linux-kernel - Re: + prctl-pr_set_mm-introduce-pr_set_mm_map-operation-v3.patch added to -mm tree

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Message-ID: <20140822192241.GA26512@redhat.com>
Date:	Fri, 22 Aug 2014 21:22:41 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Andrew Vagin <avagin@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Julien Tinnes <jln@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: + prctl-pr_set_mm-introduce-pr_set_mm_map-operation-v3.patch
	added to -mm tree

Hi Cyrill,

I think the patch is fine but I can't understand the usage of mmap_sem
and alloc_lock,

> +	stack_vma = find_vma(mm, (unsigned long)prctl_map->start_stack);

OK, find_vma() needs mmap_sem. But otherwise, why this should be called
under down_read(&mm->mmap_sem) ? What this lock tries to protect?

> +	if (prctl_map.auxv_size) {
> +		up_read(&mm->mmap_sem);
> +		memset(user_auxv, 0, sizeof(user_auxv));
> +		error = copy_from_user(user_auxv,
> +				       (const void __user *)prctl_map.auxv,
> +				       prctl_map.auxv_size);
> +		down_read(&mm->mmap_sem);

And if we actually need this lock, why it is safe to drop it temporary?
And why we can't move this copy_from_user() up before down_read) in any
case?

> +	if (prctl_map.auxv_size) {
> +		/* Last entry must be AT_NULL as specification requires */
> +		user_auxv[AT_VECTOR_SIZE - 2] = AT_NULL;
> +		user_auxv[AT_VECTOR_SIZE - 1] = AT_NULL;
> +
> +		task_lock(current);
> +		memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
> +		task_unlock(current);

Again, could you explain this task_lock() ?

Oleg.

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