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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 29 Jan 2018 08:37:14 -0800
Message-ID: <CALCETrW6SY8ktqSA-r108WkNf15js4eu0grDds=JVbNtBdSGqA@mail.gmail.com>
Subject: Re: selftests/x86/fsgsbase_64 test problem
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Dan Rue <dan.rue@linaro.org>, Shuah Khan <shuah@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Dmitry Safonov <dsafonov@virtuozzo.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Jan 29, 2018 at 1:13 AM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 01/28/18 11:21, Andy Lutomirski wrote:
>>>
>>> I think the bug is here.  I think that, when writing a NULL selector
>>> to DS, ES, FS, or GS, Intel CPUs incorrectly set DPL == RPL, whereas
>>> they should set DPL to 3.
>>
>> As an experiment, I did this:
>>
>>  DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = {
>> +       [0] = { .dpl = 3, },
>> +
>>
>> This had no apparent effect.  I was hoping that maybe loading NULL
>> into a selector would copy DPL from from gdt[0], but it seems like it
>> doesn't.
>>
>
> GDT[0] doesn't actually exist.

That's what I thought, too, and the SDM does say that, but the SDM
says all kinds of not-quite-correct things about segmentation.

> It is pretty much scratch space (I have
> suggested using it for the gsbase once all those issues get sorted out,
> because it lets the paranoid code do something like:
>
>         rdgsbase %rax
>         push %rax       /* Save old gsbase */
>         push %rax       /* Reserve space on stack */
>         sgdt -2(%rsp)   /* We don't care about the limit */
>         pop %rax        /* %rax <- gdtbase */
>         mov (%rax),%rax /* GDT[0] holds the gsbase for this cpu */
>         wrgsbase %rax

That will utterly suck on non-UMIP machines that have
hypervisor-provided UMIP emulation.