linux-kernel - Re: [PATCH v10 13/40] arm64/mm: Map pages for guarded control stack

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Message-ID: <d43f8036-cc06-430c-9e9e-b938037fc64c@sirena.org.uk>
Date: Mon, 19 Aug 2024 17:33:24 +0100
From: Mark Brown <broonie@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Arnd Bergmann <arnd@...db.de>, Oleg Nesterov <oleg@...hat.com>,
	Eric Biederman <ebiederm@...ssion.com>,
	Shuah Khan <shuah@...nel.org>,
	"Rick P. Edgecombe" <rick.p.edgecombe@...el.com>,
	Deepak Gupta <debug@...osinc.com>, Ard Biesheuvel <ardb@...nel.org>,
	Szabolcs Nagy <Szabolcs.Nagy@....com>, Kees Cook <kees@...nel.org>,
	"H.J. Lu" <hjl.tools@...il.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Florian Weimer <fweimer@...hat.com>,
	Christian Brauner <brauner@...nel.org>,
	Thiago Jung Bauermann <thiago.bauermann@...aro.org>,
	Ross Burton <ross.burton@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-mm@...ck.org,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v10 13/40] arm64/mm: Map pages for guarded control stack

On Mon, Aug 19, 2024 at 10:10:36AM +0100, Catalin Marinas wrote:
> On Thu, Aug 01, 2024 at 01:06:40PM +0100, Mark Brown wrote:

> > +	if (system_supports_gcs() && (vm_flags & VM_SHADOW_STACK)) {
> > +		/*
> > +		 * An executable GCS isn't a good idea, and the mm
> > +		 * core can't cope with a shared GCS.
> > +		 */
> > +		if (vm_flags & (VM_EXEC | VM_ARM64_BTI | VM_SHARED))
> > +			return false;
> > +	}

> I wonder whether we should clear VM_MAYEXEC early on during the vma
> creation. This way the mprotect() case will be handled in the core code.
> At a quick look, do_mmap() seems to always set VM_MAYEXEC but discard it
> for non-executable file mmap. Last time I looked (when doing MTE) there
> wasn't a way for the arch code to clear specific VM_* flags, only to
> validate them. But I think we should just clear VM_MAYEXEC and also
> return an error for VM_EXEC in the core do_mmap() if VM_SHADOW_STACK. It
> would cover the other architectures doing shadow stacks.

Yes, I think adding something generic would make sense here.  That feels
like a cleanup which could be split out?

> Regarding VM_SHARED, how do we even end up with this via the
> map_shadow_stack() syscall? I can't see how one can pass MAP_SHARED to
> do_mmap() on this path. I'm fine with a VM_WARN_ON() if you want the
> check (and there's no way a user can trigger it).

It's just a defenesive programming thing, I'm not aware of any way in
which it should be possible to trigger this.

> Is there any arch restriction with setting BTI and GCS? It doesn't make
> sense but curious if it matters. We block the exec permission anyway
> (unless the BTI pages moved to PIE as well, I don't remember).

As you say BTI should be meaningless for a non-executable page like GCS,
I'm not aware of any way in which it matters.  BTI is separate to PIE.

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