From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794047173; b=p+n7cpaLj8dQjpAnstFbT1f7VQ7afwYfvNA3A0Ldn6BMnFtLYrqCByKskrqrktj+eXlf XcrZaWNfoaiesAf+zV85pkgNnOQrhLUZsP9VHBDUX1OcONXToE3jPnoSFYBRC7UlFhBM0 8xro5AWyO5jPxyMiv3SUxaW7jIn6NQwmF0K5wv6heBpGJam8doMHyLbsn4/bqkvyN9dpK IFWrS7pi4BYa5+wUT/NGYFjyqztJzRnxLHEZyNjrXDXDFo34ZjRTClyUQ7wADMSqapEwu 74YT9aSjB/lVby1xuRO8EMPC3SmJDvnmM3+UipL8vHA/eiWjQWaRgjyUNSUYTYOBKMA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794047173; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: MIME-Version:Content-Type:Content-Transfer-Encoding; bh=jpBVlxi3AUj9yXX3AHvRzi6scjGsIxqWV5OvnNDzPcE=; b=mjsH0def+OpULkgwbrScQI1VtwCgIt/4PkUIZL8I126kc07sGNq6TtidbHPTbPiXlNSF 1AhKqro9TLkE6nXSrt6Ny8AMiNU+hjM7ZdMESGSu5tnqr+vIg88GOu55jnwD1g4t4/FiZ pM4guJMrwSVtGw9b/9BA+yegjaXRIR55ocOILhmVokyFVFucLNXc7EqB6s/l593yzrVLb dqb5dVf9lEq+NVu4semqcSzkuhpTakx1zvHRr6MyfJMoHiZK0etvy66s78URJWiufZToL nwKStE7/Ojn/mpzRACI4mrDwyfLN0OFFK8WiPGEYd+f2TqEb7ainDJKK6sHc31RgDkQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=XFC9JFJu; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A51AE606A32C for ; Sat, 7 Nov 2026 10:26:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=XFC9JFJu Received: by smtp.kernel.org (Postfix) with ESMTPSA id 45DD31F0089F; Sat, 7 Nov 2026 10:26:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794047172; bh=jpBVlxi3AUj9yXX3AHvRzi6scjGsIxqWV5OvnNDzPcE=; h=From:To:Cc:Subject:Date; b=XFC9JFJuf/i3wtiTYF/VeKgRUWH4W/eYvqujQZpL+MMIgh7wDLHsb/YaNsny0e9Hu 4znxQPXqrHJHMBIw26rLGSqe011DA6qnUsmtJHzkASsCt78RKlz2jdXLh+9p3CgwZk VgbGpKT05h4Kzxsl+jvyVhnudev3MSW2tYjuGxFETtU1GeMyZ3iLmWzZnlZuoUxvSN AR4jFpr3BE7pAvO1ckpc9chagiZoLc2SlqSEkcMBFmcAlmX9qowDjmSMT0XuUumwFw /QElW87QD6SQdefiQm9FmcuQW7qNDAAyPTWAzuIBzwfAi097pVkc7hzvU6/JlXS1g7 16+AUgAss2jvw== From: Sasha Levin To: david@kernel.org, akpm@linux-foundation.org, kylebot@openai.com, ljs@kernel.org, muchun.song@linux.dev, rppt@kernel.org, lance.yang@linux.dev, osalvador@suse.de, peterx@redhat.com, linmiaohe@huawei.com, lokeshgidra@google.com, liam@infradead.org Cc: stable@vger.kernel.org Subject: FAILED: patch "[PATCH] mm/userfaultfd: only allocate anon_vma in CoW mappings" failed to apply to 6.12-stable tree Date: Sat, 7 Nov 2026 05:26:10 -0500 Message-ID: <20261107052610-failed-6.12-514c9247db30@kernel.org> X-Mailer: git-send-email 2.53.0 X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The patch below does not apply to the 6.12-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . To reproduce the conflict and resubmit, you may use the following commands: git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y git checkout FETCH_HEAD git cherry-pick -x 514c9247db305922591d806ec65d3c3fe420538d # git commit -s git send-email --to '' --in-reply-to '20261107052610-failed-6.12-514c9247db30@kernel.org' --subject-prefix 'PATCH 6.12.y' 'HEAD^..' Thanks, Sasha ------------------ original commit in Linus's tree ------------------ >From 514c9247db305922591d806ec65d3c3fe420538d Mon Sep 17 00:00:00 2001 From: "David Hildenbrand (Arm)" Date: Wed, 14 Oct 2026 13:53:11 +0200 Subject: [PATCH] mm/userfaultfd: only allocate anon_vma in CoW mappings find_vma_and_prepare_anon() can end up allocating an anon_vma in VMAs that are not CoW mappings, which is wrong. If we have a MAP_SHARED R/O file mapped, we lack VM_SHARED (but do have VM_MAYSHARE) and lack VM_MAYWRITE. While uffd later rejects such mappings (uffd registration requires VM_MAYWRITE, see comment in validate_dst_vma) find_vma_and_prepare_anon() still wrongly allocates an anon vma before it validates the vma. This in turn later confuses copy_page_range()->vma_needs_copy() such that we end up calling copy_hugetlb_page_range() with ranges that actually do not contain *any* per-pagetable information that would require copying, which is the basis for PMD-table sharing in the first place. copy_hugetlb_page_range() should not have to worry about shared PMD tables at all: it doesn't make any sense to copy page tables that are shared, as they can be fully recreated later. Ending up in the function with shareable page tables is the real issue. If a destination PMD table is shared