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

[Tony Battersby <tonyb@cybernetics.com>]

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@cybernetics.com>