tree ce3f4d5c76202e62a1f6382b5eb618457700b21f
parent cfb34ca17cbe54514f3a77cf37afea68d2a875cc
author Vernon Mauery <vernon.mauery@linux.intel.com> 1623527543 -0700
committer Vernon Mauery <vernon.mauery@linux.intel.com> 1623981688 -0700

Be more proactive at removing stale sessions

The maximum number of sessions is set to limit total resources that
netipmid is allowed to use. But it also opens a door to DoS attacks that
would use up all the sessions and then never close them. This new
mechanism will allow extra sessions, especially if they are short and
active. As the number of sessions grows beyond the desired maximum, the
reaping time becomes shorter and shorter to ensure that only actual
active sessions are kept.

This introduces a variable max idle time that starts at 60s, according
to the IPMI spec, for up to the desired maximum number of sessions per
channel (currently 15). Beyond 15 sessions, The idle time is reduced
proportionally to the inverse^3 of the number of sessions beyond the
desired maximum.

Some sample maximum idle times for active sessions this new scheme:
  Idle time for up to 15 sessions stays at 60s
  Idle time for 16 sessions is reduced to 7.5s
  Idle time for 20 sessions is reduced to 277ms
  Idle time for 24 sessions is reduced to 60ms

For sessions in setup, the idle times are calculated the same as for
active sessions, but use the full session count (active and setup) and
are limited to a maximum idle time of 3 seconds.

One other feature added is to schedule session cleaning when a Close
Session command is received. Without this, sessions that are in the
shutDownPending state would live on for much longer than needed. Really,
the session only needs to live long enough to prepare the response
message, but curretly there is no mechanism to remove just that one
session from that context.

Tested: Open lots of sessions and wait for them to get reaped
      $ for ((i=0; i<16; i++)); do \
          ipmitool -C 17 -I lanplus -H $HOST -U $USR -P $PW sensor list & \
        done
      $ for ((i=0; i<10; i++)); do \
          ipmitool -C 17 -I lanplus -H $HOST -U $USR -P $PW mc info & \
        done

      In this case, the first 16 sessions will open just fine, but with
      a slightly shorted idle time (no problems). The next ten sessions
      may or may not all get to open, because the number of setup
      sessions open simultaneously will severely limit the idle time of
      the setup sessions, causing some of them to fail to fully open.

Change-Id: Iae2e68c7192f3f5a2cafa8e825aa025454405c84
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>