From: Oleg Nesterov <oleg@redhat.com>
To: Kees Cook <keescook@chromium.org>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
"Serge E. Hallyn" <serge@hallyn.com>,
syzbot <syzbot+a9ac39bf55329e206219@syzkaller.appspotmail.com>,
James Morris <jmorris@namei.org>,
LKML <linux-kernel@vger.kernel.org>,
linux-security-module <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 13:52:46 +0200 [thread overview]
Message-ID: <20181025115245.GB3725@redhat.com> (raw)
In-Reply-To: <CAGXu5jLxLqieLCZfAzw_xX-k3bbESUF3pjuGEDRRH1CY2O8JSA@mail.gmail.com>
On 10/25, Kees Cook wrote:
>
> task_is_descendant() is called under rcu_read_lock() in both
> ptracer_exception_found() and yama_ptrace_access_check() so I don't
> understand how any of the tasks could get freed? This is walking
> group_leader and real_parent -- are these not stable under rcu_lock()?
group_leader/real_parent/etc are no longer rcu-protected after the exiting
child calls release_task() which in particular removes the child from
children/thread_group lists.
OK. Suppose you have an rcu-protected list, and each element also has a
reference counter so you can do something
struct elt {
atomic_t ctr;
struct list_head list;
int pid;
};
rcu_read_lock();
list_for_each_entry(elt, &LIST, list) {
if (elt->pid == 100) {
atomic_inc(&elt->ctr); // get_task_struct()
break;
}
}
rcu_read_unlock();
do_something(elt);
This code is fine. This elt can't be freed, you have a reference. But once
you drop rcu lock you can't trust elt->list.next! So, for example, you can
not do
rcu_read_lock();
list_for_each_entry_continue_rcu(elt, &LIST, list) {
...
}
rcu_read_unlock();
too late, elt.list.next can be already freed, or it can be freed while you
iterate the list.
Another simple example. Suppose you have a global PTR protected by rcu. So
ignoring the necessary rcu_dereference this code is fine:
rcu_read_lock();
if (ptr = PTR)
do_something(ptr);
rcu_read_unlcok();
But this is not:
ptr = PTR;
rcu_read_lock();
if (ptr)
do_something(ptr);
rcu_read_unlock();
basically the same thing...
Oleg.
next prev parent reply other threads:[~2018-10-25 11:52 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
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 [this message]
2018-11-10 3:25 ` syzbot
2018-11-10 11:46 ` syzbot
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