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Date: Wed, 21 Feb 2024 09:58:01 -0500
From: "dalias@libc.org" <dalias@libc.org>
To: Mark Brown <broonie@kernel.org>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
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Subject: Re: [musl] Re: [PATCH v8 00/38] arm64/gcs: Provide support for GCS
 in userspace
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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 disabled,
> > > these suddenly turn into #UD generating instructions. So any other
> > > threads executing those instructions when shadow stack got disabled
> > > 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 stack
> > has been disabled, not generating SIGILL.
> 
> I'm not sure that's going to work out well, all it takes is some code
> that's looking at the shadow stack and expecting something to happen as
> a result of the instructions it's executing and we run into trouble.  A
> lot of things won't notice and will just happily carry on but I expect
> there are going to be things that care.  We also end up with an
> additional state for threads that have had shadow stacks transparently
> disabled, that's managable but still.

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.

> 
> > > > The place where it's really needed to be able to allocate the shadow
> > > > stack synchronously under userspace control, in order to harden
> > > > normal
> > > > applications that aren't doing funny things, is in pthread_create
> > > > without a caller-provided stack.
> 
> > > Yea most apps don't do anything too tricky. Mostly shadow stack "just
> > > works". But it's no excuse to just crash for the others.
> 
> > One thing to note here is that, to enable this, we're going to need
> > some way to detect "new enough kernel that shadow stack semantics are
> > all right". If there are kernels that have shadow stack support but
> > with problems that make it unsafe to use (this sounds like the case),
> > we can't turn it on without a way to avoid trying to use it on those.
> 
> If we have this automatic conversion of pages to shadow stack then we
> should have an API for enabling it, userspace should be able to use the
> presence of that API to determine if the feature is there.

Yes, or if a new prctl is needed to make disabling safe (see above)
that could probably be used.

Rich