linux-kernel - Re: [PATCH] kernel: reduce required permission for prctl_set

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Date:	Wed, 12 Feb 2014 13:32:28 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andrey Vagin <avagin@...nvz.org>
Cc:	linux-kernel@...r.kernel.org, criu@...nvz.org,
	Oleg Nesterov <oleg@...hat.com>, Robin Holt <holt@....com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Kees Cook <keescook@...omium.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Chen Gang <gang.chen@...anux.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Pavel Emelyanov <xemul@...allels.com>,
	Aditya Kali <adityakali@...gle.com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH] kernel: reduce required permission for prctl_set_mm

On Wed, 12 Feb 2014 19:40:11 +0400 Andrey Vagin <avagin@...nvz.org> wrote:

> Currently prctl_set_mm requires the global CAP_SYS_RESOURCE,
> this patch reduce requiremence to CAP_SYS_RESOURCE in the current
> namespace.
> 
> When we restore a task we need to set up text, data and data heap sizes
> from userspace to the values a task had at checkpoint time.
> 
> Currently we can not restore these parameters, if a task lives in
> a non-root user name space, because it has no capabilities in the
> parent namespace.
> 
> prctl_set_mm() changes parameters of the current task and doesn't affect
> other tasks.
> 
> This patch affects the RLIMIT_DATA limit, because a consumtiuon is
> calculated relatively to mm->end_data, mm->start_data, mm->start_brk.

I can't for the life of me work out what you were trying to say here. 
Please fix and resend this paragraph?

> rlim = rlimit(RLIMIT_DATA);
> if (rlim < RLIM_INFINITY && (brk - mm->start_brk) +
> 		(mm->end_data - mm->start_data) > rlim)
> 	goto out;
> 
> This limit affects calls to brk() and sbrk(), but it doesn't affect
> mmap. So I think requirement of CAP_SYS_RESOURCE in the current
> namespace is enough for this limit.
> 
> ...
>
> Cc: security@...nel.org

That list is for reporting kernel security bugs.

>
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1701,7 +1701,7 @@ static int prctl_set_mm(int opt, unsigned long addr,
>  	if (arg5 || (arg4 && opt != PR_SET_MM_AUXV))
>  		return -EINVAL;
>  
> -	if (!capable(CAP_SYS_RESOURCE))
> +	if (!ns_capable(current_user_ns(), CAP_SYS_RESOURCE))
>  		return -EPERM;
>  
>  	if (opt == PR_SET_MM_EXE_FILE)

This looks harmless.

My relatively-up-to-date manpages don't mention prctl(PR_SET_MM).  I
see from http://marc.info/?l=linux-man&m=133132612704130&w=2 that
manpage additions were prepared nearly three years ago.  Michael, did
this fall through a crack?

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