linux-kernel - Re: [PATCH v2 29/39] x86/cet/shstk: Support wrss for userspace

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Message-ID: <202210052011.C991E0B8C@keescook>
Date:   Wed, 5 Oct 2022 20:11:58 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "Lutomirski, Andy" <luto@...nel.org>,
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Subject: Re: [PATCH v2 29/39] x86/cet/shstk: Support wrss for userspace

On Thu, Oct 06, 2022 at 12:38:06AM +0000, Edgecombe, Rick P wrote:
> On Mon, 2022-10-03 at 21:37 -0700, Kees Cook wrote:
> > On Mon, Oct 03, 2022 at 04:00:36PM -0700, Andy Lutomirski wrote:
> > > On 10/3/22 15:28, Kees Cook wrote:
> > > > On Thu, Sep 29, 2022 at 03:29:26PM -0700, Rick Edgecombe wrote:
> > > > > For the current shadow stack implementation, shadow stacks
> > > > > contents easily
> > > > > be arbitrarily provisioned with data.
> > > > 
> > > > I can't parse this sentence.
> > > > 
> > > > > This property helps apps protect
> > > > > themselves better, but also restricts any potential apps that
> > > > > may want to
> > > > > do exotic things at the expense of a little security.
> > > > 
> > > > Is anything using this right now? Wouldn't thing be safer without
> > > > WRSS?
> > > > (Why can't we skip this patch?)
> > > > 
> > > 
> > > So that people don't write programs that need either (shstk off) or
> > > (shstk
> > > on and WRSS on) and crash or otherwise fail on kernels that support
> > > shstk
> > > but don't support WRSS, perhaps?
> > 
> > Right, yes. I meant more "what programs currently need WRSS to
> > operate
> > under shstk? (And what is it that they are doing that needs it?)"
> > 
> > All is see currently is compiler self-tests and emulators using it?
> > 
> https://codesearch.debian.net/search?q=%5Cb%28wrss%7CWRSS%29%5Cb&literal=0&perpkg=1
> 
> Most apps that weren't just automatically compiled haven't had
> implementation effort yet. (of course glibc has had a bunch) I hope we
> would see more of that when we finally get it upstream. So I think a
> better question is, how many apps will need WRSS when they go to enable
> shadow stack. I'm thinking the answer must be some and it could be nice
> to catch them when they first investigate enabling it.
> 
> But yes, except for Mike's CRIU branch, there aren't any programs that
> use it today, and we could drop it for a first implementation. I don't
> see it as something that would only make things less safe though. It
> just lets apps that can't easily work within the stricter shadow stack
> environment, at least get access to a weaker but still beneficial one.
> 
> Kees, did you catch that it can be locked off while enabling shadow
> stack?

Yup, saw that! Looks good. Thanks. :)

-- 
Kees Cook