linux-kernel - Re: [PATCH v3 1/7] x86/ldt: refresh %fs and %gs in refresh_ldt

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Date:   Wed, 27 Jun 2018 11:22:14 -0700
From:   hpa@...or.com
To:     Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <h.peter.anvin@...el.com>
CC:     LKML <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>
Subject: Re: [PATCH v3 1/7] x86/ldt: refresh %fs and %gs in refresh_ldt_segments()

On June 27, 2018 11:19:12 AM PDT, Andy Lutomirski <luto@...nel.org> wrote:
>On Fri, Jun 22, 2018 at 11:47 AM, Andy Lutomirski <luto@...capital.net>
>wrote:
>>
>>
>>
>>> On Jun 22, 2018, at 11:29 AM, H. Peter Anvin
><h.peter.anvin@...el.com> wrote:
>>>
>>>> On 06/22/18 07:24, Andy Lutomirski wrote:
>>>>
>>>> That RPL3 part is false.  The following program does:
>>>>
>>>> #include <stdio.h>
>>>>
>>>> int main()
>>>> {
>>>>    unsigned short sel;
>>>>    asm volatile ("mov %%ss, %0" : "=rm" (sel));
>>>>    sel &= ~3;
>>>>    printf("Will write 0x%hx to GS\n", sel);
>>>>    asm volatile ("mov %0, %%gs" :: "rm" (sel & ~3));
>>>>    asm volatile ("mov %%gs, %0" : "=rm" (sel));
>>>>    printf("GS = 0x%hx\n", sel);
>>>>    return 0;
>>>> }
>>>>
>>>> prints:
>>>>
>>>> Will write 0x28 to GS
>>>> GS = 0x28
>>>>
>>>> The x86 architecture is *insane*.
>>>>
>>>> Other than that, this patch seems generally sensible.  But my
>>>> objection that it's incorrect with FSGSBASE enabled for %fs and %gs
>>>> still applies.
>>>>
>>>
>>> Ugh, you're right... I misremembered.  The CPL simply overrides the
>RPL
>>> rather than trapping.
>>>
>>> We still need to give legacy applications which have zero idea about
>the
>>> separate bases that apply only to 64-bit mode a way to DTRT. 
>Requiring
>>> these old crufty applications to do something new is not an option.
>>
>>>
>>> As ugly as it is, I'm thinking the Right Thing is to simply make it
>a
>>> part of the Linux ABI that if the FS or GS selector registers point
>into
>>> the LDT then we will requalify them; if a 64-bit app does that then
>they
>>> get that behavior.  This isn't something that will happen
>>> asynchronously, and if a 64-bit process loads an LDT value into FS
>or
>>> GS, they are considered to have opted in to that behavior.
>>
>> But the old and crusty apps don’t depend on requalification because
>we never used to do it.
>>
>> I’m not convinced we ever need to refresh the base. In fact, we could
>start preserving the base of LDT-referencing FS/GS across context
>switches even without FSGSBASE at some minor performance cost, but I
>don’t really see the point. I still think my proposed semantics are
>easy to implement and preserve the ABI even if they have the sad
>property that the FSGSBASE behavior and the non-FSGSBASE behavior end
>up different.
>>
>
>There's another reasonable solution: do exactly what your patch does,
>minus the bugs.  We would need to get the RPL != 3 case right (easy)
>and the case where there's a non-running thread using the selector in
>question.  The latter is probably best handled by adding a flag to
>thread_struct that says "fsbase needs reloading from the descriptor
>table" and only applies if the selector is in the LDT or TLS area.  Or
>we could hijack a high bit in the selector.  Then we'd need to update
>everything that uses the fields.

Obviously fix the bugs.

How would you control this bit?
-- 
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