Message-ID: <ZJQR7slVHvjeCQG8@arm.com>
Date:   Thu, 22 Jun 2023 10:18:38 +0100
From:   "szabolcs.nagy@....com" <szabolcs.nagy@....com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "broonie@...nel.org" <broonie@...nel.org>
Cc:     "Xu, Pengfei" <pengfei.xu@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "nadav.amit@...il.com" <nadav.amit@...il.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "david@...hat.com" <david@...hat.com>,
        "Schimpe, Christina" <christina.schimpe@...el.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "corbet@....net" <corbet@....net>, "nd@....com" <nd@....com>,
        "dethoma@...rosoft.com" <dethoma@...rosoft.com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "debug@...osinc.com" <debug@...osinc.com>,
        "pavel@....cz" <pavel@....cz>, "bp@...en8.de" <bp@...en8.de>,
        "mike.kravetz@...cle.com" <mike.kravetz@...cle.com>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "jamorris@...ux.microsoft.com" <jamorris@...ux.microsoft.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "john.allen@....com" <john.allen@....com>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "bsingharora@...il.com" <bsingharora@...il.com>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "gorcunov@...il.com" <gorcunov@...il.com>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Syromiatnikov, Eugene" <esyr@...hat.com>,
        "Torvalds, Linus" <torvalds@...ux-foundation.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "Eranian, Stephane" <eranian@...gle.com>
Subject: Re: [PATCH v9 23/42] Documentation/x86: Add CET shadow stack
 description

The 06/21/2023 18:54, Edgecombe, Rick P wrote:
> On Wed, 2023-06-21 at 12:36 +0100, szabolcs.nagy@....com wrote:
> > > The 06/20/2023 19:34, Edgecombe, Rick P wrote:
> > > > > I actually did a POC for this, but rejected it. The problem is,
> > > > > if
> > > > > there is a shadow stack overflow at that point then the kernel
> > > > > > > can't
> > > > > push the shadow stack token to the old stack. And shadow stack
> > > > > > > overflow
> > > > > is exactly the alt shadow stack use case. So it doesn't really
> > > > > > > solve
> > > > > the problem.
> > > 
> > > the restore token in the alt shstk case does not regress anything
> > > but
> > > makes some use-cases work.
> > > 
> > > alt shadow stack is important if code tries to jump in and out of
> > > signal handlers (dosemu does this with swapcontext) and for that a
> > > restore token is needed.
> > > 
> > > alt shadow stack is important if the original shstk did not
> > > overflow
> > > but the signal handler would overflow it (small thread stack, huge
> > > sigaltstack case).
> > > 
> > > alt shadow stack is also important for crash reporting on shstk
> > > overflow even if longjmp does not work then. longjmp to a
> > > makecontext
> > > stack would still work and longjmp back to the original stack can
> > > be
> > > made to mostly work by an altshstk option to overwrite the top
> > > entry
> > > with a restore token on overflow (this can break unwinding though).
> > > 
> 
> There was previously a request to create an alt shadow stack for the
> purpose of handling shadow stack overflow. So you are now suggesting to
> to exclude that and instead target a different use case for alt shadow
> stack?

that is not what i said.

> But I'm not sure how much we should change the ABI at this point since
> we are constrained by existing userspace. If you read the history, we
> may end up needing to deprecate the whole elf bit for this and other
> reasons.

i'm not against deprecating the elf bit, but i think binary
marking will be difficult for this kind of feature no matter what
(code may be incompatible for complex runtime dependent reasons).

> So should we struggle to find a way to grow the existing ABI without
> disturbing the existing userspace? Or should we start with something,
> finally, and see where we need to grow and maybe get a chance at a
> fresh start to grow it?
> 
> Like, maybe 3 people will show up saying "hey, I *really* need to use
> shadow stack and longjmp from a ucontext stack", and no one says
> anything about shadow stack overflow. Then we know what to do. And
> maybe dosemu decides it doesn't need to implement shadow stack (highly
> likely I would think). Now that I think about it, AFAIU SS_AUTODISARM
> was created for dosemu, and the alt shadow stack patch adopted this
> behavior. So it's speculation that there is even a problem in that
> scenario.
> 
> Or maybe people just enable WRSS for longjmp() and directly jump back
> to the setjmp() point. Do most people want fast setjmp/longjmp() at the
> cost of a little security?
> 
> Even if, with enough discussion, we could optimize for all
> hypotheticals without real user feedback, I don't see how it helps
> users to hold shadow stack. So I think we should move forward with the
> current ABI.

you may not get a second chance to fix a security feature.
it will be just disabled if it causes problems.

Powered by blists - more mailing lists