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Tue, 21 Oct 2025 20:55:55 +0000 Message-ID: <144029e3-30ea-4e94-9afd-4da53ce4a657@intel.com> Date: Tue, 21 Oct 2025 13:55:51 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 03/15] x86/alternatives: Disable LASS when patching kernel alternatives Content-Language: en-US To: Borislav Petkov CC: , Dave Hansen , "Thomas Gleixner" , Ingo Molnar , "Jonathan Corbet" , "H . Peter Anvin" , Andy Lutomirski , Josh Poimboeuf , Peter Zijlstra , Ard Biesheuvel , "Kirill A . Shutemov" , Xin Li , David Woodhouse , Sean Christopherson , Rick Edgecombe , Vegard Nossum , Andrew Cooper , David Laight , Randy Dunlap , "Geert Uytterhoeven" , Kees Cook , Tony Luck , Alexander Shishkin , , , References: <20251007065119.148605-1-sohil.mehta@intel.com> <20251007065119.148605-4-sohil.mehta@intel.com> <20251021200328.GMaPfnEDibnBrhNTmQ@fat_crate.local> From: Sohil Mehta In-Reply-To: <20251021200328.GMaPfnEDibnBrhNTmQ@fat_crate.local> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BYAPR05CA0048.namprd05.prod.outlook.com (2603:10b6:a03:74::25) To DS0PR11MB7997.namprd11.prod.outlook.com (2603:10b6:8:125::14) Precedence: bulk X-Mailing-List: linux-efi@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB7997:EE_|DM6PR11MB4676:EE_ X-MS-Office365-Filtering-Correlation-Id: 0284cb21-9297-4cb7-2ebd-08de10e43a64 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr 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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>> +} >> + >> +static __always_inline void lass_stac(void) >> +{ >> + alternative("", "stac", X86_FEATURE_LASS); >> +} > > So I probably missed the whole discussion on how we arrived at > lass_{stac,clac}() but just in case, those names sound silly. > I am okay with lass_enable()/lass_disable() if we can all agree on it. PeterZ didn't like lass_enable_enforcement()/lass_enable_enforcement() when it was proposed. But your suggestion is much shorter, so maybe it would work for him. https://lore.kernel.org/lkml/20250626134921.GK1613200@noisy.programming.kicks-ass.net/ Though, there is a slight semantic difference we need to be careful about. LASS manages 2 types of kernel accesses: Data and Instruction fetch. 1) The STAC/CLAC only control the kernel *data* accesses into the lower half. 2) CR4.LASS is what truly controls the entire mechanism. If an instruction fetch needs to happen from a lower address, CR4.LASS must be cleared to disable LASS completely. (See patch 6 and 7). In the series, we directly write to the CR4 bits, so they don't have any wrappers. But in the future, lass_enable()/lass_disable() could be confusing if wrappers were added for the CR4 toggling. > IOW, I'd do this ontop: > > +#define lass_disable() stac() > +#define lass_enable() clac() > There is an issue here which you had originally objected to. https://lore.kernel.org/lkml/20240710171836.GGZo7CbFJeZwLCZUAt@fat_crate.local/ https://lore.kernel.org/lkml/20240711012333.GAZo80FU30_x77otP4@fat_crate.local/ These new versions of lass_disable()/lass_enable() will toggle the AC flag on older platforms without X86_FEATURE_LASS. It definitely makes the code easier to read and maintain if we are okay with the minor performance penalty. > static void text_poke_memcpy(void *dst, const void *src, size_t len) > { > - lass_stac(); > + lass_disable(); > __inline_memcpy(dst, src, len); > - lass_clac(); > + lass_enable(); > } > > static void text_poke_memset(void *dst, const void *src, size_t len) > { > int c = *(const int *)src; > > - lass_stac(); > + lass_disable(); > __inline_memset(dst, c, len); > - lass_clac(); > + lass_enable(); > }