From 6a3351b612b72c558910c88a43e2ef6d7d68bc97 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Mon, 25 Jan 2016 14:09:54 +0100 Subject: [PATCH] perf: Fix race in perf_event_exit_task_context() There is a race between perf_event_exit_task_context() and orphans_remove_work() which results in a use-after-free. We mark ctx->task with TASK_TOMBSTONE to indicate a context is 'dead', under ctx->lock. After which point event_function_call() on any event of that context will NOP A concurrent orphans_remove_work() will only hold ctx->mutex for the list iteration and not serialize against this. Therefore its possible that orphans_remove_work()'s perf_remove_from_context() call will fail, but we'll continue to free the event, with the result of free'd memory still being on lists and everything. Once perf_event_exit_task_context() gets around to acquiring ctx->mutex it too will iterate the event list, encounter the already free'd event and proceed to free it _again_. This fails with the WARN in free_event(). Plug the race by having perf_event_exit_task_context() hold ctx::mutex over the whole tear-down, thereby 'naturally' serializing against all other sites, including the orphan work. Signed-off-by: Peter Zijlstra (Intel) Cc: Arnaldo Carvalho de Melo Cc: Jiri Olsa Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Stephane Eranian Cc: Thomas Gleixner Cc: Vince Weaver Cc: alexander.shishkin@linux.intel.com Cc: dsahern@gmail.com Cc: namhyung@kernel.org Link: http://lkml.kernel.org/r/20160125130954.GY6357@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar --- kernel/events/core.c | 50 +++++++++++++++++++++++++------------------- 1 file changed, 29 insertions(+), 21 deletions(-) diff --git a/kernel/events/core.c b/kernel/events/core.c index 6759f2a332d7..1d243fadfd12 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -8748,14 +8748,40 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn) { struct perf_event_context *child_ctx, *clone_ctx = NULL; struct perf_event *child_event, *next; - unsigned long flags; WARN_ON_ONCE(child != current); - child_ctx = perf_lock_task_context(child, ctxn, &flags); + child_ctx = perf_pin_task_context(child, ctxn); if (!child_ctx) return; + /* + * In order to reduce the amount of tricky in ctx tear-down, we hold + * ctx::mutex over the entire thing. This serializes against almost + * everything that wants to access the ctx. + * + * The exception is sys_perf_event_open() / + * perf_event_create_kernel_count() which does find_get_context() + * without ctx::mutex (it cannot because of the move_group double mutex + * lock thing). See the comments in perf_install_in_context(). + * + * We can recurse on the same lock type through: + * + * __perf_event_exit_task() + * sync_child_event() + * put_event() + * mutex_lock(&ctx->mutex) + * + * But since its the parent context it won't be the same instance. + */ + mutex_lock(&child_ctx->mutex); + + /* + * In a single ctx::lock section, de-schedule the events and detach the + * context from the task such that we cannot ever get it scheduled back + * in. + */ + raw_spin_lock_irq(&child_ctx->lock); task_ctx_sched_out(__get_cpu_context(child_ctx), child_ctx); /* @@ -8767,14 +8793,8 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn) WRITE_ONCE(child_ctx->task, TASK_TOMBSTONE); put_task_struct(current); /* cannot be last */ - /* - * If this context is a clone; unclone it so it can't get - * swapped to another process while we're removing all - * the events from it. - */ clone_ctx = unclone_ctx(child_ctx); - update_context_time(child_ctx); - raw_spin_unlock_irqrestore(&child_ctx->lock, flags); + raw_spin_unlock_irq(&child_ctx->lock); if (clone_ctx) put_ctx(clone_ctx); @@ -8786,18 +8806,6 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn) */ perf_event_task(child, child_ctx, 0); - /* - * We can recurse on the same lock type through: - * - * __perf_event_exit_task() - * sync_child_event() - * put_event() - * mutex_lock(&ctx->mutex) - * - * But since its the parent context it won't be the same instance. - */ - mutex_lock(&child_ctx->mutex); - list_for_each_entry_safe(child_event, next, &child_ctx->event_list, event_entry) __perf_event_exit_task(child_event, child_ctx, child); -- 2.20.1