linux-kernel - Re: [PATCH RFC RFT v2 2/5] fork: Add shadow stack support to clone3()

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Date:   Thu, 16 Nov 2023 18:33:47 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH RFC RFT v2 2/5] fork: Add shadow stack support to clone3()

On Thu, 2023-11-16 at 18:14 +0000, Mark Brown wrote:
> On Thu, Nov 16, 2023 at 12:52:09AM +0000, Edgecombe, Rick P wrote:
> > On Wed, 2023-11-15 at 18:43 +0000, Mark Brown wrote:
> > > > end marker token (0) needs it i guess.
> 
> > > x86 doesn't currently have end markers.  Actually, that's a point
> > > -
> > > should we add a flag for specifying the use of end markers here?
> > > There's code in my map_shadow_stack() implementation for arm64
> > > which
> > > does that.
> 
> > Hmm, I guess there isn't a way to pass a flag for the initial exec
> > stack? So probably it should just mirror that behavior. Unless you
> > think a lot of people would like to skip the default behavior.
> 
> I don't really know that anyone would particularly want to use a flag
> on
> arm64, I was more thinking for the benefit of x86 where any
> termination
> record would be a change.  It's certainly easier to not have flags so
> I'm more than happy to leave things as they are, there's nothing
> stopping further extensions of the ABI if we decide we want them
> later.

I'm hoping that shifting the shadow stack start by one frame for thread
stacks (where there is no token to find) will not disturb anything. But
for x86, we will need a new elf bit to be fully safe in implementing
alt shadow stack. When we do that we will have a chance to add an end
of stack marker without compatibility issues on x86. So just doing
default behavior seems fine.

For map_shadow_stack, the end of stack marker will shift the token,
which userspace needs to find, so that is why I wanted a flag for that.
Appreciate the consideration.