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Message-ID: <20170711200404.GA7472@leverpostej>
Date: Tue, 11 Jul 2017 21:04:04 +0100
From: Mark Rutland <mark.rutland@....com>
To: Laura Abbott <labbott@...hat.com>
Cc: Kees Cook <keescook@...omium.org>, Alex Popov <alex.popov@...ux.com>,
kernel-hardening@...ts.openwall.com,
Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: Re: [RFC][PATCH 2/2] arm64: Clear the stack
On Tue, Jul 11, 2017 at 08:51:55PM +0100, Mark Rutland wrote:
> On Mon, Jul 10, 2017 at 03:04:43PM -0700, Laura Abbott wrote:
> > + /* Reset the lowest stack to the top of the stack */
> > +7:
> > + ldr x1, [x0, TSK_STACK]
> > + add x1, x1, #THREAD_SIZE
> > + sub x1, x1, #256
> > + str x1, [x0, #TSK_TI_LOWEST_STACK]
>
> I take it this is the offsetting you were querying?
>
> I don't think it's quite right. Our stack looks like:
>
> +---+ <- task_stack_page(p) + THREAD_SIZE
> | |
> +---+ <- task_stack_page(p) + THREAD_START_SP
> | |
> | |
> +---+ <- task_pt_regs(p)
> | |
> | |
> | |
> ~~~~~
>
> ~~~~~
> | |
> | |
> | |
> +---+ <- task_stack_page(p)
>
> At the point we return to userspace, sp == task_pt_regs(p).
>
> Judging by a generated asm-offsets.h, sizeof(struct_pt_regs) is 304
> bytes currently. THREAD_SIZE - THREAD_START_SP == 16.
>
> We probably want to give that 16 a mnemonic (e.g FRAME_PADDING), and
> have something like:
>
> ldr x1, [x0, TSK_STACK]
> add x1, x1, #THREAD_SIZE
> sub x1, x1, #(S_FRAME_SIZE + FRAME_PADDING)
> str x1, [x0, #TSK_TI_LOWEST_STACK]
Thinking about it, given that sp == task_pt_regs(p), we could just do:
mov x1, sp
str x1, [x0, #TSK_TI_LOWEST_STACK]
... unless I've managed to lose the plot here.
Mark.
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