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

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Message-Id: <5a792e77-0072-4ded-9f89-e7fcc7f7a1d6@www.fastmail.com>
Date:   Mon, 28 Feb 2022 12:30:41 -0800
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Mike Rapoport" <rppt@...nel.org>
Cc:     "Rick P Edgecombe" <rick.p.edgecombe@...el.com>,
        "Cyrill Gorcunov" <gorcunov@...il.com>,
        "Balbir Singh" <bsingharora@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Eugene Syromiatnikov" <esyr@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Randy Dunlap" <rdunlap@...radead.org>,
        "Kees Cook" <keescook@...omium.org>,
        "Dmitry Safonov" <0x7f454c46@...il.com>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Adrian Reber" <adrian@...as.de>,
        "Florian Weimer" <fweimer@...hat.com>,
        "Nadav Amit" <nadav.amit@...il.com>,
        "Jann Horn" <jannh@...gle.com>, "Andrei Vagin" <avagin@...il.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "Borislav Petkov" <bp@...en8.de>,
        "Oleg Nesterov" <oleg@...hat.com>, "H.J. Lu" <hjl.tools@...il.com>,
        "Pavel Machek" <pavel@....cz>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "Arnd Bergmann" <arnd@...db.de>,
        "Moreira, Joao" <joao.moreira@...el.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Mike Kravetz" <mike.kravetz@...cle.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Weijiang Yang" <weijiang.yang@...el.com>,
        "Dave Martin" <Dave.Martin@....com>,
        "john.allen@....com" <john.allen@....com>,
        "Ingo Molnar" <mingo@...hat.com>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Jonathan Corbet" <corbet@....net>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Linux API" <linux-api@...r.kernel.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH 00/35] Shadow stacks for userspace



On Mon, Feb 28, 2022, at 12:27 PM, Mike Rapoport wrote:
> On Wed, Feb 09, 2022 at 06:37:53PM -0800, Andy Lutomirski wrote:
>> On 2/8/22 18:18, Edgecombe, Rick P wrote:
>> > On Tue, 2022-02-08 at 20:02 +0300, Cyrill Gorcunov wrote:
>> > 
>> > Still wrapping my head around the CRIU save and restore steps, but
>> > another general approach might be to give ptrace the ability to
>> > temporarily pause/resume/set CET enablement and SSP for a stopped
>> > thread. Then injected code doesn't need to jump through any hoops or
>> > possibly run into road blocks. I'm not sure how much this opens things
>> > up if the thread has to be stopped...
>> 
>> Hmm, that's maybe not insane.
>> 
>> An alternative would be to add a bona fide ptrace call-a-function mechanism.
>> I can think of two potentially usable variants:
>> 
>> 1. Straight call.  PTRACE_CALL_FUNCTION(addr) just emulates CALL addr,
>> shadow stack push and all.
>> 
>> 2. Signal-style.  PTRACE_CALL_FUNCTION_SIGFRAME injects an actual signal
>> frame just like a real signal is being delivered with the specified handler.
>> There could be a variant to opt-in to also using a specified altstack and
>> altshadowstack.
>
> Using ptrace() will not solve CRIU's issue with sigreturn because sigreturn
> is called from the victim context rather than from the criu process that
> controls the dump and uses ptrace().

I'm not sure I follow.

>
> Even with the current shadow stack interface Rick proposed, CRIU can restore
> the victim using ptrace without any additional knobs, but we loose an
> important ability to "self-cure" the victim from the parasite in case
> anything goes wrong with criu control process.
>
> Moreover, the issue with backward compatibility is not with ptrace but with
> sigreturn and it seems that criu is not its only user.

So we need an ability for a tracer to cause the tracee to call a function and to return successfully.  Apparently a gdb branch can already do this with shstk, and my PTRACE_CALL_FUNCTION_SIGFRAME should also do the trick.  I don't see why we need a sigretur-but-dont-verify -- we just need this mechanism to create a frame such that sigreturn actually works.

--Andy