Re: KASAN: use-after-free Read in task_is_descendant

[Oleg Nesterov <oleg@redhat.com>]

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.

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

	walker = rcu_dereference(walker->real_parent);

reads the alredy freed memory.

> 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.

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

> @@ -315,6 +320,10 @@ static int ptracer_exception_found(struct task_struct *tracer,
>  	bool found = false;
>
>  	rcu_read_lock();
> +	if (!pid_alive(tracee)) {
> +		printk("tracee is dead.\n");
> +		goto unlock;

Sure, this is possible too.

> But since "child" has at least one reference, reading "child->real_parent" should
> be safe. Therefore, I think that bailing out due to pid_is_alive(child) == false
> (like above patch does) cannot avoid this problem...

Why?

OK. Lets ignore ptracer_exception_found() for the moment. Why do you think the
patch below can't help?

Oleg.

--- x/security/yama/yama_lsm.c
+++ x/security/yama/yama_lsm.c
@@ -368,7 +368,8 @@ static int yama_ptrace_access_check(stru
 			break;
 		case YAMA_SCOPE_RELATIONAL:
 			rcu_read_lock();
-			if (!task_is_descendant(current, child) &&
+			if (!pid_alive(child) ||
+			    !task_is_descendant(current, child) &&
 			    !ptracer_exception_found(current, child) &&
 			    !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
 				rc = -EPERM;