CVE-2026-53108: powerpc/64s: Fix unmap race with PMD migration entries
In the Linux kernel, the following vulnerability has been resolved:

powerpc/64s: Fix unmap race with PMD migration entries

The following race is possible with migration swap entries or
device-private THP entries. e.g. when move_pages is called on a PMD THP
page, then there maybe an intermediate state, where PMD entry acts as
a migration swap entry (pmd_present() is true). Then if an munmap
happens at the same time, then this VM_BUG_ON() can happen in
pmdp_huge_get_and_clear_full().

This patch fixes that.

Thread A: move_pages() syscall
  add_folio_for_migration()
    mmap_read_lock(mm)
    folio_isolate_lru(folio)
    mmap_read_unlock(mm)

  do_move_pages_to_node()
    migrate_pages()
      try_to_migrate_one()
        spin_lock(ptl)
        set_pmd_migration_entry()
          pmdp_invalidate()     # PMD: _PAGE_INVALID | _PAGE_PTE | pfn
          set_pmd_at()          # PMD: migration swap entry (pmd_present=0)
        spin_unlock(ptl)
        [page copy phase]       # <--- RACE WINDOW -->

Thread B: munmap()
  mmap_write_downgrade(mm)
  unmap_vmas() -> zap_pmd_range()
    zap_huge_pmd()
      __pmd_trans_huge_lock()
        pmd_is_huge():          # !pmd_present && !pmd_none -> TRUE (swap entry)
        pmd_lock() -> 		# spin_lock(ptl), waits for Thread A to release ptl
      pmdp_huge_get_and_clear_full()
        VM_BUG_ON(!pmd_present(*pmdp))  # HITS!

[  287.738700][ T1867] ------------[ cut here ]------------
[  287.743843][ T1867] kernel BUG at arch/powerpc/mm/book3s64/pgtable.c:187!
cpu 0x0: Vector: 700 (Program Check) at [c00000044037f4f0]
    pc: c000000000094ca4: pmdp_huge_get_and_clear_full+0x6c/0x23c
    lr: c000000000645dec: zap_huge_pmd+0xb0/0x868
    sp: c00000044037f790
   msr: 800000000282b033
  current = 0xc0000004032c1a00
  paca    = 0xc000000004fe0000   irqmask: 0x03   irq_happened: 0x09
    pid   = 1867, comm = a.out
kernel BUG at :187!
Linux version 6.19.0-12136-g14360d4f917c-dirty (powerpc64le-linux-gnu-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40) #27 SMP PREEMPT Sun Feb 22 10:38:56 IST 2026
enter ? for help
[link register   ] c000000000645dec zap_huge_pmd+0xb0/0x868
[c00000044037f790] c00000044037f7d0 (unreliable)
[c00000044037f7d0] c000000000645dcc zap_huge_pmd+0x90/0x868
[c00000044037f840] c0000000005724cc unmap_page_range+0x176c/0x1f40
[c00000044037fa00] c000000000572ea0 unmap_vmas+0xb0/0x1d8
[c00000044037fa90] c0000000005af254 unmap_region+0xb4/0x128
[c00000044037fb50] c0000000005af400 vms_complete_munmap_vmas+0x138/0x310
[c00000044037fbe0] c0000000005b0f1c do_vmi_align_munmap+0x1ec/0x238
[c00000044037fd30] c0000000005b3688 __vm_munmap+0x170/0x1f8
[c00000044037fdf0] c000000000587f74 sys_munmap+0x2c/0x40
[c00000044037fe10] c000000000032668 system_call_exception+0x128/0x350
[c00000044037fe50] c00000000000d05c system_call_vectored_common+0x15c/0x2ec
---- Exception: 3000 (System Call Vectored) at 0000000010064a2c
SP (7fff9b1ee9c0) is in userspace
0:mon> zh

commit a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages"),
enabled migration for device-private PMD entries. Hence this is one
other path where this warning could get trigger from.

 ------------[ cut here ]------------
 WARNING: arch/powerpc/mm/book3s64/hash_pgtable.c:199 at hash__pmd_hugepage_update+0x48/0x284, CPU#3: hmm-tests/1905
 Modules linked in: test_hmm
 CPU: 3 UID: 0 PID: 1905 Comm: hmm-tests Tainted: G    B   W    L   N  7.0.0-rc1-01438-g7e2f0ee7581c #21 PREEMPT
 Tainted: [B]=BAD_PAGE, [W]=WARN, [L]=SOFTLOCKUP, [N]=TEST
 Hardware name: IBM pSeries (emulated by qemu) POWER10 (architected) 0x801200 0xf000006 of:SLOF,git-ee03ae pSeries
 NIP [c000000000096b70] hash__pmd_hugepage_update+0x48/0x284
 LR [c000000000096e7c] hash__pmdp_huge_get_and_clear+0xd0/0xd4
 Call Trace:
 [c000000604707670] [c000000004e102b8] 0xc000000004e102b8 (unreliable)
 [c000000604707700] [c00000000064ec3c] set_pmd_migration_entry+0x414/0x498
 [c000000604707760] [c00000000063e5a4] migrate_vma_col
---truncated---