mm: clear uffd-wp PTE/PMD state on mremap()

When mremap()ing a memory region previously registered with userfaultfd as
write-protected but without UFFD_FEATURE_EVENT_REMAP, an inconsistency in
flag clearing leads to a mismatch between the vma flags (which have
uffd-wp cleared) and the pte/pmd flags (which do not have uffd-wp
cleared).  This mismatch causes a subsequent mprotect(PROT_WRITE) to
trigger a warning in page_table_check_pte_flags() due to setting the pte
to writable while uffd-wp is still set.

Fix this by always explicitly clearing the uffd-wp pte/pmd flags on any
such mremap() so that the values are consistent with the existing clearing
of VM_UFFD_WP.  Be careful to clear the logical flag regardless of its
physical form; a PTE bit, a swap PTE bit, or a PTE marker.  Cover PTE,
huge PMD and hugetlb paths.

Link: https://lkml.kernel.org/r/20250107144755.1871363-2-ryan.roberts@arm.com
Co-developed-by: Mikołaj Lenczewski <miko.lenczewski@arm.com>
Signed-off-by: Mikołaj Lenczewski <miko.lenczewski@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Closes: https://lore.kernel.org/linux-mm/810b44a8-d2ae-4107-b665-5a42eae2d948@arm.com/
Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index cb40f1a1d081..75342022d144 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -247,6 +247,13 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
 	    vma_is_shmem(vma);
 }
 
+static inline bool vma_has_uffd_without_event_remap(struct vm_area_struct *vma)
+{
+	struct userfaultfd_ctx *uffd_ctx = vma->vm_userfaultfd_ctx.ctx;
+
+	return uffd_ctx && (uffd_ctx->features & UFFD_FEATURE_EVENT_REMAP) == 0;
+}
+
 extern int dup_userfaultfd(struct vm_area_struct *, struct list_head *);
 extern void dup_userfaultfd_complete(struct list_head *);
 void dup_userfaultfd_fail(struct list_head *);
@@ -402,6 +409,11 @@ static inline bool userfaultfd_wp_async(struct vm_area_struct *vma)
 	return false;
 }
 
+static inline bool vma_has_uffd_without_event_remap(struct vm_area_struct *vma)
+{
+	return false;
+}
+
 #endif /* CONFIG_USERFAULTFD */
 
 static inline bool userfaultfd_wp_use_markers(struct vm_area_struct *vma)