linux-kernel - Re: [PATCH 3/7] arm64: uaccess: Add additional userspace GCS accessors

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Message-ID: <50a0920d-3e3d-4e96-b68a-a7a0d78c3695@sirena.org.uk>
Date: Wed, 19 Mar 2025 13:24:15 +0000
From: Mark Brown <broonie@...nel.org>
To: Jeremy Linton <jeremy.linton@....com>
Cc: linux-trace-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
	mhiramat@...nel.org, oleg@...hat.com, peterz@...radead.org,
	acme@...nel.org, namhyung@...nel.org, mark.rutland@....com,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	irogers@...gle.com, adrian.hunter@...el.com,
	kan.liang@...ux.intel.com, thiago.bauermann@...aro.org,
	yury.khrustalev@....com, kristina.martsenko@....com,
	liaochang1@...wei.com, catalin.marinas@....com, will@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] arm64: uaccess: Add additional userspace GCS
 accessors

On Tue, Mar 18, 2025 at 03:48:37PM -0500, Jeremy Linton wrote:

> +static inline u64 load_user_gcs(unsigned long __user *addr, int *err)
> +{
> +	unsigned long ret;
> +	u64 load;
> +
> +	if (!access_ok((char __user *)addr, sizeof(load))) {
> +		*err = -EFAULT;
> +		return 0;
> +	}
> +
> +	gcsb_dsync();
> +	ret = copy_from_user(&load, addr, sizeof(load));
> +	if (ret != 0)
> +		*err = ret;
> +	return load;
> +}

A GCS load done by the hardware will verify that we are loading from GCS
memory (the accesses are marked as AccessType_GCS in the pseudocode
which is then validated in for example S1CheckPermissions()).  Sadly
there's no equivalent of GCSSTR so we'd need to do the permission check
ourselves to match this behaviour.

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