From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B6095C41513 for ; Tue, 15 Aug 2023 19:59:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240037AbjHOT7H (ORCPT ); Tue, 15 Aug 2023 15:59:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40964 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234858AbjHOT6g (ORCPT ); Tue, 15 Aug 2023 15:58:36 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 68F0C1AB for ; Tue, 15 Aug 2023 12:58:34 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id F291F60DC5 for ; Tue, 15 Aug 2023 19:58:33 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 30DA8C433C7; Tue, 15 Aug 2023 19:58:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1692129513; bh=liTtQIepDemmgBKm25RqRcTit4+ntzh72R9+hldmPuk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=FHArkGL7SuhedGxZT+Q+tVL7vYEi3VdiFjUtpa7esBW/3gxI2YHGjSp5mc5CY9IEu 1zACV4FYc11+j3ftiqsvnAUfVHoqWo4rvcKECXXkBbVACapZytiJfUvhgXQLd2GDgc Rmjcy3lyfAfbDeAD2NsvQVPApHBh/D6TvHIKfWZraA325ivvMl93sVS4zTpvSgP6mN dTECccceg2AONf9EaNELtGc7ymDaA9imtdw/wfAV9J//fgenG9spSXB/5lCftKcPuj 96J8i4kTBClI0u2i1KpLb7x+LKWd4n9afSGndri4WxsZF8gMr/e1dvtZIInPef9sk1 cY8EMI51iD2nQ== Date: Tue, 15 Aug 2023 12:58:31 -0700 From: Josh Poimboeuf To: Borislav Petkov Cc: Nikolay Borisov , X86 ML , Josh Poimboeuf , LKML Subject: Re: [PATCH] x86/srso: Correct the mitigation status when SMT is disabled Message-ID: <20230815195831.2opbgrznnpszaa32@treble> References: <20230813104517.3346-1-bp@alien8.de> <1588ed00-be11-ff9d-e4c2-12db78cca06f@suse.com> <20230814200813.p5czl47zssuej7nv@treble> <20230814202545.GKZNqNybUnKv+xyrtP@fat_crate.local> <20230814205300.krikym7jeckehqik@treble> <20230814211727.GLZNqZ5+flxtyaDjMQ@fat_crate.local> <20230815095724.GBZNtMBPUJSEegviJN@fat_crate.local> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20230815095724.GBZNtMBPUJSEegviJN@fat_crate.local> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 15, 2023 at 11:57:24AM +0200, Borislav Petkov wrote: > --- a/arch/x86/kernel/cpu/bugs.c > +++ b/arch/x86/kernel/cpu/bugs.c > @@ -2417,8 +2417,7 @@ static void __init srso_select_mitigation(void) > * Zen1/2 with SMT off aren't vulnerable after the right > * IBPB microcode has been applied. > */ > - if ((boot_cpu_data.x86 < 0x19) && > - (!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED))) { > + if (boot_cpu_data.x86 < 0x19 && !cpu_smt_possible()) { > setup_force_cpu_cap(X86_FEATURE_SRSO_NO); > return; > } > @@ -2698,8 +2697,12 @@ static ssize_t retbleed_show_state(char *buf) > > static ssize_t srso_show_state(char *buf) > { > - if (boot_cpu_has(X86_FEATURE_SRSO_NO)) > - return sysfs_emit(buf, "Not affected\n"); > + if (boot_cpu_has(X86_FEATURE_SRSO_NO)) { > + if (sched_smt_active()) > + return sysfs_emit(buf, "Not affected\n"); > + else > + return sysfs_emit(buf, "Mitigation: SMT disabled\n"); > + } AFAICT, nowhere in the spec does it say the SRSO_NO bit won't get set by future (fixed) HW. In fact I'd expect it will, similar to other *_NO flags. Regardless, here SRSO_NO seems to mean two different things: "reported safe by host (or HW)" and "not reported safe on Zen1/2 with SMT not possible". Also, in this code, the SRSO_NO+SMT combo doesn't seem logically possible, as srso_show_state() only gets called if X86_BUG_SRSO is set, which only happens if SRSO_NO is not set by the HW/host in the first place. So here, if boot_cpu_has(X86_FEATURE_SRSO_NO), it means SRSO_NO was manually set by srso_select_mitigation(), and SMT can't possibly be enabled. Instead of piggybacking on SRSO_NO, which is confusing, why not just add a new mitigation type, like: diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 6c04aef4b63b..c925b98f5a15 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -2343,6 +2343,7 @@ early_param("l1tf", l1tf_cmdline); enum srso_mitigation { SRSO_MITIGATION_NONE, SRSO_MITIGATION_MICROCODE, + SRSO_MITIGATION_SMT, SRSO_MITIGATION_SAFE_RET, SRSO_MITIGATION_IBPB, SRSO_MITIGATION_IBPB_ON_VMEXIT, @@ -2359,6 +2360,7 @@ enum srso_mitigation_cmd { static const char * const srso_strings[] = { [SRSO_MITIGATION_NONE] = "Vulnerable", [SRSO_MITIGATION_MICROCODE] = "Mitigation: microcode", + [SRSO_MITIGATION_SMT] = "Mitigation: SMT disabled", [SRSO_MITIGATION_SAFE_RET] = "Mitigation: safe RET", [SRSO_MITIGATION_IBPB] = "Mitigation: IBPB", [SRSO_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT only" @@ -2407,19 +2409,15 @@ static void __init srso_select_mitigation(void) pr_warn("IBPB-extending microcode not applied!\n"); pr_warn(SRSO_NOTICE); } else { - /* - * Enable the synthetic (even if in a real CPUID leaf) - * flags for guests. - */ + /* Enable the synthetic flag, as HW doesn't set it. */ setup_force_cpu_cap(X86_FEATURE_IBPB_BRTYPE); /* * Zen1/2 with SMT off aren't vulnerable after the right * IBPB microcode has been applied. */ - if ((boot_cpu_data.x86 < 0x19) && - (!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED))) { - setup_force_cpu_cap(X86_FEATURE_SRSO_NO); + if ((boot_cpu_data.x86 < 0x19) && !cpu_smt_possible()) { + srso_mitigation = SRSO_MITIGATION_SMT; return; } } @@ -2698,9 +2696,6 @@ static ssize_t retbleed_show_state(char *buf) static ssize_t srso_show_state(char *buf) { - if (boot_cpu_has(X86_FEATURE_SRSO_NO)) - return sysfs_emit(buf, "Not affected\n"); - return sysfs_emit(buf, "%s%s\n", srso_strings[srso_mitigation], (cpu_has_ibpb_brtype_microcode() ? "" : ", no microcode"));