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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Aug 2025 09:11:39.6542 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: c656aba7-9b69-46f2-2446-08dddf006831 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB3PEPF0000885D.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DU4PR08MB10984 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, I think futex using LSUI should enable TCO bit before it calls LSUI instruction. Otherwise, this sounds have a inconsistency of allowing TCF according to SCTLR_EL1.TCF (not 0)'s configuration while kernel accescss user memory. Am I on right way? Thanks. > -- > Catalin -- Sincerely, Yeoreum Yun