linux-kernel - Re: [PATCH RFT v4 0/5] fork: Support shadow stacks in clone3()

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Message-ID: <a3a04d9c-7c53-4399-b096-dee406716193@sirena.org.uk>
Date:   Fri, 1 Dec 2023 14:00:31 +0000
From:   Mark Brown <broonie@...nel.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "catalin.marinas@....com" <catalin.marinas@....com>,
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Subject: Re: [PATCH RFT v4 0/5] fork: Support shadow stacks in clone3()

On Thu, Nov 30, 2023 at 11:37:42PM +0000, Edgecombe, Rick P wrote:
> On Thu, 2023-11-30 at 21:51 +0000, Mark Brown wrote:
> > On Thu, Nov 30, 2023 at 07:00:58PM +0000, Catalin Marinas wrote:

> > explicitly request a new shadow stack.  There was some corner case
> > with
> > IIRC posix_nspawn() mentioned where the heuristics aren't what we
> > want
> > for example.

> Can't posix_spawn() pass in a shadow stack size into clone3 to get a
> new shadow stack after this series?

Yes, the above was addressing Catalin's suggestion that we add stack
size control separately to clone3() instead - doing that would remove
the ability to explicitly request a new stack unless we add a flag to
clone3() at which point we're back to modifying clone3() anyway.

> > > Another dumb question on arm64 - is GCSPR_EL0 writeable by the
> > > user? If
> > > yes, can the libc wrapper for threads allocate a shadow stack via
> > > map_shadow_stack() and set it up in the thread initialisation
> > > handler
> > > before invoking the thread function?

> > We would need a syscall to allow GCSPR_EL0 to be written.

> I think the problem with doing this is signals. If a signal is
> delivered to the new thread, then it could push to the old shadow stack
> before userspace gets a chance to switch. So the thread needs to start
> on a new shadow/stack.

That's an issue, plus using a syscall just wouldn't work with a security
model that locked down writes to the pointer which does seem like
something people would reasonably want to deploy.

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