in copy_hugetlb_page_range(), it might get concurrently unshared as we are not holding relevant locks, resulting in us using a page table that is no longer mapped and possibly freed after calling: dst_pte = huge_pte_alloc(dst, dst_vma, addr, sz); To fix it, do not allocate an anon_vma in non-CoW mappings and add a safety guard to copy_hugetlb_page_range(), making sure that we never end up messing with shared PMD tables in there. Before commit 867a43a34ff8a ("userfaultfd: use per-vma locks in userfaultfd operations") we would always validate the VMA before allocating an anon_vma, and consequently never wrongly allocate an anon_vma. As uffd_lock_vma() is part of the same lookup machinery and also contains a misleading VM_SHARED check, change that one as well directly for consistency. A follow-up cleanup will stop taking the hugetlb_vma_lock_read(src_vma) lock in copy_hugetlb_page_range() entirely. Further, the usage of VM_SHARED in userfaultfd code should be cleaned up later to focus on CoW vs. non-CoW mappings. Link: https://lore.kernel.org/20261014-copy_hugetlb_page_range-v1-1-5b68e03fe7fe@kernel.org Fixes: 867a43a34ff8 ("userfaultfd: use per-vma locks in userfaultfd operations") Signed-off-by: David Hildenbrand (Arm) Signed-off-by: Andrew Morton Reported-by: Kyle Zeng Reviewed-by: Lorenzo Stoakes (ARM) Reviewed-by: Kyle Zeng Tested-by: Kyle Zeng Acked-by: Muchun Song Acked-by: Mike Rapoport (Microsoft) Tested-by: Lance Yang Cc: Oscar Salvador Cc: Peter Xu Cc: Miaohe Lin Cc: Lokesh Gidra Cc: Liam R. Howlett Cc: --- mm/hugetlb.c | 24 ++++++++++++------------ mm/userfaultfd.c | 5 ++--- 2 files changed, 14 insertions(+), 15 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 27e8136b18150..da5d04edf7c10 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -4973,10 +4973,11 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, raw_write_seqcount_begin(&src->write_protect_seq); } else { /* - * For shared mappings the vma lock must be held before - * calling hugetlb_walk() in the src vma. Otherwise, the - * returned ptep could go away if part of a shared pmd and - * another thread calls huge_pmd_unshare. + * Shared mappings only need copying when we could have + * information stored in page tables (e.g., userfaultfd PTE + * bits or markers), which is incompatible with PMD sharing. + * Still take the hugetlb VMA lock for now to satisfy + * hugetlb_walk()'s locking requirements. */ hugetlb_vma_lock_read(src_vma); } @@ -4984,6 +4985,13 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, last_addr_mask = hugetlb_mask_last_page(h); for (addr = src_vma->vm_start; addr < src_vma->vm_end; addr += sz) { spinlock_t *src_ptl, *dst_ptl; + + if (WARN_ON_ONCE(want_pmd_share(src_vma, addr) || + want_pmd_share(dst_vma, addr))) { + ret = -EINVAL; + break; + } + src_pte = hugetlb_walk(src_vma, addr, sz); if (!src_pte) { addr |= last_addr_mask; @@ -4995,14 +5003,6 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, break; } -#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING - /* If the pagetables are shared, there is nothing to do */ - if (ptdesc_pmd_is_shared(virt_to_ptdesc(dst_pte))) { - addr |= last_addr_mask; - continue; - } -#endif - dst_ptl = huge_pte_lock(h, dst, dst_pte); src_ptl = huge_pte_lockptr(h, src, src_pte); spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING); diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index da67d35cd9546..405f2d72f204e 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -115,8 +115,7 @@ struct vm_area_struct *find_vma_and_prepare_anon(struct mm_struct *mm, vma = vma_lookup(mm, addr); if (!vma) vma = ERR_PTR(-ENOENT); - else if (!(vma->vm_flags & VM_SHARED) && - unlikely(anon_vma_prepare(vma))) + else if (vma_is_cow_mapping(vma) && unlikely(anon_vma_prepare(vma))) vma = ERR_PTR(-ENOMEM); return vma; @@ -145,7 +144,7 @@ static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm, * We know we're going to need an anon rmap, so check that * early. */ - if (!(vma->vm_flags & VM_SHARED) && unlikely(!vma_has_anon_rmap(vma))) + if (vma_is_cow_mapping(vma) && unlikely(!vma_has_anon_rmap(vma))) vma_end_read(vma); else return vma; -- 2.53.0