• Shakeel Butt's avatar
    mm, oom: fix use-after-free in oom_kill_process · 43f7e8be
    Shakeel Butt authored
    commit cefc7ef3c87d02fc9307835868ff721ea12cc597 upstream.
    
    Syzbot instance running on upstream kernel found a use-after-free bug in
    oom_kill_process.  On further inspection it seems like the process
    selected to be oom-killed has exited even before reaching
    read_lock(&tasklist_lock) in oom_kill_process().  More specifically the
    tsk->usage is 1 which is due to get_task_struct() in oom_evaluate_task()
    and the put_task_struct within for_each_thread() frees the tsk and
    for_each_thread() tries to access the tsk.  The easiest fix is to do
    get/put across the for_each_thread() on the selected task.
    
    Now the next question is should we continue with the oom-kill as the
    previously selected task has exited? However before adding more
    complexity and heuristics, let's answer why we even look at the children
    of oom-kill selected task? The select_bad_process() has already selected
    the worst process in the system/memcg.  Due to race, the selected
    process might not be the worst at the kill time but does that matter?
    The userspace can use the oom_score_adj interface to prefer children to
    be killed before the parent.  I looked at the history but it seems like
    this is there before git history.
    
    Link: http://lkml.kernel.org/r/20190121215850.221745-1-shakeelb@google.com
    Reported-by: syzbot+7fbbfa368521945f0e3d@syzkaller.appspotmail.com
    Fixes: 6b0c81b3 ("mm, oom: reduce dependency on tasklist_lock")
    Signed-off-by: 's avatarShakeel Butt <shakeelb@google.com>
    Reviewed-by: 's avatarRoman Gushchin <guro@fb.com>
    Acked-by: 's avatarMichal Hocko <mhocko@suse.com>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    43f7e8be