Messages in this thread Patch in this message | | | From | Arnaldo Carvalho de Melo <> | Date | Thu, 4 Jul 2019 16:43:55 -0300 | Subject | Re: [PATCH v2] fix use-after-free in perf_sched__lat |
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Em Thu, Jul 04, 2019 at 07:21:28PM +0800, liwei (GF) escreveu: > Hi Arnaldo, > I found this issue has not been fixed in mainline now, please take a glance at this.
See below.
> On 2019/5/23 10:50, Namhyung Kim wrote: > > On Wed, May 22, 2019 at 08:08:23AM -0300, Arnaldo Carvalho de Melo wrote: > >> I'll try to analyse this one soon, but my first impression was that we > >> should just grab reference counts when keeping a pointer to those > >> threads instead of keeping _all_ threads alive when supposedly we could > >> trow away unreferenced data structures.
> >> But this is just a first impression from just reading the patch > >> description, probably I'm missing something.
> > No, thread refcounting is fine. We already did it and threads with the > > refcount will be accessed only.
> > But the problem is the head of the list. After using the thread, the > > refcount is gone and thread is removed from the list and destroyed. > > However the head of list is in a struct machine which was freed with > > session already.
I see, and sorry for the delay, thanks for bringing this up again, how about the following instead? I tested it with 'perf top' that exercises this code in a multithreaded fashion as well with the set of steps in your patch commit log, seems to work.
- Arnaldo
diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c index 86fede2b7507..cf826eca3aaf 100644 --- a/tools/perf/util/machine.c +++ b/tools/perf/util/machine.c @@ -210,6 +210,18 @@ void machine__exit(struct machine *machine) for (i = 0; i < THREADS__TABLE_SIZE; i++) { struct threads *threads = &machine->threads[i]; + struct thread *thread, *n; + /* + * Forget about the dead, at this point whatever threads were + * left in the dead lists better have a reference count taken + * by who is using them, and then, when they drop those references + * and it finally hits zero, thread__put() will check and see that + * its not in the dead threads list and will not try to remove it + * from there, just calling thread__delete() straight away. + */ + list_for_each_entry_safe(thread, n, &threads->dead, node) + list_del_init(&thread->node); + exit_rwsem(&threads->lock); } } @@ -1759,9 +1771,11 @@ static void __machine__remove_thread(struct machine *machine, struct thread *th, if (threads->last_match == th) threads__set_last_match(threads, NULL); - BUG_ON(refcount_read(&th->refcnt) == 0); if (lock) down_write(&threads->lock); + + BUG_ON(refcount_read(&th->refcnt) == 0); + rb_erase_cached(&th->rb_node, &threads->entries); RB_CLEAR_NODE(&th->rb_node); --threads->nr; @@ -1771,9 +1785,16 @@ static void __machine__remove_thread(struct machine *machine, struct thread *th, * will be called and we will remove it from the dead_threads list. */ list_add_tail(&th->node, &threads->dead); + + /* + * We need to do the put here because if this is the last refcount, + * then we will be touching the threads->dead head when removing the + * thread. + */ + thread__put(th); + if (lock) up_write(&threads->lock); - thread__put(th); } void machine__remove_thread(struct machine *machine, struct thread *th) diff --git a/tools/perf/util/thread.c b/tools/perf/util/thread.c index 4db8cd2a33ae..873ab505ca80 100644 --- a/tools/perf/util/thread.c +++ b/tools/perf/util/thread.c @@ -125,10 +125,27 @@ void thread__put(struct thread *thread) { if (thread && refcount_dec_and_test(&thread->refcnt)) { /* - * Remove it from the dead_threads list, as last reference - * is gone. + * Remove it from the dead threads list, as last reference is + * gone, if it is in a dead threads list. + * + * We may not be there anymore if say, the machine where it was + * stored was already deleted, so we already removed it from + * the dead threads and some other piece of code still keeps a + * reference. + * + * This is what 'perf sched' does and finally drops it in + * perf_sched__lat(), where it calls perf_sched__read_events(), + * that processes the events by creating a session and deleting + * it, which ends up destroying the list heads for the dead + * threads, but before it does that it removes all threads from + * it using list_del_init(). + * + * So we need to check here if it is in a dead threads list and + * if so, remove it before finally deleting the thread, to avoid + * an use after free situation. */ - list_del_init(&thread->node); + if (!list_empty(&thread->node)) + list_del_init(&thread->node); thread__delete(thread); } }
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