From 399f8dd9a866e107639eabd3c1979cd526ca3a98 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Tue, 22 Jun 2021 01:08:30 +0200 Subject: [PATCH] signal: Prevent sigqueue caching after task got released syzbot reported a memory leak related to sigqueue caching. The assumption that a task cannot cache a sigqueue after the signal handler has been dropped and exit_task_sigqueue_cache() has been invoked turns out to be wrong. Such a task can still invoke release_task(other_task), which cleans up the signals of 'other_task' and ends up in sigqueue_cache_or_free(), which in turn will cache the signal because task->sigqueue_cache is NULL. That's obviously bogus because nothing will free the cached signal of that task anymore, so the cached item is leaked. This happens when e.g. the last non-leader thread exits and reaps the zombie leader. Prevent this by setting tsk::sigqueue_cache to an error pointer value in exit_task_sigqueue_cache() which forces any subsequent invocation of sigqueue_cache_or_free() from that task to hand the sigqueue back to the kmemcache. Add comments to all relevant places. Fixes: 4bad58ebc8bc ("signal: Allow tasks to cache one sigqueue struct") Reported-by: syzbot+0bac5fec63d4f399ba98@syzkaller.appspotmail.com Signed-off-by: Thomas Gleixner Reviewed-by: Oleg Nesterov Acked-by: Christian Brauner Link: https://lore.kernel.org/r/878s32g6j5.ffs@nanos.tec.linutronix.de --- kernel/signal.c | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) diff --git a/kernel/signal.c b/kernel/signal.c index f7c6ffcbd044..f1ecd8f0c11d 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -435,6 +435,12 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t gfp_flags, * Preallocation does not hold sighand::siglock so it can't * use the cache. The lockless caching requires that only * one consumer and only one producer run at a time. + * + * For the regular allocation case it is sufficient to + * check @q for NULL because this code can only be called + * if the target task @t has not been reaped yet; which + * means this code can never observe the error pointer which is + * written to @t->sigqueue_cache in exit_task_sigqueue_cache(). */ q = READ_ONCE(t->sigqueue_cache); if (!q || sigqueue_flags) @@ -463,13 +469,18 @@ void exit_task_sigqueue_cache(struct task_struct *tsk) struct sigqueue *q = tsk->sigqueue_cache; if (q) { - tsk->sigqueue_cache = NULL; /* * Hand it back to the cache as the task might * be self reaping which would leak the object. */ kmem_cache_free(sigqueue_cachep, q); } + + /* + * Set an error pointer to ensure that @tsk will not cache a + * sigqueue when it is reaping it's child tasks + */ + tsk->sigqueue_cache = ERR_PTR(-1); } static void sigqueue_cache_or_free(struct sigqueue *q) @@ -481,6 +492,10 @@ static void sigqueue_cache_or_free(struct sigqueue *q) * is intentional when run without holding current->sighand->siglock, * which is fine as current obviously cannot run __sigqueue_free() * concurrently. + * + * The NULL check is safe even if current has been reaped already, + * in which case exit_task_sigqueue_cache() wrote an error pointer + * into current->sigqueue_cache. */ if (!READ_ONCE(current->sigqueue_cache)) WRITE_ONCE(current->sigqueue_cache, q);