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From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 4 Nov 2015 09:59:55 -0800
Message-ID: <CALCETrVKuAW=qYKsB_Dkmq2LDwSV1sdP8sKbuDcGzye3zn_e2A@mail.gmail.com>
Subject: Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids
To: Willy Tarreau <w@1wt.eu>
Cc: Kees Cook <keescook@chromium.org>,
        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>
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On Tue, Nov 3, 2015 at 10:58 PM, Willy Tarreau <w@1wt.eu> wrote:
> 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.

do_wp_page might be a decent bet.

--Andy
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