From: Kees Cook <keescook@chromium.org>
To: Oleg Nesterov <oleg@redhat.com>
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 12:36:08 +0100 [thread overview]
Message-ID: <CAGXu5jK5c5mf=EarFNBWeq5Gc3LB=Kd8tqDc_a2nc6Xmtu=s6g@mail.gmail.com> (raw)
In-Reply-To: <20181025111355.GA3725@redhat.com>
On Thu, Oct 25, 2018 at 12:13 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> 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.
I can't find a path for "child" to be released. I see task_lock()
always called on it before it ends up in Yama.
(Well, I can't find the lock for switch_mm(), but I assume that's safe
because it's switching to the task.)
> 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.
What does rcu_read_lock() protect actually protect here? I thought
none of the task structs would be freed until after all
rcu_read_unlock() finished.
> 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;
>
Hm, documentation there says:
* If pid_alive fails, then pointers within the task structure
* can be stale and must not be dereferenced.
What is the safe pattern for checking vs rcu?
-Kees
--
Kees Cook
next prev parent reply other threads:[~2018-10-25 11:36 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 [this message]
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
2018-11-10 3:25 ` syzbot
2018-11-10 11:46 ` syzbot
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