linux-kernel - Re: [PATCH] KVM: arm64: nested: Fix VA sign extension in VNCR/TLBI paths

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Message-ID: <86wm6ickqp.wl-maz@kernel.org>
Date: Mon, 01 Sep 2025 15:26:06 +0100
From: Marc Zyngier <maz@...nel.org>
To: Gyujeong Jin <wlsrbwjd7232@...il.com>
Cc: oliver.upton@...ux.dev,
	joey.gouly@....com,
	suzuki.poulose@....com,
	yuzenghui@...wei.com,
	catalin.marinas@....com,
	will@...nel.org,
	kvmarm@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	gyutrange <wlsrbwjd643@...er.com>,
	stable@...r.kernel.org,
	DongHa Lee <gap-dev@...mple.com>,
	Daehyeon Ko <4ncient@...mple.com>,
	Geonha Lee <leegn4a@...mple.com>,
	Hyungyu Oh <dqpc_lover@...mple.com>,
	Jaewon Yang <r4mbb1@...mple.com>
Subject: Re: [PATCH] KVM: arm64: nested: Fix VA sign extension in VNCR/TLBI paths

On Mon, 01 Sep 2025 15:15:51 +0100,
Gyujeong Jin <wlsrbwjd7232@...il.com> wrote:
> 
> From: gyutrange <wlsrbwjd643@...er.com>
> 
> VNCR/TLBI VA reconstruction currently uses bit 48 as the sign bit,
> but for 48-bit virtual addresses the correct sign bit is bit 47.

No, that's not the case. Bit 55 is used at all times to determine
which half of the address space a VA gets resolved from.

> Using 48 can mis-canonicalize addresses in the negative half and may
> cause missed invalidations.
> 
> Although VNCR_EL2 encodes other architectural fields (RESS, BADDR;
> see Arm ARM D24.2.206), sign_extend64() interprets its second argument
> as the index of the sign bit. Passing 48 prevents propagation of the
> canonical sign bit for 48-bit VAs.
> 
> Impact:
> - Incorrect canonicalization of VAs with bit47=1

No. We are not trying to make the VA canonical.

> - Potential stale VNCR pseudo-TLB entries after TLBI or MMU notifier

No. The pseudo TLB is never created the first place.

> - Possible incorrect translation/permissions or DoS when combined
>   with other issues

Please explain, as "other issues" is not a valid argument.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.