Date: Wed, 4 Nov 2015 07:58:20 +0100
From: Willy Tarreau <w@1wt.eu>
To: Kees Cook <keescook@chromium.org>
Cc: Dirk Steinmetz <public@rsjtdrjgfuzkfg.com>,
        Michael Kerrisk-manpages <mtk.manpages@gmail.com>,
        Serge Hallyn <serge.hallyn@ubuntu.com>,
        Seth Forshee <seth.forshee@canonical.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Serge Hallyn <serge.hallyn@canonical.com>,
        "security@kernel.org" <security@kernel.org>
Subject: Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids
Message-ID: <20151104065820.GF21740@1wt.eu>
References: <CALCETrUPyFMAk9uxJ8+hg0YoMN=mTM3q9jAT7zki1g9uuWtVkg@mail.gmail.com> <1446511187-9131-1-git-send-email-public@rsjtdrjgfuzkfg.com> <CAGXu5j+i6PZNMwmseZc8G6V_5=5TK=dXP=bzLteK705bLdUU2A@mail.gmail.com> <20151104002132.010ccd1d@rsjtdrjgfuzkfg.com> <CAGXu5jKCCq4hs_nerQYMnC1frA3qKkCxxQTjpS2gw-7dPZAX+w@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAGXu5jKCCq4hs_nerQYMnC1frA3qKkCxxQTjpS2gw-7dPZAX+w@mail.gmail.com>
User-Agent: Mutt/1.4.2.3i
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1148
Lines: 27

On Tue, Nov 03, 2015 at 03:29:55PM -0800, Kees Cook wrote:
> Using "write" does kill the set-gid bit. I haven't looked at
> why.
> Al or anyone else, is there a meaningful distinction here?

I remember this one, I got caught once while trying to put a shell into
a suid-writable file to get some privileges someone forgot to offer me :-)

It's done by should_remove_suid() which is called upon write() and truncate().

> Should the
> mmap MAP_SHARED-write trigger the loss of the set-gid bit too? While
> holding the file open with either open or mmap, I get a Text-in-use
> error, so I would kind of expect the same behavior between either
> close() and munmap(). I wonder if this is a bug, and if so, then your
> link patch is indeed useful again. :)

I don't see how this could be done with mmap(). Maybe we have a way to know
when the first write is performed via this path, I have no idea.

Willy

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