linux-kernel - Re: [PATCH 2/2] memcg: Close the race between migration and installing bprm->mm as mm

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Date:   Wed, 09 May 2018 22:00:00 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>, peterz@...radead.org,
        viro@...iv.linux.org.uk, mingo@...nel.org,
        paulmck@...ux.vnet.ibm.com, keescook@...omium.org, riel@...hat.com,
        tglx@...utronix.de, kirill.shutemov@...ux.intel.com,
        marcos.souza.org@...il.com, hoeun.ryu@...il.com,
        pasha.tatashin@...cle.com, gs051095@...il.com, dhowells@...hat.com,
        rppt@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
        Balbir Singh <balbir@...ux.vnet.ibm.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 2/2] memcg: Close the race between migration and installing bprm->mm as mm

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

> On 05/04, Eric W. Biederman wrote:
>>
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1044,6 +1044,8 @@ static int exec_mmap(struct mm_struct *mm)
>>  		return 0;
>>  	}
>>  	mmdrop(active_mm);
>> +	/* The tsk may have migrated before the new mm was attached */
>> +	mm_sync_memcg_from_task(tsk);
>>  	return 0;
>>  }
>
> call_usermodehelper(). perhaps it makes sense to change flush_old_exec()
> to clear PF_KTHREAD/etc before exec_mmap().

Yes.  That does look like something to be fixed.  In practice while it
is rare for anything to be migrated during exec I expect it to be even
rarer for kernel threads.  But that is a legitimate issue.

I expect all of these lines after exec_mmap actually belong
in exec_mmap.  Which would then make rearraning the lines easy to ensure
mm_sync_memcg_from_task sees PF_KTHREAD cleared.
	/*
	 * After clearing bprm->mm (to mark that current is using the
	 * prepared mm now), we have nothing left of the original
	 * process. If anything from here on returns an error, the check
	 * in search_binary_handler() will SEGV current.
	 */
	bprm->mm = NULL;

	set_fs(USER_DS);
	current->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
					PF_NOFREEZE | PF_NO_SETAFFINITY);
	flush_thread();
	current->personality &= ~bprm->per_clear;

Eric