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

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Date:	Tue, 12 Jul 2016 18:42:33 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Stanislav Kinsburskiy <skinsbursky@...tuozzo.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

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.

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