linux-kernel - Re: [RFC PATCH v4 20/27] x86/cet/shstk: Signal handling for shadow stack

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Date:   Wed, 3 Oct 2018 18:46:46 +0200
From:   Jann Horn <jannh@...gle.com>
To:     yu-cheng.yu@...el.com
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        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@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Florian Weimer <fweimer@...hat.com>, hjl.tools@...il.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>, rdunlap@...radead.org,
        ravi.v.shankar@...el.com, vedvyas.shanbhogue@...el.com
Subject: Re: [RFC PATCH v4 20/27] x86/cet/shstk: Signal handling for shadow stack

On Fri, Sep 21, 2018 at 5:09 PM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
> When setting up a signal, the kernel creates a shadow stack
> restore token at the current SHSTK address and then stores the
> token's address in the signal frame, right after the FPU state.
> Before restoring a signal, the kernel verifies and then uses the
> restore token to set the SHSTK pointer.
[...]
> +#ifdef CONFIG_X86_64
> +static int copy_ext_from_user(struct sc_ext *ext, void __user *fpu)
> +{
> +       void __user *p;
> +
> +       if (!fpu)
> +               return -EINVAL;
> +
> +       p = fpu + fpu_user_xstate_size + FP_XSTATE_MAGIC2_SIZE;
> +       p = (void __user *)ALIGN((unsigned long)p, 8);
> +
> +       if (!access_ok(VERIFY_READ, p, sizeof(*ext)))
> +               return -EFAULT;
> +
> +       if (__copy_from_user(ext, p, sizeof(*ext)))
> +               return -EFAULT;

Why do you first manually call access_ok(), then call
__copy_from_user() with the same size? Just use "if
(copy_from_user(ext, p, sizeof(*ext)))" (without underscores) and get
rid of the access_ok().

> +       if (ext->total_size != sizeof(*ext))
> +               return -EINVAL;
> +       return 0;
> +}
> +
> +static int copy_ext_to_user(void __user *fpu, struct sc_ext *ext)
> +{
> +       void __user *p;
> +
> +       if (!fpu)
> +               return -EINVAL;
> +
> +       if (ext->total_size != sizeof(*ext))
> +               return -EINVAL;
> +
> +       p = fpu + fpu_user_xstate_size + FP_XSTATE_MAGIC2_SIZE;
> +       p = (void __user *)ALIGN((unsigned long)p, 8);
> +
> +       if (!access_ok(VERIFY_WRITE, p, sizeof(*ext)))
> +               return -EFAULT;
> +
> +       if (__copy_to_user(p, ext, sizeof(*ext)))
> +               return -EFAULT;

Same as above.

> +       return 0;
> +}