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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 17 Jan 2019 12:27:23 -0800
Message-ID: <CALCETrUMAsXoZogEJg7ssv0CO56vzBV2C7VotmWcwNM7iH9Wqw@mail.gmail.com>
Subject: Re: [PATCH 06/17] x86/alternative: use temporary mm for text poking
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Nadav Amit <nadav.amit@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>, linux_dti@icloud.com,
        linux-integrity <linux-integrity@vger.kernel.org>,
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        Kristen Carlson Accardi <kristen@linux.intel.com>,
        "Dock, Deneen T" <deneen.t.dock@intel.com>,
        Nadav Amit <namit@vmware.com>,
        Kees Cook <keescook@chromium.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>
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On Wed, Jan 16, 2019 at 4:33 PM Rick Edgecombe
<rick.p.edgecombe@intel.com> wrote:
>
> From: Nadav Amit <namit@vmware.com>
>
> text_poke() can potentially compromise the security as it sets temporary
> PTEs in the fixmap. These PTEs might be used to rewrite the kernel code
> from other cores accidentally or maliciously, if an attacker gains the
> ability to write onto kernel memory.

i think this may be sufficient, but barely.

> +       pte_clear(poking_mm, poking_addr, ptep);
> +
> +       /*
> +        * __flush_tlb_one_user() performs a redundant TLB flush when PTI is on,
> +        * as it also flushes the corresponding "user" address spaces, which
> +        * does not exist.
> +        *
> +        * Poking, however, is already very inefficient since it does not try to
> +        * batch updates, so we ignore this problem for the time being.
> +        *
> +        * Since the PTEs do not exist in other kernel address-spaces, we do
> +        * not use __flush_tlb_one_kernel(), which when PTI is on would cause
> +        * more unwarranted TLB flushes.
> +        *
> +        * There is a slight anomaly here: the PTE is a supervisor-only and
> +        * (potentially) global and we use __flush_tlb_one_user() but this
> +        * should be fine.
> +        */
> +       __flush_tlb_one_user(poking_addr);
> +       if (cross_page_boundary) {
> +               pte_clear(poking_mm, poking_addr + PAGE_SIZE, ptep + 1);
> +               __flush_tlb_one_user(poking_addr + PAGE_SIZE);
> +       }

In principle, another CPU could still have the old translation.  Your
mutex probably makes this impossible, but it makes me nervous.
Ideally you'd use flush_tlb_mm_range(), but I guess you can't do that
with IRQs off.  Hmm.  I think you should add an inc_mm_tlb_gen() here.
Arguably, if you did that, you could omit the flushes, but maybe
that's silly.

If we start getting new users of use_temporary_mm(), we should give
some serious thought to the SMP semantics.

Also, you're using PAGE_KERNEL.  Please tell me that the global bit
isn't set in there.

--Andy