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Subject: Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CAGXu5jLEMy_T_5OtXLT+pUCt=Nk53nBbuRvrUgJBhq-4RZ=yCA@mail.gmail.com>
Date:   Tue, 19 Jun 2018 13:47:32 -0700
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        "H. J. Lu" <hjl.tools@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, linux-doc@vger.kernel.org,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Oleg Nesterov <oleg@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        mike.kravetz@oracle.com, Florian Weimer <fweimer@redhat.com>
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To:     Kees Cook <keescook@chromium.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jun 19, 2018, at 1:12 PM, Kees Cook <keescook@chromium.org> wrote:
>=20
>> On Tue, Jun 19, 2018 at 10:20 AM, Andy Lutomirski <luto@amacapital.net> w=
rote:
>>=20
>>> On Jun 19, 2018, at 10:07 AM, Kees Cook <keescook@chromium.org> wrote:
>>>=20
>>> Does it provide anything beyond what PR_DUMPABLE does?
>>=20
>> What do you mean?
>=20
> I was just going by the name of it. I wasn't sure what "ptrace CET
> lock" meant, so I was trying to understand if it was another "you
> can't ptrace me" toggle, and if so, wouldn't it be redundant with
> PR_SET_DUMPABLE =3D 0, etc.
>=20

No, other way around. The valid CET states are on/unlocked, off/unlocked, on=
/locked, off/locked. arch_prctl can freely the state unless locked. ptrace c=
an change it no matter what.  The lock is to prevent the existence of a gadg=
et to disable CET (unless the gadget involves ptrace, but I don=E2=80=99t th=
ink that=E2=80=99s a real concern).=