From nobody Mon Apr 13 08:21:44 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 2EC8AC3A5A7 for ; Thu, 8 Dec 2022 11:42:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229750AbiLHLmr (ORCPT ); Thu, 8 Dec 2022 06:42:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35722 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229604AbiLHLmp (ORCPT ); Thu, 8 Dec 2022 06:42:45 -0500 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 BBD5B63D67 for ; Thu, 8 Dec 2022 03:41:47 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1670499706; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=g5xBDwwbAPMXsVLc87QM3XiQi+bNxIe4VXdcPfLTTBs=; b=Ms7ahd6xHMdkhMKlMk9yHQ4Qmg/+6DhZkg1DCUVYZdZ4EL/X6Nawy+rmFeBaTgNcVfCRnO jmYYRN7a8EfQKJq+9DpYQoHSsarKtBAw+nxwqQHOjJQkvUapV0b0Gyq4x/GkZvyEzm07j4 arllSu2xBno3FuXavUN2swgENtmLNrk= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-638-EO_ZK0zzM32kIRDyvyIDqg-1; Thu, 08 Dec 2022 06:41:43 -0500 X-MC-Unique: EO_ZK0zzM32kIRDyvyIDqg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 647163803911; Thu, 8 Dec 2022 11:41:43 +0000 (UTC) Received: from t480s.in.tum.de (unknown [10.39.193.226]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9E5FA40C2064; Thu, 8 Dec 2022 11:41:40 +0000 (UTC) From: David Hildenbrand To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, David Hildenbrand , Ives van Hoorne , Peter Xu , stable@vger.kernel.org, Andrew Morton , Hugh Dickins , Alistair Popple , Mike Rapoport , Nadav Amit , Andrea Arcangeli Subject: [PATCH v1] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA Date: Thu, 8 Dec 2022 12:41:37 +0100 Message-Id: <20221208114137.35035-1-david@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Currently, we don't enable writenotify when enabling userfaultfd-wp on a shared writable mapping (for now only shmem and hugetlb). The consequence is that vma->vm_page_prot will still include write permissions, to be set as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting, page migration, ...). So far, vma->vm_page_prot is assumed to be a safe default, meaning that we only add permissions (e.g., mkwrite) but not remove permissions (e.g., wrprotect). For example, when enabling softdirty tracking, we enable writenotify. With uffd-wp on shared mappings, that changed. More details on vma->vm_page_prot semantics were summarized in [1]. This is problematic for uffd-wp: we'd have to manually check for a uffd-wp PTEs/PMDs and manually write-protect PTEs/PMDs, which is error prone. Prone to such issues is any code that uses vma->vm_page_prot to set PTE permissions: primarily pte_modify() and mk_pte(). Instead, let's enable writenotify such that PTEs/PMDs/... will be mapped write-protected as default and we will only allow selected PTEs that are definitely safe to be mapped without write-protection (see can_change_pte_writable()) to be writable. In the future, we might want to enable write-bit recovery -- e.g., can_change_pte_writable() -- at more locations, for example, also when removing uffd-wp protection. This fixes two known cases: (a) remove_migration_pte() mapping uffd-wp'ed PTEs writable, resulting in uffd-wp not triggering on write access. (b) do_numa_page() / do_huge_pmd_numa_page() mapping uffd-wp'ed PTEs/PMDs writable, resulting in uffd-wp not triggering on write access. Note that do_numa_page() / do_huge_pmd_numa_page() can be reached even without NUMA hinting (which currently doesn't seem to be applicable to shmem), for example, by using uffd-wp with a PROT_WRITE shmem VMA. On such a VMA, userfaultfd-wp is currently non-functional. Note that when enabling userfaultfd-wp, there is no need to walk page tables to enforce the new default protection for the PTEs: we know that they cannot be uffd-wp'ed yet, because that can only happen after enabling uffd-wp for the VMA in general. Also note that this makes mprotect() on ranges with uffd-wp'ed PTEs not accidentally set the write bit -- which would result in uffd-wp not triggering on later write access. This commit makes uffd-wp on shmem behave just like uffd-wp on anonymous memory (iow, less special) in that regard, even though, mixing mprotect with uffd-wp is controversial. [1] https://lkml.kernel.org/r/92173bad-caa3-6b43-9d1e-9a471fdbc184@redhat.c= om Reported-by: Ives van Hoorne Debugged-by: