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SubjectRe: [RFC 0/7] Prep code for better stack switching
On Sat, Nov 11, 2017 at 2:58 AM, Borislav Petkov <bp@suse.de> 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.

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