Re: [PATCH v3 1/7] x86/ldt: refresh %fs and %gs in refresh_ldt_segments()

[Andy Lutomirski]

On Fri, Jun 22, 2018 at 11:47 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
>
>
>> On Jun 22, 2018, at 11:29 AM, H. Peter Anvin <h.peter.anvin@xxxxxxxxx> 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 donât depend on requalification because we never used to do it.
>
> Iâm 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 donât 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.