From nobody Wed Dec 31 10:32:55 2025 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 4C732C4332F for ; Fri, 3 Nov 2023 22:44:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1376652AbjKCWoL (ORCPT ); Fri, 3 Nov 2023 18:44:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35122 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1345174AbjKCWoJ (ORCPT ); Fri, 3 Nov 2023 18:44:09 -0400 Received: from mail-yw1-x1149.google.com (mail-yw1-x1149.google.com [IPv6:2607:f8b0:4864:20::1149]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3F995D6D for ; Fri, 3 Nov 2023 15:44:06 -0700 (PDT) Received: by mail-yw1-x1149.google.com with SMTP id 00721157ae682-5a9012ab0adso36502087b3.1 for ; Fri, 03 Nov 2023 15:44:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1699051445; x=1699656245; darn=vger.kernel.org; h=cc:to:from:subject:message-id:mime-version:date:reply-to:from:to:cc :subject:date:message-id:reply-to; bh=VHUs19MXg68FJ/UikGSWl1xtUmfy1ZNYmGDVs0epASU=; b=eRdP5MYTex7drFSvnL8KqbMuun5bFCt9F19Ie+gn87Sws08TPDik6fMEhpWDYHc+Tq zj1nW1YWspv3f5JvTv1gOvybTkExyY/sczl0YsQXlTbrNl6tnb6AKVRANhaKdQGG+iVg VOPV+IGfWF7Is5mFbH9KuY2/7GQMN+uLeHlxXxK4etRrzBtwGPq13ju0tatp0KEqpcqq luPgfWcMtusTndj7JjPY/IgwxqxvIUdRzBZNemKtxs+1rQ9ZC9YBZ9XLdGPmlj0tlQ4Q Rb0QZnAeePkNn/31ORt4eC/8YJXMcosXX5awUpGrPE9kJaL/SFDBvAPuSssBGFwUE1ez y47g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1699051445; x=1699656245; h=cc:to:from:subject:message-id:mime-version:date:reply-to :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=VHUs19MXg68FJ/UikGSWl1xtUmfy1ZNYmGDVs0epASU=; b=JDkNjBTlniRcJoOHFQafoP8vhYc0DqpYzbAFy8PnV9AEhmwq75jpTvgMwnGzjXiriE aIs1ehpq42L3P2928uzfz8Dhoo25JOvTP7ds2apPAeXfKKlpG1V8ICDTRb75Mz/xys5p fsmibYevCVgd6ImRO5BMOHnyRHIgdFo+mXMjucUIkgcpVQHfEjrDc9VO440q/lh9nvEx ugK1mR7t8IzKf5uSprF2dHT0jNUpOfpAv1E0MLNvtyql+vYpJapT61mnWfUKOFVCe3DK 6rwXRedW0A4y+l/567rlBNU6KU7JHiD+zUmE15yZ7PULpiSUFSmrbW5+BQUAYP3mCCVr tS7g== X-Gm-Message-State: AOJu0YzL/mVurRK1Hjd3ds9zQ/sklIBhOcqLB9myLe//mj0VdplgY4ye XLzf2GfDI5z/Fz+gHbkjwTNA8n32Lwc= X-Google-Smtp-Source: AGHT+IHPtAwEWr+ctDqsScG5V+Q52QTEUJtI+qEU6hGf5rCAVAqdsox8vEdKuTAwtWiJ7ZeFf05HMmjUPRE= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a81:a056:0:b0:59b:ca80:919a with SMTP id x83-20020a81a056000000b0059bca80919amr78967ywg.0.1699051445462; Fri, 03 Nov 2023 15:44:05 -0700 (PDT) Reply-To: Sean Christopherson Date: Fri, 3 Nov 2023 15:44:02 -0700 Mime-Version: 1.0 X-Mailer: git-send-email 2.42.0.869.gea05f2083d-goog Message-ID: <20231103224402.347278-1-seanjc@google.com> Subject: [PATCH] x86/fpu/xstate: Always preserve non-user xfeatures/flags in __state_perm From: Sean Christopherson To: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org Cc: linux-kernel@vger.kernel.org, Maxim Levitsky , Weijiang Yang , Dave Hansen , Paolo Bonzini , Peter Zijlstra , Chao Gao , Rick Edgecombe , John Allen , kvm@vger.kernel.org, Sean Christopherson Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" When granting userspace or a KVM guest access to an xfeature, preserve the entity's existing supervisor and software-defined permissions as tracked by __state_perm, i.e. use __state_perm to track *all* permissions even though all supported supervisor xfeatures are granted to all FPUs and FPU_GUEST_PERM_LOCKED disallows changing permissions. Effectively clobbering supervisor permissions results in inconsistent behavior, as xstate_get_group_perm() will report supervisor features for process that do NOT request access to dynamic user xfeatures, whereas any and all supervisor features will be absent from the set of permissions for any process that is granted access to one or more dynamic xfeatures (which right now means AMX). The inconsistency isn't problematic because fpu_xstate_prctl() already strips out everything except user xfeatures: case ARCH_GET_XCOMP_PERM: /* * Lockless snapshot as it can also change right after the * dropping the lock. */ permitted =3D xstate_get_host_group_perm(); permitted &=3D XFEATURE_MASK_USER_SUPPORTED; return put_user(permitted, uptr); case ARCH_GET_XCOMP_GUEST_PERM: permitted =3D xstate_get_guest_group_perm(); permitted &=3D XFEATURE_MASK_USER_SUPPORTED; return put_user(permitted, uptr); and similarly KVM doesn't apply the __state_perm to supervisor states (kvm_get_filtered_xcr0() incorporates xstate_get_guest_group_perm()): case 0xd: { u64 permitted_xcr0 =3D