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From: peterz@infradead.org
To: Qian Cai <cai@lca.pw>
Cc: mark.rutland@arm.com, akpm@linux-foundation.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next] fork: silence a false postive warning in __mmdrop
Date: Wed, 22 Jul 2020 12:06:37 +0200	[thread overview]
Message-ID: <20200722100637.GR119549@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200604150344.1796-1-cai@lca.pw>

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)
> 

  parent reply	other threads:[~2020-07-22 10:06 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-04 15:03 Qian Cai
2020-07-21 17:30 ` Qian Cai
2020-07-22 10:06 ` peterz [this message]
2020-07-22 13:19   ` Qian Cai
2020-07-22 13:44     ` Peter Zijlstra
2020-09-08 16:50       ` Qian Cai
2020-09-08 16:50         ` Qian Cai
2020-09-08 17:58         ` peterz
2020-09-08 17:58           ` peterz

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