From: Peter Zijlstra peterz@infradead.org
commit 1cf8dfe8a661f0462925df943140e9f6d1ea5233 upstream.
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 alexander.shishkin@linux.intel.com Reported-by: syzbot+a24c397a29ad22d86c98@syzkaller.appspotmail.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Acked-by: Mark Rutland mark.rutland@arm.com Cc: stable@vger.kernel.org Cc: Alexander Shishkin alexander.shishkin@linux.intel.com Cc: Arnaldo Carvalho de Melo acme@redhat.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 Fixes: 82d94856fa22 ("perf/core: Fix lock inversion between perf,trace,cpuhp") Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- kernel/events/core.c | 49 +++++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 41 insertions(+), 8 deletions(-)
--- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -4459,12 +4459,20 @@ static void _free_event(struct perf_even 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);
@@ -4644,8 +4652,17 @@ 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: @@ -11506,11 +11523,11 @@ static void perf_free_event(struct perf_ }
/* - * 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) { @@ -11540,7 +11557,23 @@ void perf_event_free_task(struct task_st 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 */ } }