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Date: Fri, 30 Sep 2016 09:25:45 -0700
From: Kees Cook <keescook@...omium.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Laura Abbott <labbott@...hat.com>, AKASHI Takahiro <takahiro.akashi@...aro.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>, David Brown <david.brown@...aro.org>,
Will Deacon <will.deacon@....com>, Catalin Marinas <catalin.marinas@....com>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, LKML <linux-kernel@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 3/3] arm64: dump: Add checking for writable and exectuable pages
On Fri, Sep 30, 2016 at 8:58 AM, Mark Rutland <mark.rutland@....com> wrote:
> On Thu, Sep 29, 2016 at 02:32:57PM -0700, Laura Abbott wrote:
>> @@ -219,6 +223,15 @@ static void note_page(struct pg_state *st, unsigned long addr, unsigned level,
>> unsigned long delta;
>>
>> if (st->current_prot) {
>> + if (st->check_wx &&
>> + ((st->current_prot & PTE_RDONLY) != PTE_RDONLY) &&
>> + ((st->current_prot & PTE_PXN) != PTE_PXN)) {
>> + WARN_ONCE(1, "arm64/mm: Found insecure W+X mapping at address %p/%pS\n",
>> + (void *)st->start_address,
>> + (void *)st->start_address);
>> + st->wx_pages += (addr - st->start_address) / PAGE_SIZE;
>> + }
>> +
>
> Would it be worth verifying that all kernel mappings are UXN, too?
>
> ARMv8 allows execute-only mappings, and a !UXN mapping could result in an info
> leak (e.g. pointers in MOVZ+MOVK sequences), or potential asynchronous issues
> (e.g. user instruction fetches accessing read-destructive device registers).
> All kernel mappings *should* be UXN.
I love this idea, but based on what came up with hardened usercopy,
there are a lot of readers of kernel memory still. I think the
expectations around UXN need to be clarified so we can reason about
things like perf that want to read the kernel text.
-Kees
--
Kees Cook
Nexus Security
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