linux-kernel - Re: [PATCH RFT v8 4/9] fork: Add shadow stack support to clone3()

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Message-ID: <Zr9wSa2Yyq-MCWVq@arm.com>
Date: Fri, 16 Aug 2024 16:29:13 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Mark Brown <broonie@...nel.org>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
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Subject: Re: [PATCH RFT v8 4/9] fork: Add shadow stack support to clone3()

On Fri, Aug 16, 2024 at 11:51:57AM +0100, Mark Brown wrote:
> On Fri, Aug 16, 2024 at 09:44:46AM +0100, Catalin Marinas wrote:
> > We could, in theory, consume this token in the parent before the child
> > mm is created. The downside is that if a parent forks multiple
> > processes using the same shadow stack, it will have to set the token
> > each time. I'd be fine with this, that's really only for the mostly
> > theoretical case where one doesn't use CLONE_VM and still want a
> > separate stack and shadow stack.
> 
> I originally implemented things that way but people did complain about
> the !CLONE_VM case, which does TBH seem reasonable.  Note that the
> parent won't as standard be able to set the token again - since the
> shadow stack is not writable to userspace by default it'd instead need
> to allocate a whole new shadow stack for each child.

Ah, good point.

> I change back to parsing the token in the parent but I don't want to end
> up in a cycle of bouncing between the two implementations depending on
> who's reviewed the most recent version.

You and others spent a lot more time looking at shadow stacks than me.
I'm not necessarily asking to change stuff but rather understand the
choices made.

-- 
Catalin