linux-kernel - Re: [PATCH v4 4/8] arm64: uaccess: Add additional userspace GCS accessors

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Message-ID: <6e4cf749-d146-4f41-afec-6c07b025be93@sirena.org.uk>
Date: Wed, 23 Jul 2025 12:01:07 +0100
From: Mark Brown <broonie@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Jeremy Linton <jeremy.linton@....com>,
	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, will@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/8] arm64: uaccess: Add additional userspace GCS
 accessors

On Wed, Jul 23, 2025 at 10:50:33AM +0100, Catalin Marinas wrote:
> On Fri, Jul 18, 2025 at 11:37:36PM -0500, Jeremy Linton wrote:
> > +/*
> > + * Unlike put_user_gcs() above, the use of copy_from_user() may provide
> > + * an opening for non GCS pages to be used to source data. Therefore this
> > + * should only be used in contexts where that is acceptable.
> > + */

> Even in user space, the GCS pages can be read with normal loads, so
> already usable as a data source if one wants to (not that it's of much
> use). So not sure the comment here is needed.

The comment should probably be clarified to mention that the specific
issue is the lack of an operation that does a load with GCS permission
check to match what we have for stores, it'd be a bit of a landmine to
have the operation available without anything that flags that it didn't
validate the permissions (even assuming people read the comments in the
header...).

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