From d350956d829f4b620faf294c86116de640f1518c Mon Sep 17 00:00:00 2001 From: Gregory Price Date: Sat, 12 Sep 2026 08:28:22 -0400 Subject: [PATCH] proc/task_mmu: handle special PMDs in clear_refs and pagemap mshv_vtl_low can install a special PMD from a PFN supplied in the mmap file offset. That PFN need not have a memmap entry. Reading /proc/PID/pagemap or writing /proc/PID/clear_refs for such a mapping oopses the kernel. With a test driver mapping the PFN at the 1 TiB mark: echo 1 > /proc/self/clear_refs BUG: unable to handle page fault for address: fffffb5b80000008 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page RIP: 0010:clear_refs_pte_range+0xb1/0x1e0 Call Trace: walk_pgd_range+0x50a/0xad0 __walk_page_range+0x6a/0x1d0 walk_page_range_mm_unsafe+0x193/0x230 clear_refs_write+0x18e/0x3f0 pread(pagemap_fd, buf, 512 * 8, addr / PAGE_SIZE * 8) BUG: unable to handle page fault for address: fffff57840000008 RIP: 0010:pagemap_pmd_range+0x3cf/0x6b0 Call Trace: walk_pgd_range+0x50a/0xad0 __walk_page_range+0x6a/0x1d0 walk_page_range_mm_unsafe+0x193/0x230 pagemap_read+0x1dc/0x350 clear_refs_pte_range() passes the PMD to pmd_folio(), while pagemap_pmd_range_thp() uses pmd_page() followed by page_folio(). Both paths assume the PFN has a struct page and dereference bad address. Use vm_normal_folio_pmd() and vm_normal_page_pmd() so clear_refs skips folio operations and pagemap skips folio-derived flags for special PMDs. This matches the PTE paths, which already use vm_normal_folio() and vm_normal_page(). User-facing notes: - Pagemap still reports PM_PRESENT and the PFN to a CAP_SYS_ADMIN reader. - For a special PMD backed by a valid memmap entry and for the huge zero PMD, PM_FILE changes from set to clear. - PM_MMAP_EXCLUSIVE is already clear in both cases. - The output for an anonymous THP is unchanged. Tested in QEMU with the test driver on broken and fixed kernels. Both proc operations oops before the fix and return 0 after it. This is technically reachable via mshv_vtl_low, so I tagged it stable. Link: https://lore.kernel.org/20260912122822.3348978-1-gourry@gourry.net Fixes: 3c8e44c9b369 ("mm: mark special bits for huge pfn mappings when inject") Signed-off-by: Gregory Price Signed-off-by: Andrew Morton Reported-by: sashiko-bot Closes: https://sashiko.dev/#/patchset/20260912034833.2952750-1-gourry%40gourry.net Acked-by: David Hildenbrand (Arm) Assisted-by: LLM Cc: Jann Horn Cc: Jason Gunthorpe Cc: Liam R. Howlett Cc: Lorenzo Stoakes Cc: Pedro Falcato Cc: Peter Xu Cc: Vlastimil Babka Cc: # v6.19+ --- fs/proc/task_mmu.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index e671b4fd8ded..78e00c6d4e2d 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -1704,7 +1704,9 @@ static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr, if (!pmd_present(*pmd)) goto out; - folio = pmd_folio(*pmd); + folio = vm_normal_folio_pmd(vma, addr, *pmd); + if (!folio) + goto out; /* Clear accessed and referenced bits. */ pmdp_test_and_clear_young(vma, addr, pmd); @@ -2024,7 +2026,7 @@ static int pagemap_pmd_range_thp(pmd_t *pmdp, unsigned long addr, goto populate_pagemap; if (pmd_present(pmd)) { - page = pmd_page(pmd); + page = vm_normal_page_pmd(vma, addr, pmd); flags |= PM_PRESENT; if (pmd_soft_dirty(pmd))