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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 946D0C4320E for ; Sat, 28 Aug 2021 21:47:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 762AF60E93 for ; Sat, 28 Aug 2021 21:47:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232339AbhH1Vr5 (ORCPT ); Sat, 28 Aug 2021 17:47:57 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:44508 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230253AbhH1Vr4 (ORCPT ); Sat, 28 Aug 2021 17:47:56 -0400 From: Thomas Gleixner DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1630187224; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=636wMEgWkaGHM7Xqjl4UaJQCprbJXsUtUL3hOaFl5LE=; b=xo+lADGv3ssFUE3kclN6Zr/5XBSCUKKdydttui2NNRFl14bo2HZlKDXtov53ZTr/ssqfrK QtvNEO0pDYaq8M2LkSXl4la7tP0IshE4PiciNRAvoSDDZ2FAANGgy8KkNn5vMNkyE7D0xm B6TZ66ai54A0KJRuMKOeE0wk4x8kmSITEW3bjR6Q/IBMlc1Sbuq219yCTUqzPfpPrBGaBJ Dg8HkLSwHQhSY+FewSR5EKydC2b/Z2520zuDss3kMlbboMIDHHCWYeWK/I51/0GpkXbVZF w5Y2mqj3j55IcWAwBxadOqHmGHYgi/RLlhn+qgZbVvOpxauQTBsL1ExBsymNmg== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1630187224; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=636wMEgWkaGHM7Xqjl4UaJQCprbJXsUtUL3hOaFl5LE=; b=NAaEzdO5CAmmmO+khxg5WLkGgrwVM1AlI3iQ+B3aafBmvWpOPZ/disdLyTZ9GKLcCKkcgp n2w1F3whCtuQ8HDA== To: "Luck, Tony" , Al Viro Cc: Linus Torvalds , Andreas Gruenbacher , Christoph Hellwig , "Darrick J. Wong" , Jan Kara , Matthew Wilcox , cluster-devel , linux-fsdevel , Linux Kernel Mailing List , ocfs2-devel@oss.oracle.com, Borislav Petkov , x86@kernel.org Subject: Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable In-Reply-To: <20210827232246.GA1668365@agluck-desk2.amr.corp.intel.com> References: <20210827164926.1726765-1-agruenba@redhat.com> <20210827164926.1726765-6-agruenba@redhat.com> <20210827232246.GA1668365@agluck-desk2.amr.corp.intel.com> Date: Sat, 28 Aug 2021 23:47:03 +0200 Message-ID: <87r1edgs2w.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 27 2021 at 16:22, Tony Luck wrote: > On Fri, Aug 27, 2021 at 09:57:10PM +0000, Al Viro wrote: >> On Fri, Aug 27, 2021 at 09:48:55PM +0000, Al Viro wrote: >> >> > [btrfs]search_ioctl() >> > Broken with memory poisoning, for either variant of semantics. Same for >> > arm64 sub-page permission differences, I think. >> >> >> > So we have 3 callers where we want all-or-nothing semantics - two in >> > arch/x86/kernel/fpu/signal.c and one in btrfs. HWPOISON will be a problem >> > for all 3, AFAICS... >> > >> > IOW, it looks like we have two different things mixed here - one that wants >> > to try and fault stuff in, with callers caring only about having _something_ >> > faulted in (most of the users) and one that wants to make sure we *can* do >> > stores or loads on each byte in the affected area. >> > >> > Just accessing a byte in each page really won't suffice for the second kind. >> > Neither will g-u-p use, unless we teach it about HWPOISON and other fun >> > beasts... Looks like we want that thing to be a separate primitive; for >> > btrfs I'd probably replace fault_in_pages_writeable() with clear_user() >> > as a quick fix for now... >> > >> > Comments? >> >> Wait a sec... Wasn't HWPOISON a per-page thing? arm64 definitely does have >> smaller-than-page areas with different permissions, so btrfs search_ioctl() >> has a problem there, but arch/x86/kernel/fpu/signal.c doesn't have to deal >> with that... >> >> Sigh... I really need more coffee... > > On Intel poison is tracked at the cache line granularity. Linux > inflates that to per-page (because it can only take a whole page away). > For faults triggered in ring3 this is pretty much the same thing because > mm/memory_failure.c unmaps the page ... so while you see a #MC on first > access, you get #PF when you retry. The x86 fault handler sees a magic > signature in the page table and sends a SIGBUS. > > But it's all different if the #MC is triggerd from ring0. The machine > check handler can't unmap the page. It just schedules task_work to do > the unmap when next returning to the user. > > But if your kernel code loops and tries again without a return to user, > then your get another #MC. But that's not the case for restore_fpregs_from_user() when it hits #MC. restore_fpregs_from_user() ... ret = __restore_fpregs_from_user(buf, xrestore, fx_only) /* Try to handle #PF, but anything else is fatal. */ if (ret != -EFAULT) return -EINVAL; Now let's look at __restore_fpregs_from_user() __restore_fpregs_from_user() return $FPUVARIANT_rstor_from_user_sigframe() which all end up in user_insn(). user_insn() returns 0 or the negated trap number, which results in -EFAULT for #PF, but for #MC the negated trap number is -18 i.e. != -EFAULT. IOW, there is no endless loop. This used to be a problem before commit: aee8c67a4faa ("x86/fpu: Return proper error codes from user access functions") and as the changelog says the initial reason for this was #GP going into the fault path, but I'm pretty sure that I also discussed the #MC angle with Borislav back then. Should have added some more comments there obviously. Thanks, tglx