Peter Xu Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbf= s") Cc: stable@vger.kernel.org Cc: Andrew Morton Cc: Hugh Dickins Cc: Alistair Popple Cc: Mike Rapoport Cc: Nadav Amit Cc: Andrea Arcangeli Signed-off-by: David Hildenbrand Acked-by: Peter Xu --- As discussed in [2], this is supposed to replace the fix by Peter: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte This survives vm/selftests and my reproducers: * migrating pages that are uffd-wp'ed using mbind() on a machine with 2 NUMA nodes * Using a PROT_WRITE mapping with uffd-wp * Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and mprotect()'ing it PROT_WRITE * Using a PROT_READ|PROT_WRITE mapping with uffd-wp'ed pages and temporarily mprotect()'ing it PROT_READ uffd-wp properly triggers in all cases. On v8.1-rc8, all mre reproducers fail. It would be good to get some more testing feedback and review. [2] https://lkml.kernel.org/r/20221202122748.113774-1-david@redhat.com --- fs/userfaultfd.c | 28 ++++++++++++++++++++++------ mm/mmap.c | 4 ++++ 2 files changed, 26 insertions(+), 6 deletions(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 98ac37e34e3d..fb0733f2e623 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(struct userfaul= tfd_ctx *ctx) return ctx->features & UFFD_FEATURE_INITIALIZED; } =20 +static void userfaultfd_set_vm_flags(struct vm_area_struct *vma, + vm_flags_t flags) +{ + const bool uffd_wp =3D !!((vma->vm_flags | flags) & VM_UFFD_WP); + + vma->vm_flags =3D flags; + /* + * For shared mappings, we want to enable writenotify while + * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply + * recalculate vma->vm_page_prot whenever userfaultfd-wp is involved. + */ + if ((vma->vm_flags & VM_SHARED) && uffd_wp) + vma_set_page_prot(vma); +} + static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode, int wake_flags, void *key) { @@ -618,7 +633,8 @@ static void userfaultfd_event_wait_completion(struct us= erfaultfd_ctx *ctx, for_each_vma(vmi, vma) { if (vma->vm_userfaultfd_ctx.ctx =3D=3D release_new_ctx) { vma->vm_userfaultfd_ctx =3D NULL_VM_UFFD_CTX; - vma->vm_flags &=3D ~__VM_UFFD_FLAGS; + userfaultfd_set_vm_flags(vma, + vma->vm_flags & ~__VM_UFFD_FLAGS); } } mmap_write_unlock(mm); @@ -652,7 +668,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct = list_head *fcs) octx =3D vma->vm_userfaultfd_ctx.ctx; if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) { vma->vm_userfaultfd_ctx =3D NULL_VM_UFFD_CTX; - vma->vm_flags &=3D ~__VM_UFFD_FLAGS; + userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS); return 0; } =20 @@ -733,7 +749,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma, } else { /* Drop uffd context if remap feature not enabled */ vma->vm_userfaultfd_ctx =3D NULL_VM_UFFD_CTX; - vma->vm_flags &=3D ~__VM_UFFD_FLAGS; + userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS); } } =20 @@ -895,7 +911,7 @@ static int userfaultfd_release(struct inode *inode, str= uct file *file) prev =3D vma; } =20 - vma->vm_flags =3D new_flags; + userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx =3D NULL_VM_UFFD_CTX; } mmap_write_unlock(mm); @@ -1463,7 +1479,7 @@ static int userfaultfd_register(struct userfaultfd_ct= x *ctx, * the next vma was merged into the current one and * the current one has not been updated yet. */ - vma->vm_flags =3D new_flags; + userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx.ctx =3D ctx; =20 if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma)) @@ -1651,7 +1667,7 @@ static int userfaultfd_unregister(struct userfaultfd_= ctx *ctx, * the next vma was merged into the current one and * the current one has not been updated yet. */ - vma->vm_flags =3D new_flags; + userfaultfd_set_vm_flags(vma, new_flags); vma->vm_userfaultfd_ctx =3D NULL_VM_UFFD_CTX; =20 skip: diff --git a/mm/mmap.c b/mm/mmap.c index a5eb2f175da0..6033d20198b0 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1525,6 +1525,10 @@ int vma_wants_writenotify(struct vm_area_struct *vma= , pgprot_t vm_page_prot) if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma)) return 1; =20 + /* Do we need write faults for uffd-wp tracking? */ + if (userfaultfd_wp(vma)) + return 1; + /* Specialty mapping? */ if (vm_flags & VM_PFNMAP) return 0; base-commit: 8ed710da2873c2aeb3bb805864a699affaf1d03b --=20 2.38.1