From patchwork Tue Oct 4 00:37:03 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Peter Xu X-Patchwork-Id: 12997902 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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 971EBC4332F for ; Tue, 4 Oct 2022 00:37:14 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 2DDC06B0074; Mon, 3 Oct 2022 20:37:14 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 267056B0075; Mon, 3 Oct 2022 20:37:14 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 01B0E6B0078; Mon, 3 Oct 2022 20:37:13 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id BF8256B0074 for ; Mon, 3 Oct 2022 20:37:13 -0400 (EDT) Received: from smtpin11.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 8ACD68033A for ; Tue, 4 Oct 2022 00:37:13 +0000 (UTC) X-FDA: 79981402746.11.B42AFE3 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by imf22.hostedemail.com (Postfix) with ESMTP id 37C16C0010 for ; Tue, 4 Oct 2022 00:37:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1664843832; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=agJ87AhkrOU4WXbJTuk9++7as6a20Yx3hvs9+IM86N0=; b=b/iPtf+JA0AP1JX43pGCuUZuyxPyRpP7kbweCep9Cu3WMMtcPG+knu+6w6sfyOOHCNYBqB YwYwH38j/iLAuXZeU6pyTVSfzmrSExBHIjp++ppX1K/AW6bsEzhLIHRfkc84u2jb3DVUXf UAIznu2I/4rwYZsR6fhsWYG66I8+GxE= Received: from mail-qk1-f199.google.com (mail-qk1-f199.google.com [209.85.222.199]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_128_GCM_SHA256) id us-mta-638-o5X4LcEHNjOcW36D8mxBkg-1; Mon, 03 Oct 2022 20:37:09 -0400 X-MC-Unique: o5X4LcEHNjOcW36D8mxBkg-1 Received: by mail-qk1-f199.google.com with SMTP id h8-20020a05620a284800b006b5c98f09fbso10522340qkp.21 for ; Mon, 03 Oct 2022 17:37:09 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date; bh=agJ87AhkrOU4WXbJTuk9++7as6a20Yx3hvs9+IM86N0=; b=4WPuiNln8txV987AD7Jt7Q3THaZS3BKsK3QdAlTAIQIdnHicB0QozbLAFzXUxvAbC6 MVUikRwfmpzB6uHgXRZKAVeUUsniCphATTCp40BRkn7/rajECYpH/aD+4ffaVRYW0a/s xdYwdAzrcciAaoVxMFeqUn7p/xjgysUUxVOyXTEkDdZhKMZHDgAV/hl399LPMEQgO4ky lBEyzc7BKer0SAqa+dK+gmXE1yUPoqOblvH2Ba5FQoS6Kuhwce4C87YSRhBF5hbrH/fB iMSMy/llge3f/GP6mQxX7RuuYMmGWKnNaCZ4MkfezqEhZofMp/kMQwdbgbOtFfrheUMh shEw== X-Gm-Message-State: ACrzQf36qhCUfeKbfzuAuCrk/VOhk5RCptfqfZwe3a4ykv06QfymO/V3 idoxPhUvPcJemaTfZeRRNtKHQY26BYCi0L3ptXOrtF8wdSrlCEXh17u/XvW4GIMo8eECE554Hut kyPM9J4FjELk= X-Received: by 2002:a05:620a:2809:b0:6bc:60fa:6b93 with SMTP id f9-20020a05620a280900b006bc60fa6b93mr15045485qkp.254.1664843829350; Mon, 03 Oct 2022 17:37:09 -0700 (PDT) X-Google-Smtp-Source: AMsMyM74Q1B/bbLHC2GBtqEZOKQ/JZbvCEg1IiGQX5mCpjocQh5bJvbd6ZJW2ioDiKMXCi//B1kRGA== X-Received: by 2002:a05:620a:2809:b0:6bc:60fa:6b93 with SMTP id f9-20020a05620a280900b006bc60fa6b93mr15045473qkp.254.1664843829115; Mon, 03 Oct 2022 17:37:09 -0700 (PDT) Received: from x1n.redhat.com (bras-base-aurron9127w-grc-46-70-31-27-79.dsl.bell.ca. [70.31.27.79]) by smtp.gmail.com with ESMTPSA id u30-20020a37ab1e000000b006bb9125363fsm12228104qke.121.2022.10.03.17.37.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Oct 2022 17:37:08 -0700 (PDT) From: Peter Xu To: linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: peterx@redhat.com, Andrew Morton , David Hildenbrand , Nadav Amit , Mike Kravetz , Andrea Arcangeli , Axel Rasmussen , Mike Rapoport Subject: [PATCH v2 1/3] mm/hugetlb: Fix race condition of uffd missing/minor handling Date: Mon, 3 Oct 2022 20:37:03 -0400 Message-Id: <20221004003705.497782-2-peterx@redhat.com> X-Mailer: git-send-email 2.37.3 In-Reply-To: <20221004003705.497782-1-peterx@redhat.com> References: <20221004003705.497782-1-peterx@redhat.com> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-type: text/plain ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1664843833; a=rsa-sha256; cv=none; b=ByMEEnMitltbzYXxFtw6tmIoChuRJ1EcDtl3NLa04e0cwt0GfXYjR2fr8AOuCNSKpH7kUO 52kt+tdHb9qQfJrB0bY6yVmassKGClfdtlGKrfM7I/Apv5uW59hke8dDnJ8Xs5BD8A0eBp ovUX8EAVASby2SL2/fhVX7+5T6yv2Ec= ARC-Authentication-Results: i=1; imf22.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="b/iPtf+J"; spf=pass (imf22.hostedemail.com: domain of peterx@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1664843833; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=agJ87AhkrOU4WXbJTuk9++7as6a20Yx3hvs9+IM86N0=; b=iIbAKo/dil0EfI0XggG3b1wAM+Iwrq4h1lw8D/s0BiUvEQwY6167Ouwsx9K5Unu8NeB7yv yjLtYyfYvQIhBr+L+4ZOMRDKUx6Dxn4NRO2/8ojQb7vZCmCNdfGYAOQAPDUNsfbianVZ8H tg4R2gS74GltSiBurJEXnw9ITtRX8zw= X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 37C16C0010 X-Rspam-User: Authentication-Results: imf22.