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From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Sat, 2 Mar 2024 07:05:49 -0800
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Subject: Re: [musl] Re: [PATCH v8 00/38] arm64/gcs: Provide support for GCS in userspace
To: Mark Brown <broonie@kernel.org>, "dalias@libc.org" <dalias@libc.org>, 
	"Edgecombe, Rick P" <rick.p.edgecombe@intel.com>, 
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On Sat, Mar 2, 2024 at 6:57=E2=80=AFAM Szabolcs Nagy <nsz@port70.net> wrote=
:
>
> * Mark Brown <broonie@kernel.org> [2024-02-21 17:36:12 +0000]:
>
> > On Wed, Feb 21, 2024 at 09:58:01AM -0500, dalias@libc.org wrote:
> > > On Wed, Feb 21, 2024 at 01:53:10PM +0000, Mark Brown wrote:
> > > > On Tue, Feb 20, 2024 at 08:27:37PM -0500, dalias@libc.org wrote:
> > > > > On Wed, Feb 21, 2024 at 12:35:48AM +0000, Edgecombe, Rick P wrote=
:
> >
> > > > > > (INCSSP, RSTORSSP, etc). These are a collection of instructions=
 that
> > > > > > allow limited control of the SSP. When shadow stack gets disabl=
ed,
> > > > > > these suddenly turn into #UD generating instructions. So any ot=
her
> > > > > > threads executing those instructions when shadow stack got disa=
bled
> > > > > > would be in for a nasty surprise.
> >
> > > > > This is the kernel's problem if that's happening. It should be
> > > > > trapping these and returning immediately like a NOP if shadow sta=
ck
> > > > > has been disabled, not generating SIGILL.
> >
> > > > I'm not sure that's going to work out well, all it takes is some co=
de
> > > > that's looking at the shadow stack and expecting something to happe=
n as
> > > > a result of the instructions it's executing and we run into trouble=
  A
> >
> > > I said NOP but there's no reason it strictly needs to be a NOP. It
> > > could instead do something reasonable to convey the state of racing
> > > with shadow stack being disabled.
> >
> > This feels like it's getting complicated and I fear it may be an uphill
> > struggle to get such code merged, at least for arm64.  My instinct is
>
> the aarch64 behaviour is already nop
> for gcs instructions when gcs is disabled.
> the isa was designed so async disable is
> possible.
>
> only x86 linux would have to emulate this.

On Linux/x86, normal instructions are used to update SSP after
checking SHSTK is enabled.   If SHSTK is disabled in between,
program behavior may be undefined.

--=20
H.J.