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 X-Spam-Level: X-Spam-Status: No, score=-9.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D5F11C47255 for ; Mon, 11 May 2020 20:20:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B8B9C20736 for ; Mon, 11 May 2020 20:20:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731613AbgEKUUy (ORCPT ); Mon, 11 May 2020 16:20:54 -0400 Received: from mga18.intel.com ([134.134.136.126]:19258 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727873AbgEKUUx (ORCPT ); Mon, 11 May 2020 16:20:53 -0400 IronPort-SDR: T5ubHYuCdz/0lOb8uC9yTdY66mDZonsdGY325hTFKDetQ6d8xpnkyBKn89L3DxhF2RnlxpSEZK aznVAdiutX2g== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 11 May 2020 13:20:52 -0700 IronPort-SDR: nypXNphGh+lKqXHZpB+uuiJybEv0x3zwxO/6ScPS0+9khIxC7PjKTQkIJBZGZX5Nl5de3epUzc o4RwJiefW7WQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,381,1583222400"; d="scan'208";a="463292524" Received: from yyu32-desk.sc.intel.com ([143.183.136.146]) by fmsmga005.fm.intel.com with ESMTP; 11 May 2020 13:20:52 -0700 From: Yu-cheng Yu To: linux-kernel@vger.kernel.org, x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , Dave Hansen , Tony Luck , Andy Lutomirski , Borislav Petkov , Rik van Riel , "Ravi V. Shankar" , Sebastian Andrzej Siewior , Fenghua Yu , Peter Zijlstra Cc: Yu-cheng Yu Subject: [PATCH v3 10/10] x86/fpu/xstate: Restore supervisor states for signal return Date: Mon, 11 May 2020 13:20:06 -0700 Message-Id: <20200511202006.10295-1-yu-cheng.yu@intel.com> X-Mailer: git-send-email 2.21.0 In-Reply-To: <20200328164307.17497-11-yu-cheng.yu@intel.com> References: <20200328164307.17497-11-yu-cheng.yu@intel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org As described in the previous patch, the signal return fast path directly restores user states from the user buffer. Once that succeeds, restore supervisor states (but only when they are not yet restored). For the slow path, save supervisor states to preserve them across context switches, and restore after the user states are restored. The previous version has the overhead of an XSAVES in both the fast and the slow paths. It is addressed as the following: - In the fast path, only do an XRSTORS. - In the slow path, do a supervisor-state-only XSAVES, and relocate the buffer contents. Some thoughts in the implementation: - In the slow path, can any supervisor state become stale between save/restore? Answer: set_thread_flag(TIF_NEED_FPU_LOAD) protects the xstate buffer. - In the slow path, can any code reference a stale supervisor state register between save/restore? Answer: In the current lazy-restore scheme, any reference to xstate registers needs fpregs_lock()/fpregs_unlock() and __fpregs_load_activate(). - Are there other options? One other option is eagerly restoring all supervisor states. Currently, CET user-mode states and ENQCMD's PASID do not need to be eagerly restored. The upcoming CET kernel-mode states (24 bytes) need to be eagerly restored. To me, eagerly restoring all supervisor states adds more overhead then benefit at this point. Signed-off-by: Yu-cheng Yu Reviewed-by: Dave Hansen --- v3: - Change copy_xregs_to_kernel() to copy_supervisor_to_kernel(), which is introduced in a previous patch. - Update commit log. arch/x86/kernel/fpu/signal.c | 27 ++++++++++++++++++++++----- 1 file changed, 22 insertions(+), 5 deletions(-) diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c index 545ca4314096..4dad5afc938d 100644 --- a/arch/x86/kernel/fpu/signal.c +++ b/arch/x86/kernel/fpu/signal.c @@ -347,6 +347,23 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size) ret = copy_user_to_fpregs_zeroing(buf_fx, user_xfeatures, fx_only); pagefault_enable(); if (!ret) { + + /* + * Restore supervisor states: previous context switch + * etc has done XSAVES and saved the supervisor states + * in the kernel buffer from which they can be restored + * now. + * + * We cannot do a single XRSTORS here - which would + * be nice - because the rest of the FPU registers are + * being restored from a user buffer directly. The + * single XRSTORS happens below, when the user buffer + * has been copied to the kernel one. + */ + if (test_thread_flag(TIF_NEED_FPU_LOAD) && + xfeatures_mask_supervisor()) + copy_kernel_to_xregs(&fpu->state.xsave, + xfeatures_mask_supervisor()); fpregs_mark_activate(); fpregs_unlock(); return 0; @@ -364,14 +381,25 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size) } /* - * The current state of the FPU registers does not matter. By setting - * TIF_NEED_FPU_LOAD unconditionally it is ensured that the our xstate - * is not modified on context switch and that the xstate is considered + * By setting TIF_NEED_FPU_LOAD it is ensured that our xstate is + * not modified on context switch and that the xstate is considered * to be loaded again on return to userland (overriding last_cpu avoids * the optimisation). */ - set_thread_flag(TIF_NEED_FPU_LOAD); + fpregs_lock(); + + if (!test_thread_flag(TIF_NEED_FPU_LOAD)) { + + /* + * Supervisor states are not modified by user space input. Save + * current supervisor states first and invalidate the FPU regs. + */ + if (xfeatures_mask_supervisor()) + copy_supervisor_to_kernel(&fpu->state.xsave); + set_thread_flag(TIF_NEED_FPU_LOAD); + } __fpu_invalidate_fpregs_state(fpu); + fpregs_unlock(); if (use_xsave() && !fx_only) { u64 init_bv = xfeatures_mask_user() & ~user_xfeatures; @@ -393,7 +421,13 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size) fpregs_lock(); if (unlikely(init_bv)) copy_kernel_to_xregs(&init_fpstate.xsave, init_bv); - ret = copy_kernel_to_xregs_err(&fpu->state.xsave, user_xfeatures); + + /* + * Restore previously saved supervisor xstates along with + * copied-in user xstates. + */ + ret = copy_kernel_to_xregs_err(&fpu->state.xsave, + user_xfeatures | xfeatures_mask_supervisor()); } else if (use_fxsr()) { ret = __copy_from_user(&fpu->state.fxsave, buf_fx, state_size); -- 2.21.0