linux-kernel - Re: [PATCH 00/35] Shadow stacks for userspace

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Date:   Tue, 8 Feb 2022 20:02:32 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Mike Rapoport <rppt@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Adrian Reber <adrian@...as.de>,
        Rick P Edgecombe <rick.p.edgecombe@...el.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Moreira, Joao" <joao.moreira@...el.com>,
        "john.allen@....com" <john.allen@....com>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Dmitry Safonov <0x7f454c46@...il.com>
Subject: Re: [PATCH 00/35] Shadow stacks for userspace

On Tue, Feb 08, 2022 at 08:21:20AM -0800, Andy Lutomirski wrote:
> >> But such a knob will immediately reduce the security value of the entire
> >> thing, and I don't have good ideas how to deal with it :(
> >
> > Probably a kind of latch in the task_struct which would trigger off once
> > returt to a different address happened, thus we would be able to jump inside
> > paratite code. Of course such trigger should be available under proper
> > capability only.
> 
> I'm not fully in touch with how parasite, etc works.  Are we talking about save or restore?

We use parasite code in question during checkpoint phase as far as I remember.
push addr/lret trick is used to run "injected" code (code injection itself is
done via ptrace) in compat mode at least. Dima, Andrei, I didn't look into this code
for years already, do we still need to support compat mode at all?

> If it's restore, what exactly does CRIU need to do?  Is it just that CRIU needs to return
> out from its resume code into the to-be-resumed program without tripping CET?  Would it
> be acceptable for CRIU to require that at least one shstk slot be free at save time?
> Or do we need a mechanism to atomically switch to a completely full shadow stack at resume?
> 
> Off the top of my head, a sigreturn (or sigreturn-like mechanism) that is intended for
> use for altshadowstack could safely verify a token on the altshdowstack, possibly
> compare to something in ucontext (or not -- this isn't clearly necessary) and switch
> back to the previous stack.  CRIU could use that too.  Obviously CRIU will need a way
> to populate the relevant stacks, but WRUSS can be used for that, and I think this
> is a fundamental requirement for CRIU -- CRIU restore absolutely needs a way to write
> the saved shadow stack data into the shadow stack.
> 
> So I think the only special capability that CRIU really needs is WRUSS, and
> we need to wire that up anyway.

Thanks for these notes, Andy! I can't provide any sane answer here since didn't
read tech spec for this feature yet :-)