linux-kernel - Re: [PATCH] arm64: kvm: Fix potential overflow in len

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Message-ID: <86jzbkp1cc.wl-maz@kernel.org>
Date: Sat, 28 Dec 2024 10:07:47 +0000
From: Marc Zyngier <maz@...nel.org>
To: Steven Davis <goldside000@...look.com>
Cc: "oliver.upton@...ux.dev"
	<oliver.upton@...ux.dev>,
	"catalin.marinas@....com"
	<catalin.marinas@....com>,
	"will@...nel.org" <will@...nel.org>,
	"joey.gouly@....com" <joey.gouly@....com>,
	"suzuki.poulose@....com"
	<suzuki.poulose@....com>,
	"yuzenghui@...wei.com" <yuzenghui@...wei.com>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>,
	"kvmarm@...ts.linux.dev"
	<kvmarm@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: kvm: Fix potential overflow in len

On Sat, 28 Dec 2024 02:01:27 +0000,
Steven Davis <goldside000@...look.com> wrote:
> 
> The MMIO sign-extension logic in kvm_handle_mmio_return can
> trigger an integer overflow or undefined behavior when len
> is invalid (e.g., len == 0 or len exceeds the size of unsigned
> long). Specifically, the expression (len * 8) - 1 may result
> in an out-of-bounds shift in the computation of the mask.
> 
> This patch adds validation to ensure len is greater than
> 0 and less than the size of unsigned long before performing
> the sign-extension logic. If len falls outside this range,
> the problematic logic is skipped, preventing potential issues.

I'd be curious to understand how you came to this conclusion, given
how len is computed. If anything, we could *remove* some of the
checks, rather than adding additional ones.

Also, "skipping" things is rarely an acceptable behaviour when
emulating hardware behaviour.

	M.

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