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[70.31.27.79]) by smtp.gmail.com with ESMTPSA id bs18-20020a05620a471200b006cf38fd659asm10956732qkb.103.2022.10.03.08.56.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Oct 2022 08:56:33 -0700 (PDT) From: Peter Xu To: linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: peterx@redhat.com, Andrea Arcangeli , Mike Rapoport , David Hildenbrand , Andrew Morton , Axel Rasmussen , Mike Kravetz , Nadav Amit Subject: [PATCH 1/3] mm/hugetlb: Fix race condition of uffd missing/minor handling Date: Mon, 3 Oct 2022 11:56:28 -0400 Message-Id: <20221003155630.469263-2-peterx@redhat.com> X-Mailer: git-send-email 2.37.3 In-Reply-To: <20221003155630.469263-1-peterx@redhat.com> References: <20221003155630.469263-1-peterx@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" 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 CoW can happen on one page. CoW requires pte being cleared before being replaced with a new page for TLB coherency, but then there can be a race condition: thread 1 thread 2 -------- -------- hugetlb_fault hugetlb_fault private pte RO hugetlb_wp pgtable_lock() huge_ptep_clear_flush pte=3DNULL hugetlb_no_page generate uffd missing event even if page existed!! set_huge_pte_at 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 --- 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(st= ruct vm_area_struct *vma, return ret; } =20 +/* + * 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 =3D huge_pte_lock(h, mm, ptep); + same =3D 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 =3D 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 =3D hugetlb_handle_userfault( + vma, mapping, idx, flags, haddr, + address, VM_UFFD_MISSING); + else + /* Retry the fault */ + ret =3D 0; goto out; } =20 @@ -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 =3D 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 =3D hugetlb_handle_userfault( + vma, mapping, idx, flags, haddr, + address, VM_UFFD_MINOR); + else + /* Retry the fault */ + ret =3D 0; goto out; } } --=20 2.37.3 From nobody Mon Apr 6 14:57:14 2026 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id AA608C433FE for ; Mon, 3 Oct 2022 15:57:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229759AbiJCP5A (ORCPT ); Mon, 3 Oct 2022 11:57:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53838 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229703AbiJCP4l (ORCPT ); Mon, 3 Oct 2022 11:56:41 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1DE832F384 for ; Mon, 3 Oct 2022 08:56:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1664812599; 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=x4xYjXYzlE422UA6NYhxH1mMMh8xu1ZsHfGS6BWev5s=; b=W6BKXoMOoyaFuL7XYVYX6JVBYsxcFlnLs2kA/NBKDwmJdWdWqDU3ipcJ/nNPM6K/xeXtnj qYs3N5B5mcf+30xDCDLk1CRFPgI66tNst0uFslDFXbkiM3nK49TMlR1lEW16b8gE37x1xx u3E3NqFpKst6tWhr2bKijuPOFsAd5Hk= Received: from mail-qt1-f199.google.com (mail-qt1-f199.google.com [209.85.160.199]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_128_GCM_SHA256) id us-mta-211-i-AYDZM1Nm23zwHOHTRGrQ-1; Mon, 03 Oct 2022 11:56:36 -0400 X-MC-Unique: i-AYDZM1Nm23zwHOHTRGrQ-1 Received: by mail-qt1-f199.google.com with SMTP id ay22-20020a05622a229600b0035bbb349e79so7552005qtb.13 for ; Mon, 03 Oct 2022 08:56:36 -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=x4xYjXYzlE422UA6NYhxH1mMMh8xu1ZsHfGS6BWev5s=; b=yrHHGix8jPH6XyR/Ahe1FvPCSO557A1WwsQnYIff3Hfew/lEBzUatafByqV2nmSwEj 0X2J0HWpzftn8S5GNb1OCAYMh0wFyxlWGQbyB5C4xp3lME2Qa1oyDh95DRTpQi+KqmV1 d8BBqi01bo3iSberjDI9Y3mCajB2XUW//u61lT9wODWO/5Bw/xaRMgAEXOBMe2VzYEVT pqmypLZXB/vFPUT3EF0d/QRctBiYZjREtAB2FIOcyvPf19LKWDSMyYZ50IubvhO39bJQ ccYrhXZA8nLSR1qGy29+PrC88F5zKz6cS+KXlX+9TsLHrDcNBIKslXziAtPX0fewEa4l +eKA== X-Gm-Message-State: ACrzQf0bsbx5VzVlLgRavEBGAyse58nCVzb8qFh/C1S0zHkWdyJ3e+yn YzNJwvAOzkyBoI1GjhOoxzhi/rzR+fkZ5uL3APaEWPpivX42WV05Of+94TaHWLnC1+bagpFRtUG vznKGAZEyZ8aJ7yol75nhR3GsQywtKyarwhDwko6rq1FeYY1BKm09ZO9MDr0Zo9Dp3vwOwOnzOQ == X-Received: by 2002:a05:622a:11c8:b0:35c:e912:a8ea with SMTP id n8-20020a05622a11c800b0035ce912a8eamr16371767qtk.17.1664812595508; Mon, 03 Oct 2022 08:56:35 -0700 (PDT) X-Google-Smtp-Source: AMsMyM7jmUpIO0vv9JXx3e6EMgXhYeRsvGCHvJYDkuTWLTvnmvX6dtNKe0ySPp+Up7ucpY2Yh41crw== X-Received: by 2002:a05:622a:11c8:b0:35c:e912:a8ea with SMTP id n8-20020a05622a11c800b0035ce912a8eamr16371738qtk.17.1664812595208; Mon, 03 Oct 2022 08:56:35 -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 bs18-20020a05620a471200b006cf38fd659asm10956732qkb.103.2022.10.03.08.56.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Oct 2022 08:56:34 -0700 (PDT) From: Peter Xu To: linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: peterx@redhat.com, Andrea Arcangeli , Mike Rapoport , David Hildenbrand , Andrew Morton , Axel Rasmussen , Mike Kravetz , Nadav Amit Subject: [PATCH 2/3] mm/hugetlb: Use hugetlb_pte_stable in migration race check Date: Mon, 3 Oct 2022 11:56:29 -0400 Message-Id: <20221003155630.469263-3-peterx@redhat.com> X-Mailer: git-send-email 2.37.3 In-Reply-To: <20221003155630.469263-1-peterx@redhat.com> References: <20221003155630.469263-1-peterx@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" After hugetlb_pte_stable() introduced, we can also rewrite the migration race condition against page allocation to use the new helper too. Signed-off-by: Peter Xu --- mm/hugetlb.c | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index fa3fcdb0c4b8..e762c5369a6f 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5620,11 +5620,10 @@ static vm_fault_t hugetlb_no_page(struct mm_struct = *mm, * here. Before returning error, get ptl and make * sure there really is no pte entry. */ - ptl =3D huge_pte_lock(h, mm, ptep); - ret =3D 0; - if (huge_pte_none(huge_ptep_get(ptep))) + if (hugetlb_pte_stable(h, mm, ptep, old_pte)) ret =3D vmf_error(PTR_ERR(page)); - spin_unlock(ptl); + else + ret =3D 0; goto out; } clear_huge_page(page, address, pages_per_huge_page(h)); --=20 2.37.3 From nobody Mon Apr 6 14:57:14 2026 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id EB33FC433FE for ; Mon, 3 Oct 2022 15:56:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229716AbiJCP4t (ORCPT ); Mon, 3 Oct 2022 11:56:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53836 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229702AbiJCP4l (ORCPT ); Mon, 3 Oct 2022 11:56:41 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BC97D2DA9F for ; Mon, 3 Oct 2022 08:56:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1664812598; 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=/5RzxXUZxi/MrG066VL+DnGEo1qQl33988Rm0NK9+zA=; b=MLutX1nsPeGms5FwSsl0JmsXVFj0AoP8lzGgDk/TtB5zS/YfxRC1r47fNGYAsoBKKktAKg N2r6Sbb4l0/lDAqpA7gMlRUyNj/dNGwKazc/EStk+BLHiCfbHhaVLtvZ2i8lBV75uWy0uO eW0l5/Ksm1N7HFlAdIX8C5Jyk4dN7sQ= Received: from mail-qv1-f71.google.com (mail-qv1-f71.google.com [209.85.219.