kvm_get_filtered_xcr0(); u64 permitted_xss =3D kvm_caps.supported_xss; But if KVM in particular were to ever change, dropping supervisor permissions would result in subtle bugs in KVM's reporting of supported CPUID settings. And the above behavior also means that having supervisor xfeatures in __state_perm is correctly handled by all users. Dropping supervisor permissions also creates another landmine for KVM. If more dynamic user xfeatures are ever added, requesting access to multiple xfeatures in separate ARCH_REQ_XCOMP_GUEST_PERM calls will result in the second invocation of __xstate_request_perm() computing the wrong ksize, as as the mask passed to xstate_calculate_size() would not contain *any* supervisor features. Commit 781c64bfcb73 ("x86/fpu/xstate: Handle supervisor states in XSTATE permissions") fudged around the size issue for userspace FPUs, but for reasons unknown skipped guest FPUs. Lack of a fix for KVM "works" only because KVM doesn't yet support virtualizing features that have supervisor xfeatures, i.e. as of today, KVM guest FPUs will never need the relevant xfeatures. Simply extending the hack-a-fix for guests would temporarily solve the ksize issue, but wouldn't address the inconsistency issue and would leave another lurking pitfall for KVM. KVM support for virtualizing CET will likely add CET_KERNEL as a guest-only xfeature, i.e. CET_KERNEL will not be set in xfeatures_mask_supervisor() and would again be dropped when granting access to dynamic xfeatures. Note, the existing clobbering behavior is rather subtle. The @permitted parameter to __xstate_request_perm() comes from: permitted =3D xstate_get_group_perm(guest); which is either fpu->guest_perm.__state_perm or fpu->perm.__state_perm, where __state_perm is initialized to: fpu->perm.__state_perm =3D fpu_kernel_cfg.default_features; and copied to the guest side of things: /* Same defaults for guests */ fpu->guest_perm =3D fpu->perm; fpu_kernel_cfg.default_features contains everything except the dynamic xfeatures, i.e. everything except XFEATURE_MASK_XTILE_DATA: fpu_kernel_cfg.default_features =3D fpu_kernel_cfg.max_features; fpu_kernel_cfg.default_features &=3D ~XFEATURE_MASK_USER_DYNAMIC; When __xstate_request_perm() restricts the local "mask" variable to compute the user state size: mask &=3D XFEATURE_MASK_USER_SUPPORTED; usize =3D xstate_calculate_size(mask, false); it subtly overwrites the target __state_perm with "mask" containing only user xfeatures: perm =3D guest ? &fpu->guest_perm : &fpu->perm; /* Pairs with the READ_ONCE() in xstate_get_group_perm() */ WRITE_ONCE(perm->__state_perm, mask); Cc: Maxim Levitsky Cc: Weijiang Yang Cc: Dave Hansen Cc: Paolo Bonzini Cc: Peter Zijlstra Cc: Chao Gao Cc: Rick Edgecombe Cc: John Allen Cc: kvm@vger.kernel.org Link: https://lore.kernel.org/all/ZTqgzZl-reO1m01I@google.com Signed-off-by: Sean Christopherson --- Note, I haven't tested the PASID side of things, so someone with more know-= how definitely needs to take this for a spin. arch/x86/kernel/fpu/xstate.c | 18 +++++++++++------- 1 file changed, 11 insertions(+), 7 deletions(-) diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c index ef6906107c54..73f6bc00d178 100644 --- a/arch/x86/kernel/fpu/xstate.c +++ b/arch/x86/kernel/fpu/xstate.c @@ -1601,16 +1601,20 @@ static int __xstate_request_perm(u64 permitted, u64= requested, bool guest) if ((permitted & requested) =3D=3D requested) return 0; =20 - /* Calculate the resulting kernel state size */ + /* + * Calculate the resulting kernel state size. Note, @permitted also + * contains supervisor xfeatures even though supervisor are always + * permitted for kernel and guest FPUs, and never permitted for user + * FPUs. + */ mask =3D permitted | requested; - /* Take supervisor states into account on the host */ - if (!guest) - mask |=3D xfeatures_mask_supervisor(); ksize =3D xstate_calculate_size(mask, compacted); =20 - /* Calculate the resulting user state size */ - mask &=3D XFEATURE_MASK_USER_SUPPORTED; - usize =3D xstate_calculate_size(mask, false); + /* + * Calculate the resulting user state size. Take care not to clobber + * the supervisor xfeatures in the new mask! + */ + usize =3D xstate_calculate_size(mask & XFEATURE_MASK_USER_SUPPORTED, fals= e); =20 if (!guest) { ret =3D validate_sigaltstack(usize); base-commit: 45b890f7689eb0aba454fc5831d2d79763781677 --=20 2.42.0.869.gea05f2083d-goog