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=-8.6 required=3.0 tests=BAYES_00,DKIM_ADSP_CUSTOM_MED, DKIM_INVALID,DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 98202C433DB for ; Tue, 12 Jan 2021 19:56:34 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 0CF6D22202 for ; Tue, 12 Jan 2021 19:56:34 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0CF6D22202 Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 29AF26B00B8; Tue, 12 Jan 2021 14:56:33 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 271096B00BA; Tue, 12 Jan 2021 14:56:33 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 13A4A6B00BB; Tue, 12 Jan 2021 14:56:33 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0131.hostedemail.com [216.40.44.131]) by kanga.kvack.org (Postfix) with ESMTP id F16CE6B00B8 for ; Tue, 12 Jan 2021 14:56:32 -0500 (EST) Received: from smtpin14.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id B262C180AD81F for ; Tue, 12 Jan 2021 19:56:32 +0000 (UTC) X-FDA: 77698180224.14.walk02_1807bb827518 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin14.hostedemail.com (Postfix) with ESMTP id 820D91822987A for ; Tue, 12 Jan 2021 19:56:32 +0000 (UTC) X-HE-Tag: walk02_1807bb827518 X-Filterd-Recvd-Size: 8978 Received: from mail-io1-f47.google.com (mail-io1-f47.google.com [209.85.166.47]) by imf18.hostedemail.com (Postfix) with ESMTP for ; Tue, 12 Jan 2021 19:56:31 +0000 (UTC) Received: by mail-io1-f47.google.com with SMTP id q137so6630863iod.9 for ; Tue, 12 Jan 2021 11:56:31 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:content-transfer-encoding:in-reply-to; bh=m+OlJPIb+Y7HL3mVvnn3aPJy9H6Uy5divcbzK7j1Vac=; b=ijpBBn3/eYfeKjIklKWS1c5NBUUWU/glzhzuVqSiYrNeGFj/33i6sMGT6ABu6v8y30 6ClaiIxIJqZhO4gWRmAEMM8ki53XQp0vzKenQLOwDafAPdfAUk+Vbnq4B3TNJENciWDT MmDQ7CXTXbXr2Um9vBiU+rIbTYtlacGzKba6j2rW5bJsOWWQ2hs8mBkgg7y72tigagm/ Bm2iym+5uqEeisSbob3R540G7m4/iv7PmuZahZWdNM49malb01s1pvyViEfA+RFHZsW3 GsxZJQUp46Ei8tm92rtxLpIVxvAEsDXGuDmPDaYhrBj7dyFy+1+i7IyWsnz3R56nGBGS Uj2g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to; bh=m+OlJPIb+Y7HL3mVvnn3aPJy9H6Uy5divcbzK7j1Vac=; b=T29ofBccMNW1/OEj4Xxg9Mo17zkzIB9EhErvdrEcMod1G0GsbYNrjhEAi1A9To+/6J +QJhm+s3VRfpxtqrp5TzUJ/ZRC022GgRhqaCCZfSfmIWqTfgnOlpM8vm2OhJotT8oOOh v4X1aS2pD7LkL9V3M5CJHJy0g3fpWYsLEInsD4PdkUnzUUw2HKd1BSPKyxalkaASJOWl JC+c0CsIl/CeSrvaoVslech2zYUB5hsVXQTKjnCMBa/Rp6cjPOqqY+QvwWMUoK6QBg2u P0mvEEvd7/bWE/XZaa4VXKhfYjNDiSZ8SbwVSXAW0OZOAUBOM8D65ng8qUMbFxOoKCAz 8wgg== X-Gm-Message-State: AOAM531qHnsl6cn3bPIzpUKwWl+0HDowz3IZHqvn3aPzftBb+xA8a53U CdDbk+bm1SXaugXm71nuJq1mLQ== X-Google-Smtp-Source: ABdhPJygreYDj1N+TwKoFXkVp0vrevIYk6hR5C4174Z/QpvC//lcr0AVJTHPs8etujNQwBZ4aOmn3A== X-Received: by 2002:a92:dc8e:: with SMTP id c14mr757390iln.54.1610481391209; Tue, 12 Jan 2021 11:56:31 -0800 (PST) Received: from google.com ([2620:15c:183:200:7220:84ff:fe09:2d90]) by smtp.gmail.com with ESMTPSA id n16sm3147956ilj.19.2021.01.12.11.56.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 12 Jan 2021 11:56:30 -0800 (PST) Date: Tue, 12 Jan 2021 12:56:25 -0700 From: Yu Zhao To: Nadav Amit Cc: Laurent Dufour , Peter Zijlstra , Vinayak Menon , Linus Torvalds , Andy Lutomirski , Peter Xu , Andrea Arcangeli , linux-mm , lkml , Pavel Emelyanov , Mike Kravetz , Mike Rapoport , stable , Minchan Kim , Will Deacon , surenb@google.com Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect Message-ID: References: <20201221223041.GL6640@xz-x1> <20210105153727.GK3040@hirez.programming.kicks-ass.net> <0201238b-e716-2a3c-e9ea-d5294ff77525@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Jan 12, 2021 at 11:15:43AM -0800, Nadav Amit wrote: > > On Jan 12, 2021, at 11:02 AM, Laurent Dufour wrote: > >=20 > > Le 12/01/2021 =E0 17:57, Peter Zijlstra a =E9crit : > >> On Tue, Jan 12, 2021 at 04:47:17PM +0100, Laurent Dufour wrote: > >>> Le 12/01/2021 =E0 12:43, Vinayak Menon a =E9crit : > >>>> Possibility of race against other PTE modifiers > >>>>=20 > >>>> 1) Fork - We have seen a case of SPF racing with fork marking PTEs= RO and that > >>>> is described and fixed here https://lore.kernel.org/patchwork/patc= h/1062672/ > >> Right, that's exactly the kind of thing I was worried about. > >>>> 2) mprotect - change_protection