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

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Message-ID: <5643761f-cc38-4e41-9ddd-f0a1934f8724@sirena.org.uk>
Date: Wed, 21 Aug 2024 01:19:28 +0100
From: Mark Brown <broonie@...nel.org>
To: "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, Aug 20, 2024 at 11:57:23PM +0000, Edgecombe, Rick P wrote:
> On Wed, 2024-08-21 at 00:34 +0100, Mark Brown wrote:

> > I was doing things this way for symmetry with how we specify the normal
> > stack.  That's a bit different since the kernel will actually use the
> > size for the normal stack but it felt nicer to keep things looking
> > consistent, it saves users wondering why they work differently.  It's
> > also a bit of a help with portability given that arm64 expects to have a
> > top of stack marker above the token by default while x86 doesn't support
> > that.

> Hmm, so then on arm the kernel would look for the token down a frame. Hmm. I
> think it makes it even stranger ABI wise.

I think it's going to be strange one way or another, either you specify
a size that we don't currently really use or you have two things both
called stacks which are described differently.  I suppose we could call
a single parameter shadow_stack_pointer?  Though I do note that as you
indicated we've been going for some time and this is the first time it
came up...

> SHADOW_STACK_SET_MARKER can be optional (not on arm, but could be in the
> future). Then the shadow_stack_size to token offset behavior would depend on
> some historical originally supported combination of map_shadow_stack args.

I called it _SET_TOKEN, it's optional on arm64 - we check both potential
locations for the token in clone3().

> BTW, just to try to reduce potential future revisions, what do you think about
> the 8 byte alignment need? Did I miss the check somewhere?

I've added a check that both the base address and size are sizeof(void *)
aligned.

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