Re: [PATCH -next] fork: silence a false postive warning in __mmdrop

[peterz@infradead.org]

On Thu, Jun 04, 2020 at 11:03:44AM -0400, Qian Cai wrote:
> The linux-next commit bf2c59fce407 ("sched/core: Fix illegal RCU from
> offline CPUs") delayed,
> 
> idle->active_mm = &init_mm;
> 
> into finish_cpu() instead of idle_task_exit() which results in a false
> positive warning that was originally designed in the commit 3eda69c92d47
> ("kernel/fork.c: detect early free of a live mm").
> 
>  WARNING: CPU: 127 PID: 72976 at kernel/fork.c:697
>  __mmdrop+0x230/0x2c0
>  do_exit+0x424/0xfa0
>  Call Trace:
>  do_exit+0x424/0xfa0
>  do_group_exit+0x64/0xd0
>  sys_exit_group+0x24/0x30
>  system_call_exception+0x108/0x1d0
>  system_call_common+0xf0/0x278

Please explain; because afaict this is a use-after-free.

The thing is __mmdrop() is going to actually free the mm, so then what
is finish_cpu()'s mmdrop() going to do?

->active_mm() should have a refcount on the mm.

> Fixes: bf2c59fce407 ("sched/core: Fix illegal RCU from offline CPUs")
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
>  kernel/fork.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 142b23645d82..5334efd2a680 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -694,7 +694,6 @@ void __mmdrop(struct mm_struct *mm)
>  {
>  	BUG_ON(mm == &init_mm);
>  	WARN_ON_ONCE(mm == current->mm);
> -	WARN_ON_ONCE(mm == current->active_mm);
>  	mm_free_pgd(mm);
>  	destroy_context(mm);
>  	mmu_notifier_subscriptions_destroy(mm);
> -- 
> 2.21.0 (Apple Git-122.2)
>