From 1cf8dfe8a661f0462925df943140e9f6d1ea5233 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Sat, 13 Jul 2019 11:21:25 +0200 Subject: perf/core: Fix race between close() and fork() Syzcaller reported the following Use-after-Free bug: 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. Debugged-by: Alexander Shishkin Reported-by: syzbot+a24c397a29ad22d86c98@syzkaller.appspotmail.com Signed-off-by: Peter Zijlstra (Intel) Acked-by: Mark Rutland Cc: Cc: Alexander Shishkin Cc: Arnaldo Carvalho de Melo Cc: Jiri Olsa Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Stephane Eranian Cc: Thomas Gleixner Cc: Vince Weaver Fixes: 82d94856fa22 ("perf/core: Fix lock inversion between perf,trace,cpuhp") Signed-off-by: Ingo Molnar --- kernel/events/core.c | 49 +++++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 41 insertions(+), 8 deletions(-) (limited to 'kernel/events') diff --git a/kernel/events/core.c b/kernel/events/core.c index 785d708f8553..5dd19bedbf64 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -4465,12 +4465,20 @@ 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); @@ -4650,8 +4658,17 @@ again: 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); } no_ctx: @@ -11527,11 +11544,11 @@ 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) { @@ -11561,7 +11578,23 @@ 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 */ } } -- cgit v1.2.3