(object):
             syslog.syslog(syslog.LOG_INFO, 'ntp server update, restarting ntp-config, ntp servers configured {}'.format(self.ntp_servers))
             run_cmd(cmd)
 
+class PamLimitsCfg(object):
+    """
+    PamLimit Config Daemon
+    1) The pam_limits PAM module sets limits on the system resources that can be obtained in a user-session.
+    2) Purpose of this daemon is to render pam_limits config file.
+    """
+    def __init__(self, config_db):
+        self.config_db = config_db
+        self.hwsku = ""
+        self.type = ""
+
+    # Load config from ConfigDb and render config file/
+    def update_config_file(self):
+        device_metadata = self.config_db.get_table('DEVICE_METADATA')
+        if "localhost" not in device_metadata:
+            return
+
+        self.read_localhost_config(device_metadata["localhost"])
+        self.render_conf_file()
+
+    # Read localhost config
+    def read_localhost_config(self, localhost):
+        if "hwsku" in localhost:
+            self.hwsku = localhost["hwsku"]
+        else:
+            self.hwsku = ""
+
+        if "type" in localhost:
+            self.type = localhost["type"]
+        else:
+            self.type = ""
+
+    # Render pam_limits config files
+    def render_conf_file(self):
+        env = jinja2.Environment(loader=jinja2.FileSystemLoader('/'), trim_blocks=True)
+        env.filters['sub'] = sub
+
+        try:
+            template_file = os.path.abspath(PAM_LIMITS_CONF_TEMPLATE)
+            template = env.get_template(template_file)
+            pam_limits_conf = template.render(
+                                        hwsku=self.hwsku,
+                                        type=self.type)
+            with open(PAM_LIMITS_CONF, 'w') as f:
+                f.write(pam_limits_conf)
+
+            template_file = os.path.abspath(LIMITS_CONF_TEMPLATE)
+            template = env.get_template(template_file)
+            limits_conf = template.render(
+                                        hwsku=self.hwsku,
+                                        type=self.type)
+            with open(LIMITS_CONF, 'w') as f:
+                f.write(limits_conf)
+        except Exception as e:
+            syslog.syslog(syslog.LOG_ERR,
+                    "modify pam_limits config file failed with exception: {}"
+                    .format(e))
+
 class HostConfigDaemon:
     def __init__(self):
         # Just a sanity check to verify if the CONFIG_DB has been initialized
@@ -1007,6 +1069,9 @@ class HostConfigDaemon:
         self.hostname_cache=""
         self.aaacfg = AaaCfg()
 
+        # Initialize PamLimitsCfg
+        self.pamLimitsCfg = PamLimitsCfg(self.config_db)
+        self.pamLimitsCfg.update_config_file()
 
     def load(self):
         aaa = self.config_db.get_table('AAA')