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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sat, 22 May 2021 16:39:19 -0700
Message-ID: <CALCETrVQ70VvZPf_tH5KOspYx6KwDg07dodzobBSABxMyc1xJw@mail.gmail.com>
Subject: Re: [PATCH v27 24/31] x86/cet/shstk: Handle thread shadow stack
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
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        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
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On Fri, May 21, 2021 at 3:14 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index 5ea2b494e9f9..8e5f772181b9 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -71,6 +71,53 @@ int shstk_setup(void)
>         return 0;
>  }
>
> +int shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
> +                            unsigned long stack_size)
> +{

...

> +       state = get_xsave_addr(&tsk->thread.fpu.state.xsave, XFEATURE_CET_USER);
> +       if (!state)
> +               return -EINVAL;
> +

The get_xsave_addr() API is horrible, and we already have one
egregiously buggy instance in the kernel.  Let's not add another
dubious use case.

If state == NULL, this means that CET_USER is in its init state.
CET_USER in the init state should behave identically regardless of
whether XINUSE[CET_USER] is set.  Can you please adjust this code
accordingly?

Thanks,
Andy