mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP

Wire the fault side of read-write protection tracking and turn the
userspace interface on.

An RWP-protected PTE is PAGE_NONE with the uffd bit set.  The PROT_NONE
triggers a fault on any access; the uffd bit distinguishes it from plain
mprotect(PROT_NONE) or NUMA hinting.

Fault dispatch, per level:

  PTE     handle_pte_fault()    -> do_uffd_rwp()
  PMD     __handle_mm_fault()   -> do_huge_pmd_uffd_rwp()
  hugetlb hugetlb_fault()       -> hugetlb_handle_userfault()

The RWP branches gate on userfaultfd_pte_rwp() /
userfaultfd_huge_pmd_rwp() (VM_UFFD_RWP plus the uffd bit) and fall
through to do_numa_page() / do_huge_pmd_numa_page() otherwise.  Each
delivers a UFFD_PAGEFAULT_FLAG_RWP message through handle_userfault(); the
handler resolves it with UFFDIO_RWPROTECT clearing MODE_RWP.

userfaultfd_must_wait() and userfaultfd_huge_must_wait() add matching
protnone+uffd waiters so sync-mode fault handlers block correctly.

Expose the UAPI:

  UFFDIO_REGISTER_MODE_RWP   -> UFFD_API_REGISTER_MODES
  UFFD_FEATURE_RWP           -> UFFD_API_FEATURES
  _UFFDIO_RWPROTECT          -> UFFD_API_RANGE_IOCTLS
                                UFFD_API_RANGE_IOCTLS_BASIC

UFFD_FEATURE_RWP is masked out at UFFDIO_API time when PROT_NONE is not
available or VM_UFFD_RWP aliases VM_NONE (32-bit), so userspace never sees
an advertised-but-broken feature.

Works on anonymous, shmem, and hugetlb memory.

Link: https://lore.kernel.org/20260708111417.173443-11-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: James Houghton <jthoughton@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 31 insertions, 0 deletions

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index b34e8e5e5f9a..19a7bfce279e 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -529,6 +529,8 @@ static inline bool folio_test_pmd_mappable(struct folio *folio)
 
 vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf);
 
+vm_fault_t do_huge_pmd_uffd_rwp(struct vm_fault *vmf);
+
 vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf);
 
 extern struct folio *huge_zero_folio;
@@ -723,6 +725,11 @@ static inline spinlock_t *pud_trans_huge_lock(pud_t *pud,
 	return NULL;
 }
 
+static inline vm_fault_t do_huge_pmd_uffd_rwp(struct vm_fault *vmf)
+{
+	return 0;
+}
+
 static inline vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
 {
 	return 0;
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index d31648ba898d..6f7245609bd4 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -241,6 +241,18 @@ static inline bool userfaultfd_huge_pmd_wp(struct vm_area_struct *vma,
 	return userfaultfd_wp(vma) && pmd_uffd(pmd);
 }
 
+static inline bool userfaultfd_pte_rwp(struct vm_area_struct *vma,
+				       pte_t pte)
+{
+	return userfaultfd_rwp(vma) && pte_uffd(pte);
+}
+
+static inline bool userfaultfd_huge_pmd_rwp(struct vm_area_struct *vma,
+					    pmd_t pmd)
+{
+	return userfaultfd_rwp(vma) && pmd_uffd(pmd);
+}
+
 static inline bool userfaultfd_armed(struct vm_area_struct *vma)
 {
 	return vma_test_any_mask(vma, __VMA_UFFD_FLAGS);
@@ -371,6 +383,18 @@ static inline bool userfaultfd_huge_pmd_wp(struct vm_area_struct *vma,
 	return false;
 }
 
+static inline bool userfaultfd_pte_rwp(struct vm_area_struct *vma,
+				       pte_t pte)
+{
+	return false;
+}
+
+static inline bool userfaultfd_huge_pmd_rwp(struct vm_area_struct *vma,
+					    pmd_t pmd)
+{
+	return false;
+}
+
 static inline bool userfaultfd_armed(struct vm_area_struct *vma)
 {
 	return false;