linux-kernel - Re: [PATCH RFC RFT v2 5/5] kselftest/clone3: Test shadow stack support

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Date:   Mon, 20 Nov 2023 15:47:52 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Deepak Gupta <debug@...osinc.com>
Cc:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
        "Szabolcs.Nagy@....com" <Szabolcs.Nagy@....com>,
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        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
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        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
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        "will@...nel.org" <will@...nel.org>,
        "Pandey, Sunil K" <sunil.k.pandey@...el.com>
Subject: Re: [PATCH RFC RFT v2 5/5] kselftest/clone3: Test shadow stack
 support

On Fri, Nov 17, 2023 at 01:12:46PM -0800, Deepak Gupta wrote:
> On Tue, Nov 14, 2023 at 11:11:58PM +0000, Edgecombe, Rick P wrote:

> > It seems like there will be a need for some generic method of checking
> > if shadow stack is enabled. Maybe a more generic compiler
> > intrinsic/builtin or glibc API (something unrelated to SSP)?

> Exposing a new file under procfs would be useful?
> Something like "/proc/sys/vm/user_shadow_stack_supported"

> `map_shadow_stack` can return MAP_FAILED for other reasons.
> I think `kselftests` are fine but I don't want people to pick up this
> as test code and run with it in production :-)

> So kernel providing a way to indicate whether it supports shadow stack
> mappings in user mode via procfs would be useful and arch agnostic.

I can see that might be useful for some higher level code that wants to
tune the size and nothing else.  I'd be tempted to do it through adding
a tuneable for the maximum default shadow stack size (x86 currently uses
4G) just so it's *vaguely* useful rather than just a file.  I question
the utility of that but just a plain file doesn't feel quite idiomatic.

In any case it feels like it's off topic for this series and should be
done separately.

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