linux-kernel - Re: [PATCH] prctl: remove one-shot limitation for changing exe link

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Date:	Tue, 12 Jul 2016 18:52:29 +0200
From:	Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	<peterz@...radead.org>, <mingo@...hat.com>, <mhocko@...e.com>,
	<keescook@...omium.org>, <linux-kernel@...r.kernel.org>,
	<mguzik@...hat.com>, <bsegall@...gle.com>,
	<john.stultz@...aro.org>, <ebiederm@...ssion.com>,
	<gorcunov@...nvz.org>, <matthltc@...ibm.com>,
	<akpm@...ux-foundation.org>, <luto@...capital.net>,
	<vbabka@...e.cz>, <xemul@...tuozzo.com>
Subject: Re: [PATCH] prctl: remove one-shot limitation for changing exe link



12.07.2016 18:42, Oleg Nesterov пишет:
> On 07/12, Stanislav Kinsburskiy wrote:
>> --- a/kernel/sys.c
>> +++ b/kernel/sys.c
>> @@ -1696,16 +1696,6 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
>>   		fput(exe_file);
>>   	}
>>   
>> -	/*
>> -	 * The symlink can be changed only once, just to disallow arbitrary
>> -	 * transitions malicious software might bring in. This means one
>> -	 * could make a snapshot over all processes running and monitor
>> -	 * /proc/pid/exe changes to notice unusual activity if needed.
>> -	 */
>> -	err = -EPERM;
>> -	if (test_and_set_bit(MMF_EXE_FILE_CHANGED, &mm->flags))
>> -		goto exit;
>> -
> I didn't even try to read the changelog so I do not know why do you
> want this change ;)
>
> But I would like to ack it in any case. I never understood why do we
> want/need this MMF_EXE_FILE_CHANGED check, I suggested to remove it
> many times.
>
> And can't resist, please note the xchg() below. Currently (before this
> patch) we do not need it. I was specially added to ensure that we can
> just remove this test_and_set_bit(MMF_EXE_FILE_CHANGED) without adding
> a race.

Thanks, Oleg. I'll take a look.
But should this be addressed in this patch? Especially if it's not 
needed even now (before this patch)?


> Acked-by: Oleg Nesterov <oleg@...hat.com>
>