linux-kernel - Re: [RFC PATCH 3/3] Prevent user from writing to IBT bitmap.

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Date:   Sat, 29 Jun 2019 16:44:54 -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>,
        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>
Subject: Re: [RFC PATCH 3/3] Prevent user from writing to IBT bitmap.

On Fri, Jun 28, 2019 at 12:50 PM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
>
> The IBT bitmap is visiable from user-mode, but not writable.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
>
> ---
>  arch/x86/mm/fault.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 59f4f66e4f2e..231196abb62e 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1454,6 +1454,13 @@ void do_user_addr_fault(struct pt_regs *regs,
>          * we can handle it..
>          */
>  good_area:
> +#define USER_MODE_WRITE (FAULT_FLAG_WRITE | FAULT_FLAG_USER)
> +       if (((flags & USER_MODE_WRITE)  == USER_MODE_WRITE) &&
> +           (vma->vm_flags & VM_IBT)) {
> +               bad_area_access_error(regs, hw_error_code, address, vma);
> +               return;
> +       }
> +

Just make the VMA have VM_WRITE and VM_MAYWRITE clear.  No new code
like this should be required.