From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
To: Oleg Nesterov <oleg@redhat.com>
Cc: serge@hallyn.com,
syzbot <syzbot+a9ac39bf55329e206219@syzkaller.appspotmail.com>,
jmorris@namei.org, keescook@chromium.org,
linux-kernel@vger.kernel.org,
linux-security-module@vger.kernel.org,
syzkaller-bugs@googlegroups.com
Subject: Re: KASAN: use-after-free Read in task_is_descendant
Date: Thu, 25 Oct 2018 20:47:42 +0900 [thread overview]
Message-ID: <b0ac67e6-4279-91b3-0178-3176876abe70@i-love.sakura.ne.jp> (raw)
In-Reply-To: <20181025111355.GA3725@redhat.com>
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...
next prev parent reply other threads:[~2018-10-25 11:48 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-21 7:10 KASAN: use-after-free Read in task_is_descendant syzbot
2018-10-21 7:12 ` Tetsuo Handa
2018-10-22 9:54 ` Oleg Nesterov
2018-10-22 10:06 ` Tetsuo Handa
2018-10-22 13:46 ` Oleg Nesterov
2018-10-25 2:15 ` Tetsuo Handa
2018-10-25 11:13 ` Oleg Nesterov
2018-10-25 11:36 ` Kees Cook
2018-10-25 12:05 ` Oleg Nesterov
2018-10-25 11:47 ` Tetsuo Handa [this message]
2018-10-25 12:17 ` Oleg Nesterov
2018-10-25 13:01 ` Oleg Nesterov
2018-10-26 16:09 ` Kees Cook
2018-10-29 12:23 ` Oleg Nesterov
2018-10-29 15:05 ` yama: unsafe usage of ptrace_relation->tracer Oleg Nesterov
2019-01-10 11:05 ` Tetsuo Handa
2019-01-10 18:47 ` Kees Cook
2019-01-16 17:40 ` Oleg Nesterov
2018-10-25 13:14 ` KASAN: use-after-free Read in task_is_descendant Tetsuo Handa
2018-10-25 15:55 ` Oleg Nesterov
2018-10-25 16:25 ` Oleg Nesterov
2018-10-26 12:23 ` Tetsuo Handa
2018-10-26 13:04 ` Oleg Nesterov
2018-10-26 13:51 ` Tetsuo Handa
2018-10-26 14:39 ` Oleg Nesterov
2018-10-26 15:04 ` Tetsuo Handa
2018-10-26 15:22 ` Oleg Nesterov
2018-10-25 8:19 ` Kees Cook
2018-10-25 11:52 ` Oleg Nesterov
2018-11-10 3:25 ` syzbot
2018-11-10 11:46 ` syzbot
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