Re: [PATCH -next v2] locking/percpu-rwsem: fix a task_struct refcount

[Qian Cai]

> On Mar 30, 2020, at 5:30 PM, Qian Cai <cai@xxxxxx> wrote:
> 
> The commit 7f26482a872c ("locking/percpu-rwsem: Remove the embedded
> rwsem") introduced some task_struct memory leaks due to messing up with
> a task_struct refcount. At the beginning of
> percpu_rwsem_wake_function(), it calls get_task_struct(), but if the
> trylock failed, it will remain in the waitqueue. However, it will run
> percpu_rwsem_wake_function() again with get_task_struct() to increase
> the refcount but then only call put_task_struct() once the trylock
> succeeded.
> 
> Fix it by adjusting percpu_rwsem_wake_function() a bit to guard against
> when percpu_rwsem_wait() observing !private, terminating the wait and
> doing a quick exit() while percpu_rwsem_wake_function() then doing
> wake_up_process(p) as a use-after-free.
> 
> Fixes: 7f26482a872c ("locking/percpu-rwsem: Remove the embedded rwsem")

Peter, Ingo, can you take a look at this patch when you have a chance?

For some reasons Ingo had decided to send a pull request which is now merged
even though I had informed the commit was broken a few days earlier, it makes no
sense to leave known memory leaks in mainline like this.

> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Qian Cai <cai@xxxxxx>
> ---
> kernel/locking/percpu-rwsem.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
> index a008a1ba21a7..8bbafe3e5203 100644
> --- a/kernel/locking/percpu-rwsem.c
> +++ b/kernel/locking/percpu-rwsem.c
> @@ -118,14 +118,15 @@ static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
> 				      unsigned int mode, int wake_flags,
> 				      void *key)
> {
> -	struct task_struct *p = get_task_struct(wq_entry->private);
> 	bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
> 	struct percpu_rw_semaphore *sem = key;
> +	struct task_struct *p;
> 
> 	/* concurrent against percpu_down_write(), can get stolen */
> 	if (!__percpu_rwsem_trylock(sem, reader))
> 		return 1;
> 
> +	p = get_task_struct(wq_entry->private);
> 	list_del_init(&wq_entry->entry);
> 	smp_store_release(&wq_entry->private, NULL);
> 
> -- 
> 2.21.0 (Apple Git-122.2)
>