Re: [PATCH] coredump: Limit what can interrupt coredumps

[Jens Axboe <axboe@xxxxxxxxx>]

On 8/18/21 8:37 AM, Tony Battersby wrote:
> On 8/17/21 6:05 PM, Jens Axboe wrote:
>> On 8/17/21 3:39 PM, Tony Battersby wrote:
>>> On 8/17/21 5:28 PM, Jens Axboe wrote:
>>>> Another approach - don't allow TWA_SIGNAL task_work to get queued if
>>>> PF_SIGNALED has been set on the task. This is similar to how we reject
>>>> task_work_add() on process exit, and the callers must be able to handle
>>>> that already.
>>>>
>>>> Can you test this one on top of your 5.10-stable?
>>>>
>>>>
>>>> diff --git a/fs/coredump.c b/fs/coredump.c
>>>> index 07afb5ddb1c4..ca7c1ee44ada 100644
>>>> --- a/fs/coredump.c
>>>> +++ b/fs/coredump.c
>>>> @@ -602,6 +602,14 @@ void do_coredump(const kernel_siginfo_t *siginfo)
>>>>  		.mm_flags = mm->flags,
>>>>  	};
>>>>  
>>>> +	/*
>>>> +	 * task_work_add() will refuse to add work after PF_SIGNALED has
>>>> +	 * been set, ensure that we flush any pending TIF_NOTIFY_SIGNAL work
>>>> +	 * if any was queued before that.
>>>> +	 */
>>>> +	if (test_thread_flag(TIF_NOTIFY_SIGNAL))
>>>> +		tracehook_notify_signal();
>>>> +
>>>>  	audit_core_dumps(siginfo->si_signo);
>>>>  
>>>>  	binfmt = mm->binfmt;
>>>> diff --git a/kernel/task_work.c b/kernel/task_work.c
>>>> index 1698fbe6f0e1..1ab28904adc4 100644
>>>> --- a/kernel/task_work.c
>>>> +++ b/kernel/task_work.c
>>>> @@ -41,6 +41,12 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
>>>>  		head = READ_ONCE(task->task_works);
>>>>  		if (unlikely(head == &work_exited))
>>>>  			return -ESRCH;
>>>> +		/*
>>>> +		 * TIF_NOTIFY_SIGNAL notifications will interfere with
>>>> +		 * a core dump in progress, reject them.
>>>> +		 */
>>>> +		if ((task->flags & PF_SIGNALED) && notify == TWA_SIGNAL)
>>>> +			return -ESRCH;
>>>>  		work->next = head;
>>>>  	} while (cmpxchg(&task->task_works, head, work) != head);
>>>>  
>>>>
>>> Doesn't compile.  5.10 doesn't have TIF_NOTIFY_SIGNAL.
>> Oh right... Here's one hacked up for the 5.10 TWA_SIGNAL setup. Totally
>> untested...
>>
>> diff --git a/fs/coredump.c b/fs/coredump.c
>> index c6acfc694f65..9e899ce67589 100644
>> --- a/fs/coredump.c
>> +++ b/fs/coredump.c
>> @@ -603,6 +603,19 @@ void do_coredump(const kernel_siginfo_t *siginfo)
>>  		.mm_flags = mm->flags,
>>  	};
>>  
>> +	/*
>> +	 * task_work_add() will refuse to add work after PF_SIGNALED has
>> +	 * been set, ensure that we flush any pending TWA_SIGNAL work
>> +	 * if any was queued before that.
>> +	 */
>> +	if (signal_pending(current) && (current->jobctl & JOBCTL_TASK_WORK)) {
>> +		task_work_run();
>> +		spin_lock_irq(&current->sighand->siglock);
>> +		current->jobctl &= ~JOBCTL_TASK_WORK;
>> +		recalc_sigpending();
>> +		spin_unlock_irq(&current->sighand->siglock);
>> +	}
>> +
>>  	audit_core_dumps(siginfo->si_signo);
>>  
>>  	binfmt = mm->binfmt;
>> diff --git a/kernel/task_work.c b/kernel/task_work.c
>> index 8d6e1217c451..93b3f262eb4a 100644
>> --- a/kernel/task_work.c
>> +++ b/kernel/task_work.c
>> @@ -39,6 +39,12 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
>>  		head = READ_ONCE(task->task_works);
>>  		if (unlikely(head == &work_exited))
>>  			return -ESRCH;
>> +		/*
>> +		 * TWA_SIGNAL notifications will interfere with
>> +		 * a core dump in progress, reject them.
>> +		 */
>> +		if ((task->flags & PF_SIGNALED) && notify == TWA_SIGNAL)
>> +			return -ESRCH;
>>  		work->next = head;
>>  	} while (cmpxchg(&task->task_works, head, work) != head);
>>  
>>
> Tested with 5.10.59 + backport 06af8679449d + the patch above.  That
> fixes it for me.  I tested a couple of variations to make sure.
> 
> Thanks!
> 
> Tested-by: Tony Battersby <tonyb@xxxxxxxxxxxxxxx>

Great, thanks for testing! The 5.10 version is a bit uglier due to how
TWA_SIGNAL used to work, but it's the most straight forward backport of
the other version I sent.

-- 
Jens Axboe