Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
From:   Andy Lutomirski <luto@amacapital.net>
Mime-Version: 1.0 (1.0)
Subject: Re: hv_hypercall_pg page permissios
Date:   Tue, 7 Apr 2020 14:01:25 -0700
Message-Id: <C311EB52-A796-4B94-AADD-CCABD19B377E@amacapital.net>
References: <20200407073830.GA29279@lst.de>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>, x86@kernel.org,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Stephen Hemminger <stephen@networkplumber.org>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
In-Reply-To: <20200407073830.GA29279@lst.de>
To:     Christoph Hellwig <hch@lst.de>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Apr 7, 2020, at 12:38 AM, Christoph Hellwig <hch@lst.de> wrote:
>=20
> =EF=BB=BFOn Tue, Apr 07, 2020 at 09:28:01AM +0200, Vitaly Kuznetsov wrote:=

>> Christoph Hellwig <hch@lst.de> writes:
>>=20
>>> Hi all,
>>>=20
>>> The x86 Hyper-V hypercall page (hv_hypercall_pg) is the only allocation
>>> in the kernel using __vmalloc with exectutable persmissions, and the
>>> only user of PAGE_KERNEL_RX.  Is there any good reason it needs to
>>> be readable?  Otherwise we could use vmalloc_exec and kill off
>>> PAGE_KERNEL_RX.  Note that before 372b1e91343e6 ("drivers: hv: Turn off
>>> write permission on the hypercall page") it was even mapped writable..
>>=20
>> [There is nothing secret in the hypercall page, by reading it you can
>> figure out if you're running on Intel or AMD (VMCALL/VMMCALL) but it's
>> likely not the only possible way :-)]
>>=20
>> I see no reason for hv_hypercall_pg to remain readable. I just
>> smoke-tested
>=20
> Thanks, I have the same in my WIP tree, but just wanted to confirm this
> makes sense.

Just to make sure we=E2=80=99re all on the same page: x86 doesn=E2=80=99t no=
rmally have an execute-only mode. Executable memory in the kernel is readabl=
e unless you are using fancy hypervisor-based XO support.=