linux-kernel - Re: [PATCH 1/8] arm64: Enable VMAP

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Message-ID: <aG0Lpy1c2LuSPdj2@willie-the-truck>
Date: Tue, 8 Jul 2025 13:14:31 +0100
From: Will Deacon <will@...nel.org>
To: Breno Leitao <leitao@...ian.org>
Cc: Mark Rutland <mark.rutland@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Ard Biesheuvel <ardb@...nel.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-efi@...r.kernel.org, leo.yan@....com, kernel-team@...a.com
Subject: Re: [PATCH 1/8] arm64: Enable VMAP_STACK support

Hey Breno,

Thanks for doing this.

On Tue, Jul 08, 2025 at 02:39:56AM -0700, Breno Leitao wrote:
> On Mon, Jul 07, 2025 at 06:23:09PM +0100, Mark Rutland wrote:
> > On Mon, Jul 07, 2025 at 09:01:01AM -0700, Breno Leitao wrote:
> > > Enable virtually mapped kernel stacks for ARM64. This provides better
> > > stack overflow detection and improved security by mapping kernel stacks
> > > in vmalloc space rather than using direct mapping.
> > > 
> > > VMAP_STACK helps catch stack overflows early by placing guard pages
> > > around kernel stacks, and also provides better isolation between
> > > kernel stacks and other kernel data structures.
> > > 
> > > All dependencies are satisfied for arm64: HAVE_ARCH_VMAP_STACK is
> > > already selected above, and KASAN_VMALLOC is selected when KASAN is
> > > enabled, meeting the KASAN dependency requirements.
> > 
> > I reckon it might be better to say something like:
> > 
> > | arm64: Mandate VMAP_STACK
> > |
> > | On arm64, VMAP_STACK has been enabled by default for a while now, and
> > | the only reason to disable it was a historical lack of support for
> > | KASAN_VMALLOC. Today there's no good reason to disable VMAP_STACK.
> > |
> > | Mandate VMAP_STACK, which will allow code to be simplified in
> > | subsequent patches.
> > 
> > ... to make it clear that we're not changing the default, and we are
> > removing the ability to deselect VMAP_STACK.
> > 
> > Either way, the patch itself looks good to me.
> 
> Thanks for the suggestion. I will update and respin.

No need to respin just for that; I can fold the above when I apply.

Cheers,

Will