linux-kernel - Re: [PATCH 14/14] arm64: add VMAP

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Date:   Tue, 15 Aug 2017 12:10:32 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, keescook@...omium.org,
        ard.biesheuvel@...aro.org, matt@...eblueprint.co.uk,
        kernel-hardening@...ts.openwall.com, will.deacon@....com,
        linux-kernel@...r.kernel.org, luto@...capital.net,
        james.morse@....com, labbott@...hat.com
Subject: Re: [PATCH 14/14] arm64: add VMAP_STACK overflow detection

On Mon, Aug 07, 2017 at 07:36:05PM +0100, Mark Rutland wrote:
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index e5aa866..44a27c3 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -72,6 +72,37 @@
>  	.macro kernel_ventry	label
>  	.align 7
>  	sub	sp, sp, #S_FRAME_SIZE
> +#ifdef CONFIG_VMAP_STACK
> +	add	sp, sp, x0			// sp' = sp + x0
> +	sub	x0, sp, x0			// x0' = sp' - x0 = (sp + x0) - x0 = sp
> +	tbnz	x0, #THREAD_SHIFT, 0f
> +	sub	x0, sp, x0			// sp' - x0' = (sp + x0) - sp = x0
> +	sub	sp, sp, x0			// sp' - x0 = (sp + x0) - x0 = sp
> +	b	\label

Maybe a small comment before this hunk just to tell the user that it's
trying to test a bit in SP without corrupting a gpr. It's obvious once
you read it but not you see it for the first time ;).

> +
> +	/* Stash the original SP value in tpidr_el0 */
> +0:	msr	tpidr_el0, x0

And a comment here that on this path we no longer care about the user
tpidr_el0 as we are never returning there.

Otherwise I'm fine with the series (I'm not a fan of the complexity it
adds but I don't have a better suggestion).

-- 
Catalin