linux-kernel - Re: [PATCH v6 5/5] arm64: futex: support futex with FEAT

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Message-ID: <aKSKS2__HEZDfyoi@arm.com>
Date: Tue, 19 Aug 2025 15:29:31 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: will@...nel.org, broonie@...nel.org, maz@...nel.org,
	oliver.upton@...ux.dev, shameerali.kolothum.thodi@...wei.com,
	joey.gouly@....com, james.morse@....com, ardb@...nel.org,
	scott@...amperecomputing.com, suzuki.poulose@....com,
	yuzenghui@...wei.com, mark.rutland@....com,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 5/5] arm64: futex: support futex with FEAT_LSUI

On Tue, Aug 19, 2025 at 10:11:02AM +0100, Yeoreum Yun wrote:
> On Tue, Aug 19, 2025 at 09:38:54AM +0100, Catalin Marinas wrote:
> > On Mon, Aug 18, 2025 at 08:53:57PM +0100, Yeoreum Yun wrote:
> > > > On Sat, Aug 16, 2025 at 03:57:49PM +0100, Yeoreum Yun wrote:
> > > > > why we need to care about the different settings for tag checking when
> > > > > we use uaccess_disable_privileged()?
> > [...]
> > > > > But, although tag check fault happens in kernel side,
> > > > > It seems to be handled by fixup code if user address is wrong.
> > > >
> > > > The user may know it is wrong and not care (e.g. one wants to keep using
> > > > a buggy application).
> > >
> > > Then Does this example -- ignoring wrong and keep using a buggy
> > > application shows us that we need to enable TCO when
> > > we runs the LSUI instruction?
> > >
> > > AFAIK, LSUI instruction also check memory tag -- i.e) ldtadd.
> > > if passed user address which has unmatched tag and if user isn't
> > > interested in tah check, It can meet the unexpected report from KASAN.
> >
> > That's a valid point w.r.t. PSTATE.TCO that applies to copy_to/from_user
> > as well. I don't think we documented it but we don't expect the user
> > PSTATE.TCO state to be taken into account while doing uaccess from the
> > kernel. We do, however, expect SCTLR_EL1.TCF0 to be honoured and that's
> > what the user normally tweaks via a prctl(). The TCO is meant to
> > disable tag checking briefly when TCF enabled the tag check faults.
> 
> So, IMHO, as copy_to/from_user (ldt/sttr) enable tco before it operates,

They don't enable TCO.

-- 
Catalin