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cgroup: fix exit() vs rmdir() race
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In cgroup_exit() put_css_set_taskexit() is called without any lock,
which might lead to accessing a freed cgroup:

thread1                           thread2
---------------------------------------------
exit()
  cgroup_exit()
    put_css_set_taskexit()
      atomic_dec(cgrp->count);
                                   rmdir();
      /* not safe !! */
      check_for_release(cgrp);

rcu_read_lock() can be used to make sure the cgroup is alive.

Signed-off-by: Li Zefan <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
Cc: [email protected]
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lizf-os authored and htejun committed Feb 18, 2013
1 parent 9ed8a65 commit 71b5707
Showing 1 changed file with 8 additions and 0 deletions.
8 changes: 8 additions & 0 deletions kernel/cgroup.c
Original file line number Diff line number Diff line change
Expand Up @@ -422,12 +422,20 @@ static void __put_css_set(struct css_set *cg, int taskexit)
struct cgroup *cgrp = link->cgrp;
list_del(&link->cg_link_list);
list_del(&link->cgrp_link_list);

/*
* We may not be holding cgroup_mutex, and if cgrp->count is
* dropped to 0 the cgroup can be destroyed at any time, hence
* rcu_read_lock is used to keep it alive.
*/
rcu_read_lock();
if (atomic_dec_and_test(&cgrp->count) &&
notify_on_release(cgrp)) {
if (taskexit)
set_bit(CGRP_RELEASABLE, &cgrp->flags);
check_for_release(cgrp);
}
rcu_read_unlock();

kfree(link);
}
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