[Bug 219701] crash in camperiphfree()

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=219701

--- Comment #9 from commit-hook at freebsd.org ---
A commit references this bug:

Author: ken
Date: Mon Jul  3 18:20:45 UTC 2017
New revision: 320608
URL: https://svnweb.freebsd.org/changeset/base/320608

Log:
  Merge r320602 from stable/11 into releng/11.1:
    ------------------------------------------------------------------------
    r320602 | ken | 2017-07-03 09:34:21 -0600 (Mon, 03 Jul 2017) | 45 lines

    MFC r320421:

      ------------------------------------------------------------------------
      r320421 | ken | 2017-06-27 13:26:02 -0600 (Tue, 27 Jun 2017) | 37 lines

      Fix a panic in camperiphfree().

      If a peripheral driver (e.g. da, sa, cd) is added or removed from the
      peripheral driver list while an unrelated peripheral driver instance
(e.g.
      da0, sa5, cd2) is going away and is inside camperiphfree(), we could
      dereference an invalid pointer.

      When peripheral drivers are added or removed (see periphdriver_register()
      and periphdriver_unregister()), the peripheral driver array is resized
      and existing entries are moved.

      Although we hold the topology lock while we traverse the peripheral
driver
      list, we retain a pointer to the location of the peripheral driver
pointer
      and then drop the topology lock.  So we are still vulnerable to the list
      getting moved around while the lock is dropped.

      To solve the problem, cache a copy of the peripheral driver pointer.  If
      its storage location in the list changes while we have the lock dropped,
it
      won't have any effect.

      This doesn't solve the issue that peripheral drivers ("da", "cd", as
opposed
      to individual instances like "da0", "cd0") are not generally part of a
      reference counting scheme to guard against deregistering them while there
      are instances active.  The caller (generally the person unloading a
module)
      has to be aware of active drivers and not unload something that is in
use.

      sys/cam/cam_periph.c:
        In camperiphfree(), cache a pointer to the peripheral driver
        instance to avoid holding a pointer to an invalid memory location
        in the event that the peripheral driver list changes while we have
        the topology lock dropped.

      PR:               kern/219701
      Submitted by:     avg
      Sponsored by:     Spectra Logic

      ------------------------------------------------------------------------
    ------------------------------------------------------------------------

  Approved by:  re (gjb)

Changes:
_U  releng/11.1/
  releng/11.1/sys/cam/cam_periph.c

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