linux-kernel - Re: [PATCH v27 24/31] x86/cet/shstk: Handle thread shadow stack

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Message-ID: <CALCETrVQ70VvZPf_tH5KOspYx6KwDg07dodzobBSABxMyc1xJw@mail.gmail.com>
Date:   Sat, 22 May 2021 16:39:19 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>
Subject: Re: [PATCH v27 24/31] x86/cet/shstk: Handle thread shadow stack

On Fri, May 21, 2021 at 3:14 PM Yu-cheng Yu <yu-cheng.yu@...el.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