linux-kernel - Re: [PATCH 7/9] ptrace: Simplify the wait_task_inactive call in ptrace_check

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Message-ID: <87o80m8uu1.fsf@email.froward.int.ebiederm.org>
Date:   Wed, 27 Apr 2022 09:27:50 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     linux-kernel@...r.kernel.org
Cc:     rjw@...ysocki.net, Oleg Nesterov <oleg@...hat.com>,
        mingo@...nel.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, mgorman@...e.de,
        bigeasy@...utronix.de, Will Deacon <will@...nel.org>,
        tj@...nel.org, linux-pm@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        inux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 7/9] ptrace: Simplify the wait_task_inactive call in
 ptrace_check_attach

"Eric W. Biederman" <ebiederm@...ssion.com> writes:

> "Eric W. Biederman" <ebiederm@...ssion.com> writes:
>
>> Asking wait_task_inactive to verify that tsk->__state == __TASK_TRACED
>> was needed to detect the when ptrace_stop would decide not to stop
>> after calling "set_special_state(TASK_TRACED)".  With the recent
>> cleanups ptrace_stop will always stop after calling set_special_state.
>>
>> Take advatnage of this by no longer asking wait_task_inactive to
>> verify the state.  If a bug is hit and wait_task_inactive does not
>> succeed warn and return -ESRCH.
>
> As Oleg noticed upthread there are more reasons than simply
> !current->ptrace for wait_task_inactive to fail.  In particular a fatal
> signal can be received any time before JOBCTL_DELAY_SIGKILL.
>
> So this change is not safe.  I will respin this one.

Bah.  I definitely need to update the description so there is going to
be a v2.

I confused myself.  This change is safe because ptrace_freeze_traced
fails if there is a pending fatal signal, and arranges that no new fatal
signals will wake up the task.

Eric

>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>> ---
>>  kernel/ptrace.c | 14 +++-----------
>>  1 file changed, 3 insertions(+), 11 deletions(-)
>>
>> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
>> index 16d1a84a2cae..0634da7ac685 100644
>> --- a/kernel/ptrace.c
>> +++ b/kernel/ptrace.c
>> @@ -265,17 +265,9 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
>>  	}
>>  	read_unlock(&tasklist_lock);
>>  
>> -	if (!ret && !ignore_state) {
>> -		if (!wait_task_inactive(child, __TASK_TRACED)) {
>> -			/*
>> -			 * This can only happen if may_ptrace_stop() fails and
>> -			 * ptrace_stop() changes ->state back to TASK_RUNNING,
>> -			 * so we should not worry about leaking __TASK_TRACED.
>> -			 */
>> -			WARN_ON(READ_ONCE(child->__state) == __TASK_TRACED);
>> -			ret = -ESRCH;
>> -		}
>> -	}
>> +	if (!ret && !ignore_state &&
>> +	    WARN_ON_ONCE(!wait_task_inactive(child, 0)))
>> +		ret = -ESRCH;
>>  
>>  	return ret;
>>  }
>
> Eric