linux-kernel - Re: [PATCH] ptrace: fix PTRACE_LISTEN race corrupting task->state

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Message-ID: <xm26y3wyhzxj.fsf@bsegall-linux.mtv.corp.google.com>
Date:   Wed, 22 Feb 2017 09:56:40 -0800
From:   bsegall@...gle.com
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Roland McGrath <roland@...k.frob.com>
Subject: Re: [PATCH] ptrace: fix PTRACE_LISTEN race corrupting task->state

Oleg Nesterov <oleg@...hat.com> writes:

> On 02/21, bsegall@...gle.com wrote:
>>
>> In PT_SEIZED + LISTEN mode SIGSTOP/SIGCONT signals cause a wakeup
>> against __TASK_TRACED. If this races with the ptrace_unfreeze_traced at
>> the end of a PTRACE_LISTEN, this can wake the task /after/ the check
>> against __TASK_TRACED, but before the reset of state to TASK_TRACED.
>
> Oh, thanks...
>
> note also that PTRACE_LISTEN itself can do ptrace_signal_wake_up(true),
>
>> This causes it to instead clobber TASK_WAKING,
>
> even if it is already TASK_RUNNING it is simply wrong to set TASK_TRACED
> in both cases, right?

Yeah, that's also wrong and could possibly lead to different errors, but
is likely to work out by accident when say ttwu checks on_rq and sees true.

>
> Thanks. The patch looks good at first glance, but let me think a bit...
> perhaps we should change PTRACE_LISTEN instead, not sure.
>
> Oleg.