linux-kernel - Re: [RFC PATCH v2 22/27] x86/cet/ibt: User-mode indirect branch tracking support

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Date:   Wed, 11 Jul 2018 14:07:09 -0700
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>, 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>, keescook@...omiun.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>,
        ravi.v.shankar@...el.com, vedvyas.shanbhogue@...el.com
Subject: Re: [RFC PATCH v2 22/27] x86/cet/ibt: User-mode indirect branch
 tracking support

On Tue, Jul 10, 2018 at 3:31 PM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
>
> Add user-mode indirect branch tracking enabling/disabling
> and supporting routines.
>
> Signed-off-by: H.J. Lu <hjl.tools@...il.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
[...]
> diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> index 4eba7790c4e4..8bbd63e1a2ba 100644
> --- a/arch/x86/kernel/cet.c
> +++ b/arch/x86/kernel/cet.c
[...]
> +static unsigned long ibt_mmap(unsigned long addr, unsigned long len)
> +{
> +       struct mm_struct *mm = current->mm;
> +       unsigned long populate;
> +
> +       down_write(&mm->mmap_sem);
> +       addr = do_mmap(NULL, addr, len, PROT_READ | PROT_WRITE,
> +                      MAP_ANONYMOUS | MAP_PRIVATE,
> +                      VM_DONTDUMP, 0, &populate, NULL);
> +       up_write(&mm->mmap_sem);
> +
> +       if (populate)
> +               mm_populate(addr, populate);
> +
> +       return addr;
> +}

Is this thing going to stay writable? Will any process with an IBT
bitmap be able to disable protections by messing with the bitmap even
if the lock-out mode is active? If so, would it perhaps make sense to
forbid lock-out mode if an IBT bitmap is active, to make it clear that
effective lock-out is impossible in that state?