On 2018/10/25 20:13, Oleg Nesterov wrote:

On 10/25, Tetsuo Handa wrote:

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Oleg Nesterov wrote:

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On 10/22, Tetsuo Handa wrote:

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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.

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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...

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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...