linux-kernel - Re: [PATCH] move cred_guard_mutex from task_struct to signal

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Message-ID: <20100910172457.GA23393@redhat.com>
Date:	Fri, 10 Sep 2010 19:24:57 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Roland McGrath <roland@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, oss-security@...ts.openwall.com,
	Solar Designer <solar@...nwall.com>,
	Kees Cook <kees.cook@...onical.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Neil Horman <nhorman@...driver.com>,
	linux-fsdevel@...r.kernel.org, pageexec@...email.hu,
	"Brad Spengler <spender@...ecurity.net>, Eugene Teo" 
	<eugene@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] move cred_guard_mutex from task_struct to signal_struct

On 09/10, KOSAKI Motohiro wrote:
>
> 1) moving cread_guard_mutex itself
>    - no increase execve overhead
> 	-> very good
>    - it also prevent parallel ptrace

No, it doesn't. Only PTRACE_ATTACH needs this mutex, and as Roland
pointed out it also needs write_lock(tasklist) which is worse. So
this change doesn't make any practical harm for ptrace.

> 2) move in_exec_mm to signal_struct too
>    -> very hard. oom-killer can use very few lock because it's called
>       from various place. now both ->mm and ->in_exec_mm are protected
>       task_lock() and it help to avoid messy.

Yes. But, if ->in_exec_mm is only used by oom_badness(), then I think
you can use task_lock(tsk->group_leader). oom_badness() needs tasklist
anyway, this means it can't race with de_thread() changing the leader.
But up to you.

Another very minor nit (but again, up to you). Perhaps exec_mmap()
could clear ->in_exec_mm (in task_struct or signal_struct, this doesnt
matter), it takes task_lock(current) anyway (and at this point current
is always the group leader).

> Let's move ->cred_guard_mutex from task_struct to signal_struct. It
> naturally prevent multiple-threads-inside-exec.

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


This is very minor, but perhaps you can also fix a couple of comments
which mention task->cred_guard_mutex,

	fs/exec.c:1109		the caller must hold current->cred_guard_mutex
	kernel/cred.c:328	The caller must hold current->cred_guard_mutex
	include/linux/tracehook.h:153	@task->cred_guard_mutex

Oleg.

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