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=-6.0 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=unavailable 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 A80F1C04E87 for ; Mon, 20 May 2019 12:46:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7E987204FD for ; Mon, 20 May 2019 12:46:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1558356374; bh=hcCWdm8G1C81OZqG9T6IE40vqZ2g7WBEfGBUW8Mt+Bg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=WgYO10v7EvEhIXgb1IKGEJ2dU9QBsRvOsE3DMl1VQqoXazjhBM729pE06AOMYviRU ueSuXMFeaf/IT0y272VwTRObtp9YVlQ3DN6sClfhHM0XfAVteCJgTAYPQuroTWDHTL gp7cAmpASuFHT3QqCupowX74aD7By/eOr79Y+ZvU= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388786AbfETM0E (ORCPT ); Mon, 20 May 2019 08:26:04 -0400 Received: from mail.kernel.org ([198.145.29.99]:41382 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388273AbfETM0D (ORCPT ); Mon, 20 May 2019 08:26:03 -0400 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 811E6214DA; Mon, 20 May 2019 12:26:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1558355162; bh=hcCWdm8G1C81OZqG9T6IE40vqZ2g7WBEfGBUW8Mt+Bg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=sUzVLYp8SsdgQqUpNg3pGRJHDY9p2Ibr/9A6we3+ClJf9aC/iTHTn9m0FSnLaw8Td zx+KxM7pEb3czhjdrPpHc2B57FHyeav1CtaNA4pZ6TwWV6oYitm59fLPZc8Bgj01r6 uBoX6iWc0/D74oFVn1ZGQy0NsPrOuNnZRRhP+35o= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Andy Lutomirski , Borislav Petkov , Frederic Weisbecker , Jon Masters , Linus Torvalds , Peter Zijlstra , Thomas Gleixner , Ingo Molnar Subject: [PATCH 5.0 003/123] x86/speculation/mds: Improve CPU buffer clear documentation Date: Mon, 20 May 2019 14:13:03 +0200 Message-Id: <20190520115245.587746818@linuxfoundation.org> X-Mailer: git-send-email 2.21.0 In-Reply-To: <20190520115245.439864225@linuxfoundation.org> References: <20190520115245.439864225@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Andy Lutomirski commit 9d8d0294e78a164d407133dea05caf4b84247d6a upstream. On x86_64, all returns to usermode go through prepare_exit_to_usermode(), with the sole exception of do_nmi(). This even includes machine checks -- this was added several years ago to support MCE recovery. Update the documentation. Signed-off-by: Andy Lutomirski Cc: Borislav Petkov Cc: Frederic Weisbecker Cc: Greg Kroah-Hartman Cc: Jon Masters Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: stable@vger.kernel.org Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user") Link: http://lkml.kernel.org/r/999fa9e126ba6a48e9d214d2f18dbde5c62ac55c.1557865329.git.luto@kernel.org Signed-off-by: Ingo Molnar Signed-off-by: Greg Kroah-Hartman --- Documentation/x86/mds.rst | 39 +++++++-------------------------------- 1 file changed, 7 insertions(+), 32 deletions(-) --- a/Documentation/x86/mds.rst +++ b/Documentation/x86/mds.rst @@ -142,38 +142,13 @@ Mitigation points mds_user_clear. The mitigation is invoked in prepare_exit_to_usermode() which covers - most of the kernel to user space transitions. There are a few exceptions - which are not invoking prepare_exit_to_usermode() on return to user - space. These exceptions use the paranoid exit code. - - - Non Maskable Interrupt (NMI): - - Access to sensible data like keys, credentials in the NMI context is - mostly theoretical: The CPU can do prefetching or execute a - misspeculated code path and thereby fetching data which might end up - leaking through a buffer. - - But for mounting other attacks the kernel stack address of the task is - already valuable information. So in full mitigation mode, the NMI is - mitigated on the return from do_nmi() to provide almost complete - coverage. - - - Machine Check Exception (#MC): - - Another corner case is a #MC which hits between the CPU buffer clear - invocation and the actual return to user. As this still is in kernel - space it takes the paranoid exit path which does not clear the CPU - buffers. So the #MC handler repopulates the buffers to some - extent. Machine checks are not reliably controllable and the window is - extremly small so mitigation would just tick a checkbox that this - theoretical corner case is covered. To keep the amount of special - cases small, ignore #MC. - - - Debug Exception (#DB): - - This takes the paranoid exit path only when the INT1 breakpoint is in - kernel space. #DB on a user space address takes the regular exit path, - so no extra mitigation required. + all but one of the kernel to user space transitions. The exception + is when we return from a Non Maskable Interrupt (NMI), which is + handled directly in do_nmi(). + + (The reason that NMI is special is that prepare_exit_to_usermode() can + enable IRQs. In NMI context, NMIs are blocked, and we don't want to + enable IRQs with NMIs blocked.) 2. C-State transition