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References: <20230515130553.2311248-1-jeffxu@chromium.org> <2bcffc9f-9244-0362-2da9-ece230055320@intel.com>
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From:   =?UTF-8?Q?Stephen_R=C3=B6ttger?= <sroettger@google.com>
Date:   Tue, 16 May 2023 09:06:32 +0200
Message-ID: <CAEAAPHYdRyZEMp97919errF7SDuYBJoSrD5i1wrTx1sMdr_ZdQ@mail.gmail.com>
Subject: Re: [PATCH 0/6] Memory Mapping (VMA) protection using PKU - set 1
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     jeffxu@chromium.org, luto@kernel.org, jorgelo@chromium.org,
        keescook@chromium.org, groeck@chromium.org, jannh@google.com,
        akpm@linux-foundation.org, jeffxu@google.com,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-mm@kvack.org, linux-hardening@vger.kernel.org
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On Mon, May 15, 2023 at 4:28=E2=80=AFPM Dave Hansen <dave.hansen@intel.com>=
 wrote:
>
> On 5/15/23 06:05, jeffxu@chromium.org wrote:
> > We're using PKU for in-process isolation to enforce control-flow integr=
ity
> > 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 sigretur=
n?

(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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