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    Subject[PATCH 3.16 287/306] perf: Fix race in swevent hash
    3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

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

    From: Peter Zijlstra <peterz@infradead.org>

    commit 12ca6ad2e3a896256f086497a7c7406a547ee373 upstream.

    There's a race on CPU unplug where we free the swevent hash array
    while it can still have events on. This will result in a
    use-after-free which is BAD.

    Simply do not free the hash array on unplug. This leaves the thing
    around and no use-after-free takes place.

    When the last swevent dies, we do a for_each_possible_cpu() iteration
    anyway to clean these up, at which time we'll free it, so no leakage
    will occur.

    Reported-by: Sasha Levin <sasha.levin@oracle.com>
    Tested-by: Sasha Levin <sasha.levin@oracle.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Jiri Olsa <jolsa@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Stephane Eranian <eranian@google.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Vince Weaver <vincent.weaver@maine.edu>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    kernel/events/core.c | 20 +-------------------
    1 file changed, 1 insertion(+), 19 deletions(-)

    --- a/kernel/events/core.c
    +++ b/kernel/events/core.c
    @@ -5595,9 +5595,6 @@ struct swevent_htable {

    /* Recursion avoidance in each contexts */
    int recursion[PERF_NR_CONTEXTS];
    -
    - /* Keeps track of cpu being initialized/exited */
    - bool online;
    };

    static DEFINE_PER_CPU(struct swevent_htable, swevent_htable);
    @@ -5844,14 +5841,8 @@ static int perf_swevent_add(struct perf_
    hwc->state = !(flags & PERF_EF_START);

    head = find_swevent_head(swhash, event);
    - if (!head) {
    - /*
    - * We can race with cpu hotplug code. Do not
    - * WARN if the cpu just got unplugged.
    - */
    - WARN_ON_ONCE(swhash->online);
    + if (WARN_ON_ONCE(!head))
    return -EINVAL;
    - }

    hlist_add_head_rcu(&event->hlist_entry, head);

    @@ -5918,7 +5909,6 @@ static int swevent_hlist_get_cpu(struct
    int err = 0;

    mutex_lock(&swhash->hlist_mutex);
    -
    if (!swevent_hlist_deref(swhash) && cpu_online(cpu)) {
    struct swevent_hlist *hlist;

    @@ -8050,7 +8040,6 @@ static void perf_event_init_cpu(int cpu)
    struct swevent_htable *swhash = &per_cpu(swevent_htable, cpu);

    mutex_lock(&swhash->hlist_mutex);
    - swhash->online = true;
    if (swhash->hlist_refcount > 0) {
    struct swevent_hlist *hlist;

    @@ -8103,14 +8092,7 @@ static void perf_event_exit_cpu_context(

    static void perf_event_exit_cpu(int cpu)
    {
    - struct swevent_htable *swhash = &per_cpu(swevent_htable, cpu);
    -
    perf_event_exit_cpu_context(cpu);
    -
    - mutex_lock(&swhash->hlist_mutex);
    - swhash->online = false;
    - swevent_hlist_release(swhash);
    - mutex_unlock(&swhash->hlist_mutex);
    }
    #else
    static inline void perf_event_exit_cpu(int cpu) { }
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