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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 4 Nov 2015 19:28:13 +0100
From:	Willy Tarreau <w@....eu>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Kees Cook <keescook@...omium.org>,
	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 Wed, Nov 04, 2015 at 10:17:06AM -0800, Andy Lutomirski wrote:
> > That said I never feel completely comfortable with changing a file's
> > permissions this way, I always fear it could break backup/restore
> > applications. Let's imagine for a minute that a restore does this :
> >
> >  extract(const char *file_name, int file_perms) {
> >    fd = open(".tmpfile", O_CREAT, file_perms);
> >    mmap(fd);
> >    /* actually write file */
> >    close(fd);
> >    unlink(real_file_name);
> >    rename(".tmpfile", file_name);
> >  }
> >
> > Yes I know it's not safe to do the chmod before writing to the file
> > but we could imagine some situations where it makes sense to be done
> > this way (eg: if the file is put into a protected directory), and
> > anyway this possibility is provided by open() and creat() so it is
> > legitimate to imagine these ones could exist.
> >
> > Such a change would slightly modify semantics and affect such use cases
> > *if they exist*, just like using write() instead of mmap() would fail.
> > We could imagine having a sysctl to disable this strengthening, but it
> > is probably not the best solution for the long term either.
> 
> I'd say that this is an acceptable breakage risk.

Yes probably.

> In any event, the
> potential for data loss is limited to a bit of the file mode,

When this bit is the one sudo's setuid, it becomes one of the most important
bits on the whole system :-)

> and
> restore apps like that really don't deserve to work in the first
> place.

I absolutely agree for this specific case. I just wanted to raise this
case so that we're sure not to oversee anything related to other similar
but more justified use cases.

Willy

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/