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Howlett" , Sasha Levin Subject: [PATCH 6.12.y] mm/userfaultfd: only allocate anon_vma in CoW mappings Date: Tue, 10 Nov 2026 23:38:36 -0500 Message-ID: <20261111043611.1129191-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261107052610-failed-6.12-514c9247db30@kernel.org> References: <20261107052610-failed-6.12-514c9247db30@kernel.org> 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-Transfer-Encoding: 8bit From: "David Hildenbrand (Arm)" [ Upstream commit 514c9247db305922591d806ec65d3c3fe420538d ] 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: [ sashal: adapted VMA checks to use is_cow_mapping(vma->vm_flags) with direct vma->anon_vma access ] Signed-off-by: Sasha Levin --- 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 d9443bdebc068..4c59efc87a355 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5275,10 +5275,11 @@ 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); } @@ -5286,6 +5287,13 @@ 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; @@ -5297,14 +5305,6 @@ 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 9951b4f42c65a..417c4f5265fff 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -48,8 +48,7 @@ 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 (is_cow_mapping(vma->vm_flags) && unlikely(anon_vma_prepare(vma))) vma = ERR_PTR(-ENOMEM); return vma; @@ -77,7 +76,7 @@ static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm, * We know we're going to need to use anon_vma, so check * that early. */ - if (!(vma->vm_flags & VM_SHARED) && unlikely(!vma->anon_vma)) + if (is_cow_mapping(vma->vm_flags) && unlikely(!vma->anon_vma)) vma_end_read(vma); else return vma; -- 2.53.0