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From:   "H.J. Lu" <hjl.tools@gmail.com>
Date:   Sat, 5 Feb 2022 05:29:55 -0800
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Subject: Re: [PATCH 00/35] Shadow stacks for userspace
To:     David Laight <David.Laight@aculab.com>
Cc:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
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On Sat, Feb 5, 2022 at 5:27 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Edgecombe, Rick P
> > Sent: 04 February 2022 01:08
> > Hi Thomas,
> >
> > Thanks for feedback on the plan.
> >
> > On Thu, 2022-02-03 at 22:07 +0100, Thomas Gleixner wrote:
> > > > Until now, the enabling effort was trying to support both Shadow
> > > > Stack and IBT.
> > > > This history will focus on a few areas of the shadow stack
> > > > development history
> > > > that I thought stood out.
> > > >
> > > >        Signals
> > > >        -------
> > > >        Originally signals placed the location of the shadow stack
> > > > restore
> > > >        token inside the saved state on the stack. This was
> > > > problematic from a
> > > >        past ABI promises perspective. So the restore location was
> > > > instead just
> > > >        assumed from the shadow stack pointer. This works because in
> > > > normal
> > > >        allowed cases of calling sigreturn, the shadow stack pointer
> > > > should be
> > > >        right at the restore token at that time. There is no
> > > > alternate shadow
> > > >        stack support. If an alt shadow stack is added later we
> > > > would
> > > >        need to
> > >
> > > So how is that going to work? altstack is not an esoteric corner
> > > case.
> >
> > My understanding is that the main usages for the signal stack were
> > handling stack overflows and corruption. Since the shadow stack only
> > contains return addresses rather than large stack allocations, and is
> > not generally writable or pivotable, I thought there was a good
> > possibility an alt shadow stack would not end up being especially
> > useful. Does it seem like reasonable guesswork?
>
> The other 'problem' is that it is valid to longjump out of a signal handler.
> These days you have to use siglongjmp() not longjmp() but it is still used.
>
> It is probably also valid to use siglongjmp() to jump from a nested
> signal handler into the outer handler.
> Given both signal handlers can have their own stack, there can be three
> stacks involved.
>
> I think the shadow stack pointer has to be in ucontext - which also
> means the application can change it before returning from a signal.
> In much the same way as all the segment registers can be changed
> leading to all the nasty bugs when the final 'return to user' code
> traps in kernel when loading invalid segment registers or executing iret.
>
> Hmmm... do shadow stacks mean that longjmp() has to be a system call?

No.  setjmp/longjmp save and restore shadow stack pointer.

--
H.J.