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

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Message-ID: <20151110150829.GC3156@quack.suse.cz>
Date:	Tue, 10 Nov 2015 16:08:29 +0100
From:	Jan Kara <jack@...e.cz>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Kees Cook <keescook@...omium.org>,
	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 Sat 07-11-15 21:02:06, Ted Tso wrote:
> 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()?

page_mkwrite() callbacks are IMHO the right place for this check (and
change).  Just next to file_update_time() call. You get proper filesystem
freezing protection etc. As Ted properly mentions filemap_page_mkwrite() is
one place you want to hook into but quite a few filesystems (most notably
ext4, xfs, btrfs) overload ->page_mkwrite() callbacks to their own
functions so each filesystem that does this needs to be updated
separately...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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