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

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Date:   Tue, 08 Feb 2022 08:15:12 -0800
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Thomas Gleixner" <tglx@...utronix.de>,
        "Rick P Edgecombe" <rick.p.edgecombe@...el.com>,
        "H.J. Lu" <hjl.tools@...il.com>,
        "David Laight" <David.Laight@...lab.com>,
        "Adrian Reber" <adrian@...as.de>,
        "Cyrill Gorcunov" <gorcunov@...nvz.org>,
        "Eugene Syromiatnikov" <esyr@...hat.com>,
        "Dmitry Safonov" <0x7f454c46@...il.com>
Cc:     "Balbir Singh" <bsingharora@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Randy Dunlap" <rdunlap@...radead.org>,
        "Kees Cook" <keescook@...omium.org>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Florian Weimer" <fweimer@...hat.com>,
        "Nadav Amit" <nadav.amit@...il.com>,
        "Jann Horn" <jannh@...gle.com>, "kcc@...gle.com" <kcc@...gle.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "Pavel Machek" <pavel@....cz>, "Oleg Nesterov" <oleg@...hat.com>,
        "Weijiang Yang" <weijiang.yang@...el.com>,
        "Borislav Petkov" <bp@...en8.de>, "Arnd Bergmann" <arnd@...db.de>,
        "Moreira, Joao" <joao.moreira@...el.com>,
        "Mike Kravetz" <mike.kravetz@...cle.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "Dave Martin" <Dave.Martin@....com>,
        "john.allen@....com" <john.allen@....com>,
        "Ingo Molnar" <mingo@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Jonathan Corbet" <corbet@....net>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Linux API" <linux-api@...r.kernel.org>,
        "Cyrill Gorcunov" <gorcunov@...il.com>
Subject: Re: [PATCH 00/35] Shadow stacks for userspace

On Tue, Feb 8, 2022, at 1:31 AM, Thomas Gleixner wrote:
> On Mon, Feb 07 2022 at 17:31, Andy Lutomirski wrote:
>> So this leaves altshadowstack.  If we want to allow userspace to handle 
>> a shstk overflow, I think we need altshadowstack.  And I can easily 
>> imagine signal handling in a coroutine or user-threading evironment (Go? 
>> UMCG or whatever it's called?) wanting this.  As noted, this obnoxious 
>> Andy person didn't like putting any shstk-related extensions in the FPU 
>> state.
>>
>> For better or for worse, altshadowstack is (I think) fundamentally a new 
>> API.  No amount of ucontext magic is going to materialize an entire 
>> shadow stack out of nowhere when someone calls sigaltstack().  So the 
>> questions are: should we support altshadowstack from day one and, if so, 
>> what should it look like?
>
> I think we should support them from day one.
>
>> So I don't have a complete or even almost complete design in mind, but I 
>> think we do need to make a conscious decision either to design this 
>> right or to skip it for v1.
>
> Skipping it might create a fundamental design fail situation as it might
> require changes to the shadow stack signal handling in general which
> becomes a nightmare once a non-altstack API is exposed.

It would also expose a range of kernels in which shstk is on but programs that want altshadowstack don't have it.  That would be annoying.

>
>> As for CRIU, I don't think anyone really expects a new kernel, running 
>> new userspace that takes advantage of features in the new kernel, to 
>> work with old CRIU.
>
> Yes, CRIU needs updates, but what ensures that CRIU managed user space
> does not use SHSTK if CRIU is not updated yet?

In some sense this is like any other feature.  If a program uses timerfd but CRIU doesn't support timerfd, then it won't work.  SHSTK is a bit unique because it's likely that all programs on a system will start using it all at once.