linux-kernel - Re: [PATCH RFT v9 4/8] fork: Add shadow stack support to clone3()

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Message-ID: <6bf15851-03fe-40cd-b95c-f7e2ca40ac54@sirena.org.uk>
Date: Tue, 1 Oct 2024 18:33:14 +0100
From: Mark Brown <broonie@...nel.org>
To: Christian Brauner <brauner@...nel.org>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
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Subject: Re: [PATCH RFT v9 4/8] fork: Add shadow stack support to clone3()

On Tue, Oct 01, 2024 at 05:12:38PM +0200, Christian Brauner wrote:
> On Fri, Sep 27, 2024 at 03:21:59PM GMT, Edgecombe, Rick P wrote:

> > Did you catch that a token can be at a different offsets location on the stack
> > depending on args passed to map_shadow_stack? So userspace will need something
> > like the code above, but that adjusts the 'shadow_stack_size' such that the
> > kernel looks for the token in the right place. It will be even weirder if
> > someone uses clone3 to switch to a stack that has already been used, and pivoted
> > off of, such that a token was left in the middle of the stack. In that case
> > userspace would have to come up with args disconnected from the actual size of
> > the shadow stack such that the kernel would be cajoled into looking for the
> > token in the right place.
> > 
> > A shadow stack size is more symmetric on the surface, but I'm not sure it will
> > be easier for userspace to handle. So I think we should just have a pointer to
> > the token. But it will be a usable implementation either way.

My suspicion would be that if we're doing the pivot to a previously used
shadow stack we'd also be pivoting the regular stack along with it which
would face similar issues with having an unusual method for specifying
the stack top so I don't know how much we're really winning.  Like we
both keep saying either of the interfaces works though, it's just a
taste question with both having downsides.

> Maybe it's best to let glibc folks decide what is better/more ergonomic for them.

The relevant people are on the thread I think.  

I've rebased onto v6.12-rc1, assuming I don't notice anything horrible
in testing I'll post that with the ABI unchanged for now.

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