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Peter Anvin" , Jann Horn , kasan-dev@googlegroups.com Subject: Re: [REGRESSION] x86_32 boot hang in 6.19-rc7 caused by b505f1944535 ("x86/kfence: avoid writing L1TF-vulnerable PTEs") To: Ryusuke Konishi References: <20260106180426.710013-1-andrew.cooper3@citrix.com> <20260107151700.c7b9051929548391e92cfb3e@linux-foundation.org> Content-Language: en-GB From: Andrew Cooper In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: LO4P123CA0503.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:1ab::22) To CH8PR03MB8275.namprd03.prod.outlook.com (2603:10b6:610:2b9::7) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CH8PR03MB8275:EE_|DM6PR03MB5113:EE_ X-MS-Office365-Filtering-Correlation-Id: 2fab5a24-b747-435b-02f1-08de5d12abfa X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|7416014|366016|376014; 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The kernel hangs immediately after "Booting the kernel" > and does not produce any early console output. > > A git bisect identified the following commit as the first bad commit: > b505f1944535 ("x86/kfence: avoid writing L1TF-vulnerable PTEs") > > Environment and Config: > - Guest Arch: x86_32 (one of my test VMs) > - Memory Config: # CONFIG_X86_PAE is not set > - KFENCE Config: CONFIG_KFENCE=y > - Host/Hypervisor: x86_64 host running KVM > > The system fails to boot at a very early stage. I have confirmed that > reverting commit b505f1944535 on top of v6.19-rc7 completely resolves > the issue, and the kernel boots normally. > > Could you please verify if this change is compatible with x86_32 > (non-PAE) configurations? > I am happy to provide my full .config or test any potential fixes. Hmm.  To start with, does this fix the crash? diff --git a/arch/x86/include/asm/kfence.h b/arch/x86/include/asm/kfence.h index acf9ffa1a171..2fe454722e54 100644 --- a/arch/x86/include/asm/kfence.h +++ b/arch/x86/include/asm/kfence.h @@ -67,8 +67,6 @@ static inline bool kfence_protect_page(unsigned long addr, bool protect)          * If the page was protected (non-present) and we're making it          * present, there is no need to flush the TLB at all.          */ -       if (!protect) -               return true;           /*          * We need to avoid IPIs, as we may get KFENCE allocations or faults Re-reading, I can't spot anything obvious. Architecturally, x86 explicitly does not need a TLB flush when turning a non-present mapping present, and it's strictly 4k leaf mappings we're handling here. I wonder if something else is missing a flush, and was being covered by this. ~Andrew