linux-kernel - Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids

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Message-ID: <20151108020206.GA3880@thunk.org>
Date:	Sat, 7 Nov 2015 21:02:06 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Kees Cook <keescook@...omium.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Theodore Tso <tytso@...gle.com>, Willy Tarreau <w@....eu>,
	Dirk Steinmetz <public@...tdrjgfuzkfg.com>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Seth Forshee <seth.forshee@...onical.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Eric W . Biederman" <ebiederm@...ssion.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	"security@...nel.org" <security@...nel.org>
Subject: Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids

On Fri, Nov 06, 2015 at 09:05:57PM -0800, Kees Cook wrote:
> >>>> They're certainly not used early enough -- we need to remove suid when
> >>>> the page becomes writable via mmap (wp_page_shared), not when
> >>>> writeback happens, or at least not only when writeback happens.
> >>>
> >>> Well, I'm shy about the change there. For example, we don't strip in
> >>> on open(RDWR), just on write().
> >>
> >> I take it back. Hooking wp_page_shared looks expensive. :) Maybe we do
> >> need to hook the mmap?
> >
> > But file_update_time already pokes at the same (or nearby) cachelines,
> > I think -- why would it be expensive?  The whole thing could be
> > guarded by if (unlikely(is setuid)), right?
> 
> Yeah, true. I added file_remove_privs calls near all the
> file_update_time calls, to no effect. Added to wp_page_shared too,
> nothing. Hmmm.

Why not put the the should_remove_suid() call in
filemap_page_mkwrite(), or maybe do_page_mkwrite()?

Cheers,

					- Ted
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