Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Content-Type: text/plain;
        charset=utf-8
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH v6 5/6] binfmt_*: scope path resolution of interpreters
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190512133549.ymx5yg5rdqvavzyq@yavin>
Date:   Sun, 12 May 2019 07:34:26 -0700
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Lutomirski <luto@kernel.org>,
        Jann Horn <jannh@google.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Jeff Layton <jlayton@kernel.org>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Arnd Bergmann <arnd@arndb.de>,
        David Howells <dhowells@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Christian Brauner <christian@brauner.io>,
        Tycho Andersen <tycho@tycho.ws>,
        David Drysdale <drysdale@google.com>,
        Chanho Min <chanho.min@lge.com>,
        Oleg Nesterov <oleg@redhat.com>, Aleksa Sarai <asarai@suse.de>,
        Linux Containers <containers@lists.linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>
Content-Transfer-Encoding: quoted-printable
Message-Id: <0ED963D0-4C31-45B1-B361-E4A75DFBF7C1@amacapital.net>
References: <20190506191735.nmzf7kwfh7b6e2tf@yavin> <20190510204141.GB253532@google.com> <CALCETrW2nn=omqJb4p+m-BDsCOhg+YZQ3ELd4BdhODV3G44gfA@mail.gmail.com> <20190510225527.GA59914@google.com> <C60DC580-854D-478D-AF23-5F29FB7C3E50@amacapital.net> <CAHk-=wh1JJD_RabMaFfinsAQp1vHGJOQ1rKqihafY=r7yHc8sQ@mail.gmail.com> <CALCETrUOj=4VWp=B=QT0BQ8X_Ds_b+pt68oDwfjGb+K0StXmWA@mail.gmail.com> <CAHk-=wg3+3GfHsHdB4o78jNiPh_5ShrzxBuTN-Y8EZfiFMhCvw@mail.gmail.com> <9CD2B97D-A6BD-43BE-9040-B410D996A195@amacapital.net> <CAHk-=wh3dT7=SMjvSZreXSu36Cg7gsfSipLhfTz5ioDKXV5uHg@mail.gmail.com> <20190512133549.ymx5yg5rdqvavzyq@yavin>
To:     Aleksa Sarai <cyphar@cyphar.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On May 12, 2019, at 6:35 AM, Aleksa Sarai <cyphar@cyphar.com> wrote:
>=20
>> On 2019-05-12, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>>> On Sat, May 11, 2019 at 7:37 PM Andy Lutomirski <luto@amacapital.net> wr=
ote:
>>> I bet this will break something that already exists. An execveat()
>>> flag to turn off /proc/self/exe would do the trick, though.
>>=20
>> Thinking more about it, I suspect it is (once again) wrong to let the
>> thing that does the execve() control that bit.
>>=20
>> Generally, the less we allow people to affect the lifetime and
>> environment of a suid executable, the better off we are.
>>=20
>> But maybe we could limit /proc/*/exe to at least not honor suid'ness
>> of the target? Or does chrome/runc depend on that too?
>=20
> Speaking on the runc side, we don't depend on this. It's possible
> someone depends on this for fexecve(3) -- but as mentioned before in
> newer kernels glibc uses execve(AT_EMPTY_PATH).

Why are we concerned about suid?  Don=E2=80=99t we already block suid if the=
 path being execed doesn=E2=80=99t come from the current mountns?  That shou=
ld mostly cover the things we care about, no?

I suppose we could also block suid for deleted files, so that deleting a kno=
wn-buggy suid binary becomes more reliable. But every sensible package tool s=
hould already be chmoding the suid away before unlinking.=