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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 7 Jun 2018 11:23:18 -0700
Message-ID: <CALCETrU0gXZOZ90vUSMV_csop2O4TpKW_8D17no-wp-X3nutxA@mail.gmail.com>
Subject: Re: [PATCH 01/10] x86/cet: User-mode shadow stack support
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Florian Weimer <fweimer@redhat.com>
Cc:     Andrew Lutomirski <luto@kernel.org>,
        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
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On Thu, Jun 7, 2018 at 10:50 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> On Thu, 2018-06-07 at 09:37 -0700, Andy Lutomirski wrote:
> > On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> > >
> > > This patch adds basic shadow stack enabling/disabling routines.
> > > A task's shadow stack is allocated from memory with VM_SHSTK
> > > flag set and read-only protection.  The shadow stack is
> > > allocated to a fixed size and that can be changed by the system
> > > admin.
> >
> > How do threads work?  Can a user program mremap() its shadow stack to
> > make it bigger?
>
> A pthread's shadow stack is allocated/freed by the kernel.  This patch
> has the supporting routines that handle both non-pthread and pthread.
>
> In [PATCH 04/10] "Handle thread shadow stack", we allocate pthread
> shadow stack in copy_thread_tls(), and free it in deactivate_mm().
>
> If clone of a pthread fails, shadow stack is freed in
> cet_disable_free_shstk() below (I will add more comments):
>
> If (Current thread existing)
>         Disable and free shadow stack
>
> If (Clone of a pthread fails)
>         Free the pthread shadow stack
>
> We block mremap, mprotect, madvise, and munmap on a vma that has
> VM_SHSTK (in separate patches).

Why?  mremap() seems like a sensible way to enlarge a shadow stack.
munmap() seems like a good way to get rid of one, and mmap() seems
like a nice way to create a new shadow stack if one were needed (for
green threads or similar).