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="b/iPtf+J"; spf=pass (imf22.hostedemail.com: domain of peterx@redhat.com designates 170.10.133.124 as permitted sender) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Stat-Signature: 9ga73mctaudaqsc41y65aqu94n8zo49u X-HE-Tag: 1664843833-188808 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: After the recent rework patchset of hugetlb locking on pmd sharing, kselftest for userfaultfd sometimes fails on hugetlb private tests with unexpected write fault checks. It turns out there's nothing wrong within the locking series regarding this matter, but it could have changed the timing of threads so it can trigger an old bug. The real bug is when we call hugetlb_no_page() we're not with the pgtable lock. It means we're reading the pte values lockless. It's perfectly fine in most cases because before we do normal page allocations we'll take the lock and check pte_same() again. However before that, there are actually two paths on userfaultfd missing/minor handling that may directly move on with the fault process without checking the pte values. It means for these two paths we may be generating an uffd message based on an unstable pte, while an unstable pte can legally be anything as long as the modifier holds the pgtable lock. One example, which is also what happened in the failing kselftest and caused the test failure, is that for private mappings wr-protection changes can happen on one page. While hugetlb_change_protection() generally requires pte being cleared before being changed, then there can be a race condition like: thread 1 thread 2 -------- -------- UFFDIO_WRITEPROTECT hugetlb_fault hugetlb_change_protection pgtable_lock() huge_ptep_modify_prot_start pte==NULL hugetlb_no_page generate uffd missing event even if page existed!! huge_ptep_modify_prot_commit pgtable_unlock() Fix this by recheck the pte after pgtable lock for both userfaultfd missing & minor fault paths. This bug should have been around starting from uffd hugetlb introduced, so attaching a Fixes to the commit. Also attach another Fixes to the minor support commit for easier tracking. Note that userfaultfd is actually fine with false positives (e.g. caused by pte changed), but not wrong logical events (e.g. caused by reading a pte during changing). The latter can confuse the userspace, so the strictness is very much preferred. E.g., MISSING event should never happen on the page after UFFDIO_COPY has correctly installed the page and returned. Cc: Andrea Arcangeli Cc: Mike Kravetz Cc: Axel Rasmussen Cc: Nadav Amit Fixes: 1a1aad8a9b7b ("userfaultfd: hugetlbfs: add userfaultfd hugetlb hook") Fixes: 7677f7fd8be7 ("userfaultfd: add minor fault registration mode") Co-developed-by: Mike Kravetz Signed-off-by: Peter Xu Reviewed-by: Mike Kravetz --- mm/hugetlb.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 49 insertions(+), 6 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 9679fe519b90..fa3fcdb0c4b8 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5521,6 +5521,23 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma, return ret; } +/* + * Recheck pte with pgtable lock. Returns true if pte didn't change, or + * false if pte changed or is changing. + */ +static bool hugetlb_pte_stable(struct hstate *h, struct mm_struct *mm, + pte_t *ptep, pte_t old_pte) +{ + spinlock_t *ptl; + bool same; + + ptl = huge_pte_lock(h, mm, ptep); + same = pte_same(huge_ptep_get(ptep), old_pte); + spin_unlock(ptl); + + return same; +} + static vm_fault_t hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma, struct address_space *mapping, pgoff_t idx, @@ -5562,9 +5579,30 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm, goto out; /* Check for page in userfault range */ if (userfaultfd_missing(vma)) { - ret = hugetlb_handle_userfault(vma, mapping, idx, - flags, haddr, address, - VM_UFFD_MISSING); + /* + * Since hugetlb_no_page() was examining pte + * without pgtable lock, we need to re-test under + * lock because the pte may not be stable and could + * have changed from under us. Try to detect + * either changed or during-changing ptes and retry + * properly when needed. + * + * Note that userfaultfd is actually fine with + * false positives (e.g. caused by pte changed), + * but not wrong logical events (e.g. caused by + * reading a pte during changing). The latter can + * confuse the userspace, so the strictness is very + * much preferred. E.g., MISSING event should + * never happen on the page after UFFDIO_COPY has + * correctly installed the page and returned. + */ + if (hugetlb_pte_stable(h, mm, ptep, old_pte)) + ret = hugetlb_handle_userfault( + vma, mapping, idx, flags, haddr, + address, VM_UFFD_MISSING); + else + /* Retry the fault */ + ret = 0; goto out; } @@ -5634,9 +5672,14 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm, if (userfaultfd_minor(vma)) { unlock_page(page); put_page(page); - ret = hugetlb_handle_userfault(vma, mapping, idx, - flags, haddr, address, - VM_UFFD_MINOR); + /* See comment in userfaultfd_missing() block above */ + if (hugetlb_pte_stable(h, mm, ptep, old_pte)) + ret = hugetlb_handle_userfault( + vma, mapping, idx, flags, haddr, + address, VM_UFFD_MINOR); + else + /* Retry the fault */ + ret = 0; goto out; } }