linux-hardening - Re: [PATCH 0/6] Memory Mapping (VMA) protection using PKU

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Date:   Tue, 16 May 2023 09:06:32 +0200
From:   Stephen Röttger <sroettger@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     jeffxu@...omium.org, luto@...nel.org, jorgelo@...omium.org,
        keescook@...omium.org, groeck@...omium.org, jannh@...gle.com,
        akpm@...ux-foundation.org, jeffxu@...gle.com,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-mm@...ck.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 0/6] Memory Mapping (VMA) protection using PKU - set 1

On Mon, May 15, 2023 at 4:28 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 5/15/23 06:05, jeffxu@...omium.org wrote:
> > We're using PKU for in-process isolation to enforce control-flow integrity
> > for a JIT compiler. In our threat model, an attacker exploits a
> > vulnerability and has arbitrary read/write access to the whole process
> > space concurrently to other threads being executed. This attacker can
> > manipulate some arguments to syscalls from some threads.
>
> This all sounds like it hinges on the contents of PKRU in the attacker
> thread.
>
> Could you talk a bit about how the attacker is prevented from running
> WRPKRU, XRSTOR or compelling the kernel to write to PKRU like at sigreturn?

(resending without html)

Since we're using the feature for control-flow integrity, we assume
the control-flow is still intact at this point. I.e. the attacker
thread can't run arbitrary instructions.
* For JIT code, we're going to scan it for wrpkru instructions before
writing it to executable memory
* For regular code, we only use wrpkru around short critical sections
to temporarily enable write access

Sigreturn is a separate problem that we hope to solve by adding pkey
support to sigaltstack

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