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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AE3CAC4332F for ; Mon, 15 Nov 2021 17:21:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 99CA160F22 for ; Mon, 15 Nov 2021 17:21:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236321AbhKORYW (ORCPT ); Mon, 15 Nov 2021 12:24:22 -0500 Received: from mail.kernel.org ([198.145.29.99]:46616 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236873AbhKORN5 (ORCPT ); Mon, 15 Nov 2021 12:13:57 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 9BD3C6323F; Mon, 15 Nov 2021 17:10:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1636996238; bh=DMHq3tMX7mhsLf751wcYGEZiqnNt2TgbiYgPgqyP4xo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=EAmlFxbef2XQY2fWhpievImMCGcu/Pnyz0wAXc5uCaEi1j4E6kvmFLgpst3KhR+HF zvC3x1xUpsWXnhw6eDVyDaKCRGNS/w73SGK1DWSr9ivibYLjtwi2xJKSFSCRUJSsGs gx9gNqK8Q1GuvsZjrczBR5qq0KXy+MmwN73EpsmM= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Jane Malalane , Borislav Petkov Subject: [PATCH 5.4 034/355] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL Date: Mon, 15 Nov 2021 17:59:18 +0100 Message-Id: <20211115165314.649503931@linuxfoundation.org> X-Mailer: git-send-email 2.33.1 In-Reply-To: <20211115165313.549179499@linuxfoundation.org> References: <20211115165313.549179499@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Jane Malalane commit 415de44076640483648d6c0f6d645a9ee61328ad upstream. Currently, Linux probes for X86_BUG_NULL_SEL unconditionally which makes it unsafe to migrate in a virtualised environment as the properties across the migration pool might differ. To be specific, the case which goes wrong is: 1. Zen1 (or earlier) and Zen2 (or later) in a migration pool 2. Linux boots on Zen2, probes and finds the absence of X86_BUG_NULL_SEL 3. Linux is then migrated to Zen1 Linux is now running on a X86_BUG_NULL_SEL-impacted CPU while believing that the bug is fixed. The only way to address the problem is to fully trust the "no longer affected" CPUID bit when virtualised, because in the above case it would be clear deliberately to indicate the fact "you might migrate to somewhere which has this behaviour". Zen3 adds the NullSelectorClearsBase CPUID bit to indicate that loading a NULL segment selector zeroes the base and limit fields, as well as just attributes. Zen2 also has this behaviour but doesn't have the NSCB bit. [ bp: Minor touchups. ] Signed-off-by: Jane Malalane Signed-off-by: Borislav Petkov CC: Link: https://lkml.kernel.org/r/20211021104744.24126-1-jane.malalane@citrix.com Signed-off-by: Greg Kroah-Hartman --- arch/x86/kernel/cpu/amd.c | 2 + arch/x86/kernel/cpu/common.c | 44 ++++++++++++++++++++++++++++++++++++------- arch/x86/kernel/cpu/cpu.h | 1 arch/x86/kernel/cpu/hygon.c | 2 + 4 files changed, 42 insertions(+), 7 deletions(-) --- a/arch/x86/kernel/cpu/amd.c +++ b/arch/x86/kernel/cpu/amd.c @@ -987,6 +987,8 @@ static void init_amd(struct cpuinfo_x86 if (cpu_has(c, X86_FEATURE_IRPERF) && !cpu_has_amd_erratum(c, amd_erratum_1054)) msr_set_bit(MSR_K7_HWCR, MSR_K7_HWCR_IRPERF_EN_BIT); + + check_null_seg_clears_base(c); } #ifdef CONFIG_X86_32 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1336,9 +1336,8 @@ void __init early_cpu_init(void) early_identify_cpu(&boot_cpu_data); } -static void detect_null_seg_behavior(struct cpuinfo_x86 *c) +static bool detect_null_seg_behavior(void) { -#ifdef CONFIG_X86_64 /* * Empirically, writing zero to a segment selector on AMD does * not clear the base, whereas writing zero to a segment @@ -1359,10 +1358,43 @@ static void detect_null_seg_behavior(str wrmsrl(MSR_FS_BASE, 1); loadsegment(fs, 0); rdmsrl(MSR_FS_BASE, tmp); - if (tmp != 0) - set_cpu_bug(c, X86_BUG_NULL_SEG); wrmsrl(MSR_FS_BASE, old_base); -#endif + return tmp == 0; +} + +void check_null_seg_clears_base(struct cpuinfo_x86 *c) +{ + /* BUG_NULL_SEG is only relevant with 64bit userspace */ + if (!IS_ENABLED(CONFIG_X86_64)) + return; + + /* Zen3 CPUs advertise Null Selector Clears Base in CPUID. */ + if (c->extended_cpuid_level >= 0x80000021 && + cpuid_eax(0x80000021) & BIT(6)) + return; + + /* + * CPUID bit above wasn't set. If this kernel is still running + * as a HV guest, then the HV has decided not to advertize + * that CPUID bit for whatever reason. For example, one + * member of the migration pool might be vulnerable. Which + * means, the bug is present: set the BUG flag and return. + */ + if (cpu_has(c, X86_FEATURE_HYPERVISOR)) { + set_cpu_bug(c, X86_BUG_NULL_SEG); + return; + } + + /* + * Zen2 CPUs also have this behaviour, but no CPUID bit. + * 0x18 is the respective family for Hygon. + */ + if ((c->x86 == 0x17 || c->x86 == 0x18) && + detect_null_seg_behavior()) + return; + + /* All the remaining ones are affected */ + set_cpu_bug(c, X86_BUG_NULL_SEG); } static void generic_identify(struct cpuinfo_x86 *c) @@ -1398,8 +1430,6 @@ static void generic_identify(struct cpui get_model_name(c); /* Default name */ - detect_null_seg_behavior(c); - /* * ESPFIX is a strange bug. All real CPUs have it. Paravirt * systems that run Linux at CPL > 0 may or may not have the --- a/arch/x86/kernel/cpu/cpu.h +++ b/arch/x86/kernel/cpu/cpu.h @@ -73,6 +73,7 @@ extern int detect_extended_topology_earl extern int detect_extended_topology(struct cpuinfo_x86 *c); extern int detect_ht_early(struct cpuinfo_x86 *c); extern void detect_ht(struct cpuinfo_x86 *c); +extern void check_null_seg_clears_base(struct cpuinfo_x86 *c); unsigned int aperfmperf_get_khz(int cpu); --- a/arch/x86/kernel/cpu/hygon.c +++ b/arch/x86/kernel/cpu/hygon.c @@ -350,6 +350,8 @@ static void init_hygon(struct cpuinfo_x8 /* Hygon CPUs don't reset SS attributes on SYSRET, Xen does. */ if (!cpu_has(c, X86_FEATURE_XENPV)) set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS); + + check_null_seg_clears_base(c); } static void cpu_detect_tlb_hygon(struct cpuinfo_x86 *c)