in mprotect which does the deferre= d flush is > >>>> marked under vm_write_begin/vm_write_end, thus SPF bails out on fa= ults > >>>> on those VMAs. > >> Sure, mprotect also changes vm_flags, so it really needs that anyway= . > >>>> 3) userfaultfd - mwriteprotect_range is not protected unlike in (2= ) above. > >>>> But SPF does not take UFFD faults. > >>>> 4) hugetlb - hugetlb_change_protection - called from mprotect and = covered by > >>>> (2) above. > >>>> 5) Concurrent faults - SPF does not handle all faults. Only anon p= age faults. > >> What happened to shared/file-backed stuff? ISTR I had that working. > >=20 > > File-backed mappings are not processed in a speculative way, there we= re options to manage some of them depending on the underlying file system= but that's still not done. > >=20 > > Shared anonymous mapping, are also not yet handled in a speculative w= ay (vm_ops is not null). > >=20 > >>>> Of which do_anonymous_page and do_swap_page are NONE/NON-PRESENT->= PRESENT > >>>> transitions without tlb flush. And I hope do_wp_page with RO->RW i= s fine as well. > >> The tricky one is demotion, specifically write to non-write. > >>>> I could not see a case where speculative path cannot see a PTE upd= ate done via > >>>> a fault on another CPU. > >> One you didn't mention is the NUMA balancing scanning crud; although= I > >> think that's fine, loosing a PTE update there is harmless. But I've = not > >> thought overly hard on it. > >=20 > > That's a good point, I need to double check on that side. > >=20 > >>> You explained it fine. Indeed SPF is handling deferred TLB invalida= tion by > >>> marking the VMA through vm_write_begin/end(), as for the fork case = you > >>> mentioned. Once the PTL is held, and the VMA's seqcount is checked,= the PTE > >>> values read are valid. > >> That should indeed work, but are we really sure we covered them all? > >> Should we invest in better TLBI APIs to make sure we can't get this > >> wrong? > >=20 > > That may be a good option to identify deferred TLB invalidation but I= 've no clue on what this API would look like. >=20 > I will send an RFC soon for per-table deferred TLB flushes tracking. > The basic idea is to save a generation in the page-struct that tracks > when deferred PTE change took place, and track whenever a TLB flush > completed. In addition, other users - such as mprotect - would use > the tlb_gather interface. >=20 > Unfortunately, due to limited space in page-struct this would only > be possible for 64-bit (and my implementation is only for x86-64). I don't want to discourage you but I don't think this would end up well. PPC doesn't necessarily follow one-page-struct-per-table rule, and I've run into problems with this before while trying to do something similar. I'd recommend per-vma and per-category (unmapping, clearing writable and clearing dirty) tracking, which only rely on arch-independent data structures, i.e., vm_area_struct and mm_struct. > It would still require to do the copying while holding the PTL though. IMO, this is unacceptable. Most archs don't support per-table PTL, and even x86_64 can be configured to use per-mm PTL. What if we want to support a larger page size in the feature? It seems to me the only way to solve the problem with self-explanatory code and without performance impact is to check mm_tlb_flush_pending and the writable bit (and two other cases I mentioned above) at the same time. Of course, this requires a lot of effort to audit the existing uses, clean them up and properly wrap them up with new primitives, BUG_ON all invalid cases and document the exact workflow to prevent misuses. I've mentioned the following before -- it only demonstrates the rough idea. diff --git a/mm/memory.c b/mm/memory.c index 5e9ca612d7d7..af38c5ee327e 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4403,8 +4403,11 @@ static vm_fault_t handle_pte_fault(struct vm_fault= *vmf) goto unlock; } if (vmf->flags & FAULT_FLAG_WRITE) { - if (!pte_write(entry)) + if (!pte_write(entry)) { + if (mm_tlb_flush_pending(vmf->vma->vm_mm)) + flush_tlb_page(vmf->vma, vmf->address); return do_wp_page(vmf); + } entry =3D pte_mkdirty(entry); } entry =3D pte_mkyoung(entry);