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

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Date:   Sat, 11 Nov 2017 11:58:21 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC 0/7] Prep code for better stack switching

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)
--