linux-kernel - Re: [Xen-devel] Does __KERNEL

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Message-ID: <CALCETrXeU7oi9F+k+mrjV=jo9TW9ic8tOA094j9mbyrX12Jazw@mail.gmail.com>
Date:	Fri, 8 Apr 2016 09:00:21 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andrew Cooper <andrew.cooper3@...rix.com>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>, kvm list <kvm@...r.kernel.org>,
	xen-devel <Xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] Does __KERNEL_DS serve a purpose?

On Fri, Apr 8, 2016 at 1:01 AM, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
> On 08/04/2016 01:24, Andy Lutomirski wrote:
>> I can't see any reason that we need the __KERNEL_DS segment at all --
>> I think that everything that uses __KERNEL_DS could use __USER_DS
>> instead.  Am I missing anything?  This has been bugging me for a
>> while.
>>
>> I mulled over this a bit when trying to understand the sysret_ss_attrs
>> bug and then forgot about it.
>
> Linux doesn't have a separate __KERNEL_SS.  For the plain data segments,
> the dpl is not interesting.
>
> However, %ss is also loaded with __KERNEL_DS, and %ss.dpl is somewhat
> important.

But %ss can be loaded with 0 on 64-bit kernels.  (I assume that
loading 0 into %ss sets SS.DPL to 0 if done at CPL0, but I'm vague on
this, since it only really matters to hypervisor code AFAIK.)

32-bit kernels need __KERNEL_DS, I think.

--Andy