linux-kernel - Re: [PATCH 02/35] prctl: Add flag for shadow stack writeability and push/pop

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Date:   Tue, 18 Jul 2023 20:10:35 +0100
From:   Mark Brown <broonie@...nel.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "corbet@....net" <corbet@....net>,
        "ardb@...nel.org" <ardb@...nel.org>,
        "maz@...nel.org" <maz@...nel.org>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Szabolcs.Nagy@....com" <Szabolcs.Nagy@....com>,
        "keescook@...omium.org" <keescook@...omium.org>,
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Subject: Re: [PATCH 02/35] prctl: Add flag for shadow stack writeability and
 push/pop

On Tue, Jul 18, 2023 at 05:47:32PM +0000, Edgecombe, Rick P wrote:
> On Sun, 2023-07-16 at 22:50 +0100, Mark Brown wrote:

> > On arm64 the kernel can separately control if userspace is able to
> > pop
> > and push values directly onto the shadow stack via GCS push and pop
> > instructions, supporting many scenarios where userspace needs to
> > write
> > to the stack with less security exposure than full write access.  Add
> > a
> > flag to allow this to be selected when changing the shadow stack
> > status.

> Is this correct? I thought Szabolcs was saying pop was always
> supported, but push was optional.

It's not, I wrote this right after looking at hypervisor controls which
do control push and pop.

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