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 <gourry@gourry.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260912034833.2952750-1-gourry%40gourry.net
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Assisted-by: LLM
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org> # v6.19+

0 files changed, 0 insertions, 0 deletions