From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6366B49CF40; Thu, 15 Oct 2026 20:10:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792095036; cv=none; b=MDIpcoLW8JyXj0hPVlyVXmySNZB9/JSCEWPRjtpmoqTE2+R8hDB1MgmP/Z+xjjUswvYyHBwUFPa2ipJqLusSZ1/qQPX05Hv0xsRJL6jqYPf1bYf+1sDeyzwbBfhNnaUkLrjvFXOjCjVRyribijgp2JC9NpUij6e7KqzM98UVDBA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792095036; c=relaxed/simple; bh=Y8wzWW15iPX0w/KAfHwtmZmo2Ehi+gj1ghdmxwnUOXI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=q/0jgitKYBuTt2fO65889SZWQdR9YsxV6vRxoNXKC2Y58GZIZWU5S+x11hdbFfiO1kBFhj4wWTOZag+Lbu/IxOHEpcI6Sb9SsLb8UMSBFTmdSu9I6b132gI25ga8BUAX6/8naMEuXFFxZnqX8Nz8VgpLd5MYWx2UO4gYo15CHx8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=gxhkLSP1; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="gxhkLSP1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 47DDC1F000C4; Thu, 15 Oct 2026 20:10:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792095034; bh=Bm2ERFyVvwHy3KL3GkZjGCypavkWH4W9XQY9JXBOp38=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=gxhkLSP1ZNSGV8UU5saxrx0QHAqHeo8ZiuEPhoF5s7P570x19ZLLx/+1hYWjllG3A uTfOjm2GqP2hGVC1vT2M7tc0qf0kx0D8PpllSnxKPqAFhvKYwAAR+sbFcxsQbUwkA1 atznjnS9TsmxwCILJJFNVTDqc6MDJF603RyAn2FK7UBYoMyRHiZmvUaDxm1N5Qn82x D6GGDEuRUBNVLommG2TkyePjUjK2On7X1EYYJ19e/Oaz/Dir4OON5iBzh901RRhKmp EatjIdApRPLEolKK2VkjzZlga9mhWhUyHiczizT3hcSoqezppKW7qdxB9wNnR3dckn ZgHwgwKq2+uzQ== Date: Thu, 15 Oct 2026 21:10:26 +0100 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Muchun Song , Oscar Salvador , Miaohe Lin , Naoya Horiguchi , Adam Litke , Mel Gorman , Rik van Riel , Peter Xu , "Kiryl Shutsemau (Meta)" , "Mike Rapoport (Microsoft)" , Josef Bacik , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Chris Mason , stable@vger.kernel.org Subject: Re: [PATCH 06/10] mm/hugetlb: fix endless write fault on a leftover uffd bit Message-ID: References: <20261015-urgent_fixes-v1-0-42796911043b@kernel.org> <20261015-urgent_fixes-v1-6-42796911043b@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261015-urgent_fixes-v1-6-42796911043b@kernel.org> On Thu, Oct 15, 2026 at 06:07:12PM +0200, David Hildenbrand (Arm) wrote: > From: Josef Bacik > > hugetlb_fault() resolves the uffd bit on a write fault only if the VMA is > registered for userfaultfd write protection, and hugetlb_wp() bails out > for any entry with the uffd bit set assuming hugetlb_fault() has already > resolved it. > > When a read-only hugetlb entry with the uffd bit set is mapped in a VMA > that is not registered for uffd-wp, a write fault to it never makes > progress. If the access comes from the kernel, e.g. read(2) from > /dev/zero into such page, the task spins in the page fault handler, > ignores SIGKILL and keeps holding the locks taken by the system call. > > Such entries should not exist, but an unprivileged user can create them, > for example with MADV_DONTNEED of a page protected with UFFDIO_RWPROTECT > or with UFFDIO_COPY in MODE_WP racing with UFFDIO_UNREGISTER. Ugh yuck. Uffd is another gift that keeps giving... > > Clear the uffd bit when the VMA is not registered for uffd-wp and let the > fault proceed to hugetlb_wp(), like do_wp_page() does for regular pages. > > Note that a FOLL_FORCE write to a page protected with UFFDIO_RWPROTECT in > a private VMA that was made PROT_NONE now drops the RWP protection of > that page. > > Original fix generated by an LLM and sent in private by Josef. > > Fixes: 60d5b473d61b ("mm/hugetlb: fix uffd wr-protection for CoW optimization path") > Reported-by: Chris Mason > Cc: stable@vger.kernel.org > Signed-off-by: Josef Bacik > [rppt: massage commit message] > Signed-off-by: Mike Rapoport (Microsoft) > [ david: convert comment into a TODO ] > Signed-off-by: David Hildenbrand (Arm) Looks sensible to me so: Acked-by: Lorenzo Stoakes (ARM) > --- > mm/hugetlb.c | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index f8ad2efd472cb..c3823f1958568 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -6184,15 +6184,19 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, > goto out_ptl; > > /* Handle userfault-wp first, before trying to lock more pages */ > - if (userfaultfd_wp(vma) && huge_pte_uffd(huge_ptep_get(mm, vmf.address, vmf.pte)) && > + if (huge_pte_uffd(huge_ptep_get(mm, vmf.address, vmf.pte)) && > (flags & FAULT_FLAG_WRITE) && !huge_pte_write(vmf.orig_pte)) { > - if (!userfaultfd_wp_async(vma)) { > + if (userfaultfd_wp(vma) && !userfaultfd_wp_async(vma)) { > spin_unlock(vmf.ptl); > hugetlb_vma_unlock_read(vma); > mutex_unlock(&hugetlb_fault_mutex_table[hash]); > return handle_userfault(&vmf, VM_UFFD_WP); > } > > + /* > + * TODO: fix all cases where uffd PTE bits remain in VMAs that > + * are no longer registered with uffd. > + */ > vmf.orig_pte = huge_pte_clear_uffd(vmf.orig_pte); > set_huge_pte_at(mm, vmf.address, vmf.pte, vmf.orig_pte, > huge_page_size(hstate_vma(vma))); > > -- > 2.43.0 > -- Cheers, Lorenzo