Messages in this thread | | | Date | Sat, 11 Nov 2017 11:58:21 +0100 | From | Borislav Petkov <> | Subject | Re: [RFC 0/7] Prep code for better stack switching |
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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?
-- Regards/Gruss, Boris.
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) --
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