Re: [RFC 0/7] Prep code for better stack switching

[Andy Lutomirski]

On Sat, Nov 11, 2017 at 2:58 AM, Borislav Petkov <bp@xxxxxxx> wrote:
> On Fri, Nov 10, 2017 at 08:05:19PM -0800, Andy Lutomirski wrote:
>> This isn't quite done (the TSS remap patch is busted on 32-bit, but
>> that's a straightforward fix), but it should be ready for at least a
>> conceptual review.
>>
>> The idea here is to prepare us to have all kernel data needed for
>> user mode execution and early entry located in the fixmap.  To do
>> this, I hijack the GDT remap mechanism and make it more general.  I
>> add a struct cpu_entry_area.  This struct is never instantiated
>> directly.  Instead, it represents the layout of a per-cpu portion of
>> the fixmap.  That portion contains the GDT, the TSS (including IO
>> bitmap), and the entry stack (for now just a part of the TSS
>> region).  It should also end up containing the PEBS and BTS buffers.
>>
>> If this works, then the idea would be to add a magic *executable* page
>> to cpu_entry_area.  That page would contain a stub like this:
>>
>> ENTRY(entry_SYSCALL_64_trampoline)
>>       UNWIND_HINT_EMPTY
>>       movq    %rsp, 0x1000+entry_SYSCALL_64_trampoline-1f(%rip)
>> 1:
>>       movq    0x1008+entry_SYSCALL_64_trampoline-1f(%rip), %rsp
>> 1:
>>       pushq   %rdi
>>       pushq   %rsi
>
>>       movq    0x1000+entry_SYSCALL_64_trampoline-1f(%rip), %rsi
>> 1:
>>       movq    $entry_SYSCALL_64, %rdi
>>       jmp     *%rdi
>
> So I'm wondering: r12-r15 are callee-preserved so why can't you
> scratch into those on entry and leave rsi and rdi pristine so that
> entry_SYSCALL_64 can get to work directly?

I'm not sure I understand your suggestion.  SYSCALL has always
preserved all regs except rcx, r11, flags, rax, and, depending on what
signals are involved, the argument registers.  r12-r15 are definitely
preserved, and existing userspace relies on that.

Anyway, I'm halfway through actually implementing this, and it looks a
wee bit different, but not much different.