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_128_GCM_SHA256) id us-mta-656-Qv4bF7p1OWmFjIE3K2quXQ-1; Mon, 03 Oct 2022 11:56:37 -0400 X-MC-Unique: Qv4bF7p1OWmFjIE3K2quXQ-1 Received: by mail-qv1-f71.google.com with SMTP id ks15-20020a056214310f00b004b18ea4bc05so1621558qvb.22 for ; Mon, 03 Oct 2022 08:56:37 -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=/5RzxXUZxi/MrG066VL+DnGEo1qQl33988Rm0NK9+zA=; b=Z9lsZzdnw1iLShr3ZARV7gKCwBJ5u0fhiPNzXDlDo/VjIJ34pWgjtZ1ueYUvqQPlL+ MqakKr5Fi/D/ZVbR9tX4npafIBB5NEamrxwpYSPqeYlA9mslRBVM6D89neA8rixM4ENj cpFl/25v6OuqelLGAeXIVgAuCkcevG1GRL/s4uuWig3G1I9Jtne9ZzH6ISwmd5HHBTj3 ES9RUk/ooHGLt8e6ZSqstnuO8VUPbkqx/FZLkRmFfyYRt1IKeTlxJdvoGkE9UQWiQeRB Qzrk4hQrYQ7bNF6ODA0JfwCbDHpqSsCbBr67F8yn/mec5RsSwMcFUsPfWO+dVg/Po1mY so1g== X-Gm-Message-State: ACrzQf3CmJCuV1pEnL1oeZWZwN9l/pBmFNU3yIwHP+D8a564SwQUzq9+ n/p/Q6/bc6y6HDWiQtpMpPbPbW0j2AaWngKF05kHIUMbai4p3OHwVQGagGtXNpt4OtgONJhC+2j KoGFM9KY45lpbskTNVZwnNUrl3EuLlEH9PfUXASTDPtp1vuljhUqwVjZHOTF+tFuPG0VjRUvEDw == X-Received: by 2002:a05:620a:15d2:b0:6cf:2d38:9c0d with SMTP id o18-20020a05620a15d200b006cf2d389c0dmr14084021qkm.426.1664812596943; Mon, 03 Oct 2022 08:56:36 -0700 (PDT) X-Google-Smtp-Source: AMsMyM5YdLVN5mgUHqMX9gMAFXtuu3DO1SuWxQNyQcY/m8UTnt2zEXiE/KDGL7J7kdUZbNlU/tPU6w== X-Received: by 2002:a05:620a:15d2:b0:6cf:2d38:9c0d with SMTP id o18-20020a05620a15d200b006cf2d389c0dmr14083998qkm.426.1664812596639; Mon, 03 Oct 2022 08:56:36 -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 bs18-20020a05620a471200b006cf38fd659asm10956732qkb.103.2022.10.03.08.56.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Oct 2022 08:56:36 -0700 (PDT) From: Peter Xu To: linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: peterx@redhat.com, Andrea Arcangeli , Mike Rapoport , David Hildenbrand , Andrew Morton , Axel Rasmussen , Mike Kravetz , Nadav Amit Subject: [PATCH 3/3] mm/selftest: uffd: Explain the write missing fault check Date: Mon, 3 Oct 2022 11:56:30 -0400 Message-Id: <20221003155630.469263-4-peterx@redhat.com> X-Mailer: git-send-email 2.37.3 In-Reply-To: <20221003155630.469263-1-peterx@redhat.com> References: <20221003155630.469263-1-peterx@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" It's not obvious why we had a write check for each of the missing messages, especially when it should be a locking op. Add a rich comment for that, and also try to explain its good side and limitations, so that if someone hit it again for either a bug or a different glibc impl there'll be some clue to start with. Signed-off-by: Peter Xu --- tools/testing/selftests/vm/userfaultfd.c | 22 +++++++++++++++++++++- 1 file changed, 21 insertions(+), 1 deletion(-) diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selft= ests/vm/userfaultfd.c index 74babdbc02e5..297f250c1d95 100644 --- a/tools/testing/selftests/vm/userfaultfd.c +++ b/tools/testing/selftests/vm/userfaultfd.c @@ -774,7 +774,27 @@ static void uffd_handle_page_fault(struct uffd_msg *ms= g, continue_range(uffd, msg->arg.pagefault.address, page_size); stats->minor_faults++; } else { - /* Missing page faults */ + /* + * Missing page faults. + * + * Here we force a write check for each of the missing mode + * faults. It's guaranteed because the only threads that + * will trigger uffd faults are the locking threads, and + * their first instruction to touch the missing page will + * always be pthread_mutex_lock(). + * + * Note that here we relied on an NPTL glibc impl detail to + * always read the lock type at the entry of the lock op + * (pthread_mutex_t.__data.__type, offset 0x10) before + * doing any locking operations to guarantee that. It's + * actually not good to rely on this impl detail because + * logically a pthread-compatible lib can implement the + * locks without types and we can fail when linking with + * them. However since we used to find bugs with this + * strict check we still keep it around. Hopefully this + * could be a good hint when it fails again. If one day + * it'll break on some other impl of glibc we'll revisit. + */ if (msg->arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WRITE) err("unexpected write fault"); =20 --=20 2.37.3