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From:   Florian Weimer <fweimer@redhat.com>
To:     "H.J. Lu" <hjl.tools@gmail.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
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Subject: Re: [PATCH 34/35] x86/cet/shstk: Support wrss for userspace
References: <20220130211838.8382-1-rick.p.edgecombe@intel.com>
        <20220130211838.8382-35-rick.p.edgecombe@intel.com>
        <87wnig8hj6.fsf@oldenburg.str.redhat.com>
        <CAMe9rOrVvjL1F3UgOWL-gAGRyyiG6r20TWUusEUFhZMEEAjH7w@mail.gmail.com>
Date:   Mon, 31 Jan 2022 19:45:10 +0100
In-Reply-To: <CAMe9rOrVvjL1F3UgOWL-gAGRyyiG6r20TWUusEUFhZMEEAjH7w@mail.gmail.com>
        (H. J. Lu's message of "Mon, 31 Jan 2022 10:26:49 -0800")
Message-ID: <87a6fb7nih.fsf@oldenburg.str.redhat.com>
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* H. J. Lu:

> On Sun, Jan 30, 2022 at 11:57 PM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * Rick Edgecombe:
>>
>> > For the current shadow stack implementation, shadow stacks contents cannot
>> > be arbitrarily provisioned with data. This property helps apps protect
>> > themselves better, but also restricts any potential apps that may want to
>> > do exotic things at the expense of a little security.
>> >
>> > The x86 shadow stack feature introduces a new instruction, wrss, which
>> > can be enabled to write directly to shadow stack permissioned memory from
>> > userspace. Allow it to get enabled via the prctl interface.
>>
>> Why can't this be turned on unconditionally?
>
> WRSS can be a security risk since it defeats the whole purpose of
> Shadow Stack.  If an application needs to write to shadow stack,
> it can make a syscall to enable it.  After the CET patches are checked
> in Linux kernel, I will make a proposal to allow applications or shared
> libraries to opt-in WRSS through a linker option, a compiler option or
> a function attribute.

Ahh, that makes sense.  I assumed that without WRSS, the default was to
allow plain writes. 8-)

Thanks,
Florian