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

[Jens Axboe]

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);
 

-- 
Jens Axboe