linux-kernel - Re: [PATCH RFT v4 2/5] fork: Add shadow stack support to clone3()

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Message-ID: <ed665d6f-66b0-4eeb-8cf8-db852e017d6a@sirena.org.uk>
Date:   Tue, 5 Dec 2023 15:51:19 +0000
From:   Mark Brown <broonie@...nel.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH RFT v4 2/5] fork: Add shadow stack support to clone3()

On Tue, Dec 05, 2023 at 12:26:57AM +0000, Edgecombe, Rick P wrote:
> On Tue, 2023-11-28 at 18:22 +0000, Mark Brown wrote:

> > -       size = adjust_shstk_size(stack_size);
> > +       size = adjust_shstk_size(size);
> >         addr = alloc_shstk(0, size, 0, false);

> Hmm. I didn't test this, but in the copy_process(), copy_mm() happens
> before this point. So the shadow stack would get mapped in current's MM
> (i.e. the parent). So in the !CLONE_VM case with shadow_stack_size!=0
> the SSP in the child will be updated to an area that is not mapped in
> the child. I think we need to pass tsk->mm into alloc_shstk(). But such
> an exotic clone usage does give me pause, regarding whether all of this
> is premature.

Hrm, right.  And we then can't use do_mmap() either.  I'd be somewhat
tempted to disallow that specific case for now rather than deal with it
though that's not really in the spirit of just always following what the
user asked for.

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