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

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Date:	Thu, 13 Feb 2014 02:11:20 +0400
From:	Andrew Vagin <avagin@...allels.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Andrey Vagin <avagin@...nvz.org>, <linux-kernel@...r.kernel.org>,
	<criu@...nvz.org>, Oleg Nesterov <oleg@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Kees Cook <keescook@...omium.org>,
	"Eric W. Biederman" <ebiederm@...ssion.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, Feb 12, 2014 at 01:32:28PM -0800, Andrew Morton wrote:
> 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?

A task can exceed the RLIMIT_DATA limit by changing mm->start_brk,
so this patch reduces required permission for RLIMIT_DATA too

> 
> > 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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