Re: [PATCH v5] mm/oom_kill.c: futex: Close a race between do_exit and the oom_reaper

[Michal Hocko]

On Thu 17-03-22 21:36:21, Nico Pache wrote:
> The pthread struct is allocated on PRIVATE|ANONYMOUS memory [1] which can
> be targeted by the oom reaper. This mapping is used to store the futex
> robust list; the kernel does not keep a copy of the robust list and instead
> references a userspace address to maintain the robustness during a process
> death. A race can occur between exit_mm and the oom reaper that allows
> the oom reaper to free the memory of the futex robust list before the
> exit path has handled the futex death:
> 
>     CPU1                               CPU2
> ------------------------------------------------------------------------
>     page_fault
>     out_of_memory
>     do_exit "signal"
>     wake_oom_reaper
> 					oom_reaper
>                         		oom_reap_task_mm (invalidates mm)
>     exit_mm
>     exit_mm_release
>     futex_exit_release
>     futex_cleanup
>     exit_robust_list
>     get_user (EFAULT- can't access memory)

I still think it is useful to be explicit about the consequences of the
EFAULT here. Did you want to mention that a failing get_user in this
path would result in a hang because nobody is woken up when the current
holder of the lock terminates.

> While in the oom reaper thread, we must handle the futex cleanup without
> sleeping. To achieve this, add the boolean `try` to futex_exit_begin().
> This will control weather or not we use a trylock. Also introduce
> try_futex_exit_release() which will utilize the trylock version of the
> futex_cleanup_begin(). Also call kthread_use_mm in this context to assure
> the get_user call in futex_cleanup() does not return with EFAULT.

This alone is not sufficient. get_user can sleep in the #PF handler path
(e.g. by waiting for swap in). Or is there any guarantee that the page
is never swapped out? If we cannot rule out #PF then this is not a
viable way to address the problem I am afraid.

[...]
> @@ -587,6 +588,18 @@ static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
>  		goto out_unlock;
>  	}
>  
> +	/* We can't reap a process holding a robust_list; the pthread
> +	 * struct is allocated in userspace using PRIVATE | ANONYMOUS
> +	 * memory which when reaped before futex_cleanup() can leave
> +	 * the waiting process stuck. Try to perform the futex_cleanup,
> +	 * and if unsuccessful, skip the OOM reaping.
> +	 */
> +	if (task_has_robust_list(tsk) && !try_futex_exit_release(tsk)) {
> +		trace_skip_task_reaping(tsk->pid);
> +		pr_info("oom_reaper: skipping task as it contains a robust list");
> +		goto out_finish;
> +	}
> +
>  	trace_start_task_reaping(tsk->pid);
>  
>  	/* failed to reap part of the address space. Try again later */

Please also note that this all is done after mmap_lock has been already
taken so a page fault could deadlock on the mmap_lock.

The more I am thinking about this the more I am getting convinced that
we should rather approach this differently and skip over vmas which can
be holding the list. Have you considered this option?
-- 
Michal Hocko
SUSE Labs