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Subject: Re: [PATCH 00/10] Control Flow Enforcement - Part (3)
From:   Balbir Singh <bsingharora@gmail.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>,
        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>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. J. Lu" <hjl.tools@gmail.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
Date:   Tue, 19 Jun 2018 18:52:29 +1000
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On Mon, 2018-06-18 at 14:44 -0700, Andy Lutomirski wrote:
> On Sat, Jun 16, 2018 at 8:16 PM Balbir Singh <bsingharora@gmail.com> wrote:
> > 
> > On Thu, 2018-06-14 at 07:56 -0700, Yu-cheng Yu wrote:
> > > On Thu, 2018-06-14 at 11:07 +1000, Balbir Singh wrote:
> > > > On Tue, 2018-06-12 at 08:03 -0700, Yu-cheng Yu wrote:
> > > > > On Tue, 2018-06-12 at 20:56 +1000, Balbir Singh wrote:
> > > > > > 
> > > > > > On 08/06/18 00:37, Yu-cheng Yu wrote:
> > > > > > > This series introduces CET - Shadow stack
> > > > > > > 
> > > > > > > At the high level, shadow stack is:
> > > > > > > 
> > > > > > >       Allocated from a task's address space with vm_flags VM_SHSTK;
> > > > > > >       Its PTEs must be read-only and dirty;
> > > > > > >       Fixed sized, but the default size can be changed by sys admin.
> > > > > > > 
> > > > > > > For a forked child, the shadow stack is duplicated when the next
> > > > > > > shadow stack access takes place.
> > > > > > > 
> > > > > > > For a pthread child, a new shadow stack is allocated.
> > > > > > > 
> > > > > > > The signal handler uses the same shadow stack as the main program.
> > > > > > > 
> > > > > > 
> > > > > > Even with sigaltstack()?
> > > > > > 
> > > > > 
> > > > > Yes.
> > > > 
> > > > I am not convinced that it would work, as we switch stacks, oveflow might
> > > > be an issue. I also forgot to bring up setcontext(2), I presume those
> > > > will get new shadow stacks
> > > 
> > > Do you mean signal stack/sigaltstack overflow or swapcontext in a signal
> > > handler?
> > > 
> > 
> > I meant any combination of that. If there is a user space threads implementation that uses sigaltstack for switching threads
> > 
> 
> Anyone who does that is nuts.  The whole point of user space threads
> is speed, and signals are very slow.  For userspace threads to work,
> we need an API to allocate new shadow stacks, and we need to use the
> extremely awkwardly defined RSTORSSP stuff to switch.  (I assume this
> is possible on an ISA level.  The docs are bad, and the mnemonics for
> the relevant instructions are nonsensical.)

The whole point was to ensure we don't break applications/code that work
today. I think as long as there is a shadow stack allocated corresponding
to the user space stack and we can Restore SSP as we switch things should be
fine.

Balbir Singh.