Re: [PATCH] perf: Fix race between close() and fork()

Mark Rutland
2019-06-28T16:50:12Z

On Tue, Jun 25, 2019 at 12:43:20PM +0200, Peter Zijlstra wrote:
>
> Syzcaller reported the following Use-after-Free issue:
>
> close() clone()
>
> copy_process()
> perf_event_init_task()
> perf_event_init_context()
> mutex_lock(parent_ctx->mutex)
> inherit_task_group()
> inherit_group()
> inherit_event()
> mutex_lock(event->child_mutex)
> // expose event on child list
> list_add_tail()
> mutex_unlock(event->child_mutex)
> mutex_unlock(parent_ctx->mutex)
>
> ...
> goto bad_fork_*
>
> bad_fork_cleanup_perf:
> perf_event_free_task()
>
> perf_release()
> perf_event_release_kernel()
> list_for_each_entry()
> mutex_lock(ctx->mutex)
> mutex_lock(event->child_mutex)
> // event is from the failing inherit
> // on the other CPU
> perf_remove_from_context()
> list_move()
> mutex_unlock(event->child_mutex)
> mutex_unlock(ctx->mutex)
>
> mutex_lock(ctx->mutex)
> list_for_each_entry_safe()
> // event already stolen
> mutex_unlock(ctx->mutex)
>
> delayed_free_task()
> free_task()
>
> list_for_each_entry_safe()
> list_del()
> free_event()
> _free_event()
> // and so event->hw.target
> // is the already freed failed clone()
> if (event->hw.target)
> put_task_struct(event->hw.target)
> // WHOOPSIE, already quite dead
>
>
> Which puts the lie to the the comment on perf_event_free_task():
> 'unexposed, unused context' not so much.
>
> Which is a 'fun' confluence of fail; copy_process() doing an
> unconditional free_task() and not respecting refcounts, and perf having
> creative locking. In particular:
>
> 82d94856fa22 ("perf/core: Fix lock inversion between perf,trace,cpuhp")
>
> seems to have overlooked this 'fun' parade.
>
> Solve it by using the fact that detached events still have a reference
> count on their (previous) context. With this perf_event_free_task()
> can detect when events have escaped and wait for their destruction.
>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Fixes: 82d94856fa22 ("perf/core: Fix lock inversion between perf,trace,cpuhp")
> Reported-by: syzbot+a24c397a29ad22d86c98@syzkaller.appspotmail.com
> Debugged-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> kernel/events/core.c | 49 +++++++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 41 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 10c1dba9068c..5302c19e9892 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4463,12 +4463,20 @@ static void _free_event(struct perf_event *event)
> if (event->destroy)
> event->destroy(event);
>
> - if (event->ctx)
> - put_ctx(event->ctx);
> -
> + /*
> + * Must be after ->destroy(), due to uprobe_perf_close() using
> + * hw.target.
> + */
> if (event->hw.target)
> put_task_struct(event->hw.target);
>
> + /*
> + * perf_event_free_task() relies on put_ctx() being 'last', in particular
> + * all task references must be cleaned up.
> + */
> + if (event->ctx)
> + put_ctx(event->ctx);
> +
> exclusive_event_destroy(event);
> module_put(event->pmu->module);
>
> @@ -4648,8 +4656,17 @@ int perf_event_release_kernel(struct perf_event *event)
> mutex_unlock(&event->child_mutex);
>
> list_for_each_entry_safe(child, tmp, &free_list, child_list) {
> + void *var = &child->ctx->refcount;
> +
> list_del(&child->child_list);
> free_event(child);
> +
> + /*
> + * Wake any perf_event_free_task() waiting for this event to be
> + * freed.
> + */
> + smp_mb(); /* pairs with wait_var_event() */
> + wake_up_var(var);

Huh, so wake_up_var() doesn't imply a RELEASE?

As an aside, doesn't that mean all callers of wake_up_var() have to do
likewise to ensure it isn't re-ordered with whatever prior stuff they're
trying to notify waiters about? Several do an smp_store_release() then a
wake_up_var(), but IIUC the wake_up_var() could get pulled before that
release...

I'm likely missing a subtlety there (I guess in practice the
implementation of wake_up_var prevents that), or maybe I have more
vocabulary than I have a clue. ;)

Other than that tangent, this looks sane to me, so FWIW:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Ideally, we'd throw some CPU hours at testing this. I can drop this in
my fuzzing queue next week, but I don't have a reproducer that I can
soak-test this with. :(

Thanks,
Mark.

> }
>
> no_ctx:
> @@ -11512,11 +11529,11 @@ static void perf_free_event(struct perf_event *event,
> }
>
> /*
> - * Free an unexposed, unused context as created by inheritance by
> - * perf_event_init_task below, used by fork() in case of fail.
> + * Free a context as created by inheritance by perf_event_init_task() below,
> + * used by fork() in case of fail.
> *
> - * Not all locks are strictly required, but take them anyway to be nice and
> - * help out with the lockdep assertions.
> + * Even though the task has never lived, the context and events have been
> + * exposed through the child_list, so we must take care tearing it all down.
> */
> void perf_event_free_task(struct task_struct *task)
> {
> @@ -11546,7 +11563,23 @@ void perf_event_free_task(struct task_struct *task)
> perf_free_event(event, ctx);
>
> mutex_unlock(&ctx->mutex);
> - put_ctx(ctx);
> +
> + /*
> + * perf_event_release_kernel() could've stolen some of our
> + * child events and still have them on its free_list. In that
> + * case we must wait for these events to have been freed (in
> + * particular all their references to this task must've been
> + * dropped).
> + *
> + * Without this copy_process() will unconditionally free this
> + * task (irrespective of its reference count) and
> + * _free_event()'s put_task_struct(event->hw.target) will be a
> + * use-after-free.
> + *
> + * Wait for all events to drop their context reference.
> + */
> + wait_var_event(&ctx->refcount, refcount_read(&ctx->refcount) == 1);
> + put_ctx(ctx); /* must be last */
> }
> }
>
>