[PATCH] proc/task_mmu: handle special PMDs in clear_refs and pagemap

[Gregory Price]

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.

Reported-by: sashiko-bot <sashiko-bot@xxxxxxxxxx>
Closes: https://sashiko.dev/#/patchset/20260912034833.2952750-1-gourry%40gourry.net
Fixes: 3c8e44c9b369 ("mm: mark special bits for huge pfn mappings when inject")
Cc: <stable@xxxxxxxxxxxxxxx> # v6.19+
Assisted-by: LLM
Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
---
 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 24425e230895..501cd038b4e4 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1671,7 +1671,9 @@ 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);
@@ -1991,7 +1993,7 @@ 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))
-- 
2.55.0