KASAN: use-after-free Read in task_is_descendant

[Tetsuo Handa]

On 2018/10/25 20:13, Oleg Nesterov wrote:
> On 10/25, Tetsuo Handa wrote:
>>
>> Oleg Nesterov wrote:
>>> On 10/22, Tetsuo Handa wrote:
>>>>> And again, I do not know how/if yama ensures that child is rcu-protected, perhaps
>>>>> task_is_descendant() needs to check pid_alive(child) right after rcu_read_lock() ?
>>>>
>>>> Since the caller (ptrace() path) called get_task_struct(child), child itself can't be
>>>> released. Do we still need pid_alive(child) ?
>>>
>>> get_task_struct(child) can only ensure that this task_struct can't be freed.
>>
>> The report says that it is a use-after-free read at
>>
>>   walker = rcu_dereference(walker->real_parent);
>>
>> which means that walker was already released.
> 
> quite possibly I missed something, but I am not sure I understand your concerns...
> 
> So again, suppose that "child" is already dead. Its task_struct can't be freed,
> but child->real_parent can point to the already freed memory.

Yes.

But if child->real_parent is pointing to the already freed memory,
why does pid_alive(child) == true help?

> 
> This means that the 1st walker = rcu_dereference(walker->real_parent) is fine,
> this simply reads the child->real_parent pointer,

Yes.

>                                                   but on the second iteration
> 
> 	walker = rcu_dereference(walker->real_parent);
> 
> reads the alredy freed memory.

Yes.

> 
>> I wonder whether pid_alive() test helps.
>>
>> We can get
>>
>> [   40.620318] parent or walker is dead.
>> [   40.624146] tracee is dead.
>>
>> messages using below patch and reproducer.
> 
> again, I do not understand, this all looks correct...
> 
>> ----------
>> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
>> index 99cfddd..0d9d786 100644
>> --- a/kernel/ptrace.c
>> +++ b/kernel/ptrace.c
>> @@ -385,6 +385,7 @@ static int ptrace_attach(struct task_struct *task, long request,
>>  	if (mutex_lock_interruptible(&task->signal->cred_guard_mutex))
>>  		goto out;
>>
>> +	schedule_timeout_killable(HZ);
>>  	task_lock(task);
>>  	retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS);
>>  	task_unlock(task);
>> diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
>> index ffda91a..a231ec6 100644
>> --- a/security/yama/yama_lsm.c
>> +++ b/security/yama/yama_lsm.c
>> @@ -283,6 +283,11 @@ static int task_is_descendant(struct task_struct *parent,
>>  		return 0;
>>
>>  	rcu_read_lock();
>> +	if (!pid_alive(parent) || !pid_alive(walker)) {
>> +		rcu_read_unlock();
>> +		printk("parent or walker is dead.\n");
> 
> This is what we need to do, except I think we should change yama_ptrace_access_check().
> And iiuc parent == current, pid_alive(parent) looks unnecessary. Although we need to
> check ptracer_exception_found(), may be it needs some changes too.

There are two task_is_descendant() callers, and one of them is not passing current.

> 
> And yes, task_is_descendant() can hit the dead child, if nothing else it can
> be killed. This can explain the kasan report.

The kasan is reporting that child->real_parent (or maybe child->real_parent->real_parent
or child->real_parent->real_parent->real_parent ...) was pointing to already freed memory,
isn't it?

How can we check that that pointer is pointing to already freed memory? As soon as

  walker = rcu_dereference(walker->real_parent);

is executed, task_alive(walker) will try to read from already freed memory...