From: Andy Lutomirski <luto@kernel.org>
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>
Subject: Re: selftests/x86/fsgsbase_64 test problem
Date: Mon, 29 Jan 2018 08:37:14 -0800 [thread overview]
Message-ID: <CALCETrW6SY8ktqSA-r108WkNf15js4eu0grDds=JVbNtBdSGqA@mail.gmail.com> (raw)
In-Reply-To: <46328204-e363-e517-f30c-c8c94ac1442c@zytor.com>
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.
next prev parent reply other threads:[~2018-01-29 16:37 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-26 15:36 selftests/x86/fsgsbase_64 test problem Dan Rue
2018-01-26 16:22 ` Andy Lutomirski
2018-01-26 18:59 ` Andy Lutomirski
2018-01-26 19:46 ` Andy Lutomirski
2018-01-26 22:38 ` Andy Lutomirski
2018-01-26 22:42 ` Andy Lutomirski
2018-01-28 19:21 ` Andy Lutomirski
2018-01-29 9:13 ` H. Peter Anvin
2018-01-29 16:37 ` Andy Lutomirski [this message]
2018-01-29 18:12 ` H. Peter Anvin
2018-01-29 18:26 ` Andy Lutomirski
2018-01-29 18:30 ` H. Peter Anvin
2018-02-27 22:59 ` Dan Rue
2018-01-26 22:56 ` Borislav Petkov
2018-01-28 19:21 ` Andy Lutomirski
2018-01-26 22:51 ` H. Peter Anvin
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