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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     "Nicholas Piggin" <npiggin@gmail.com>
Cc:     <linux-kernel@vger.kernel.org>,
        "Michael Ellerman" <mpe@ellerman.id.au>
References: <20221004094401.708299-1-npiggin@gmail.com>
        <CP6GJNNB62LY.N38DBL6D2S79@bobo>
Date:   Fri, 23 Dec 2022 22:28:27 -0600
In-Reply-To: <CP6GJNNB62LY.N38DBL6D2S79@bobo> (Nicholas Piggin's message of
        "Tue, 20 Dec 2022 17:14:05 +1000")
Message-ID: <874jtlo26s.fsf@email.froward.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)
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Content-Type: text/plain
Subject: Re: [PATCH] exit: Detect and fix irq disabled state in oops
Precedence: bulk

"Nicholas Piggin" <npiggin@gmail.com> writes:

> On Tue Oct 4, 2022 at 7:44 PM AEST, Nicholas Piggin wrote:
>> If a task oopses with irqs disabled, this can cause various cascading
>> problems in the oops path such as sleep-from-invalid warnings, and
>> potentially worse.
>>
>> Since commit 0258b5fd7c712 ("coredump: Limit coredumps to a single
>> thread group"), the unconditional irq enable in coredump_task_exit()
>> will "fix" the irq state to be enabled early in do_exit(), so currently
>> this may not be triggerable, but that is coincidental and fragile.
>>
>> Detect and fix the irqs_disabled() condition in the oops path before
>> calling do_exit(), similarly to the way in_atomic() is handled.
>>
>> Reported-by: Michael Ellerman <mpe@ellerman.id.au>
>> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
>
> Hey Eric, did you have any thoughts on this?

No strong thoughts.

I agree that the unconditionally disabling then enabling irqs in
coredump_task_exit will mean there is likely to be little change in real
behavior.

I also agree that is something fragile to depend upon so we making
our assumptions explicit seems good.

Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>

>
> Thanks,
> Nick
>
>> ---
>>  kernel/exit.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/kernel/exit.c b/kernel/exit.c
>> index 84021b24f79e..fa696765f694 100644
>> --- a/kernel/exit.c
>> +++ b/kernel/exit.c
>> @@ -738,6 +738,7 @@ void __noreturn do_exit(long code)
>>  	struct task_struct *tsk = current;
>>  	int group_dead;
>>  
>> +	WARN_ON(irqs_disabled());
>>  	WARN_ON(tsk->plug);
>>  
>>  	kcov_task_exit(tsk);
>> @@ -865,6 +866,11 @@ void __noreturn make_task_dead(int signr)
>>  	if (unlikely(!tsk->pid))
>>  		panic("Attempted to kill the idle task!");
>>  
>> +	if (unlikely(irqs_disabled())) {
>> +		pr_info("note: %s[%d] exited with irqs disabled\n",
>> +			current->comm, task_pid_nr(current));
>> +		local_irq_enable();
>> +	}
>>  	if (unlikely(in_atomic())) {
>>  		pr_info("note: %s[%d] exited with preempt_count %d\n",
>>  			current->comm, task_pid_nr(current),
>> -- 
>> 2.37.2