[PATCH 06/10] mm/hugetlb: fix endless write fault on a leftover uffd bit

[David Hildenbrand (Arm)]

From: Josef Bacik <josef@xxxxxxxxxxxxxx>

hugetlb_fault() resolves the uffd bit on a write fault only if the VMA is
registered for userfaultfd write protection, and hugetlb_wp() bails out
for any entry with the uffd bit set assuming hugetlb_fault() has already
resolved it.

When a read-only hugetlb entry with the uffd bit set is mapped in a VMA
that is not registered for uffd-wp, a write fault to it never makes
progress. If the access comes from the kernel, e.g. read(2) from
/dev/zero into such page, the task spins in the page fault handler,
ignores SIGKILL and keeps holding the locks taken by the system call.

Such entries should not exist, but an unprivileged user can create them,
for example with MADV_DONTNEED of a page protected with UFFDIO_RWPROTECT
or with UFFDIO_COPY in MODE_WP racing with UFFDIO_UNREGISTER.

Clear the uffd bit when the VMA is not registered for uffd-wp and let the
fault proceed to hugetlb_wp(), like do_wp_page() does for regular pages.

Note that a FOLL_FORCE write to a page protected with UFFDIO_RWPROTECT in
a private VMA that was made PROT_NONE now drops the RWP protection of
that page.

Original fix generated by an LLM and sent in private by Josef.

Fixes: 60d5b473d61b ("mm/hugetlb: fix uffd wr-protection for CoW optimization path")
Reported-by: Chris Mason <mason@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
[rppt: massage commit message]
Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
[ david: convert comment into a TODO ]
Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
---
 mm/hugetlb.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f8ad2efd472cb..c3823f1958568 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6184,15 +6184,19 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 		goto out_ptl;
 
 	/* Handle userfault-wp first, before trying to lock more pages */
-	if (userfaultfd_wp(vma) && huge_pte_uffd(huge_ptep_get(mm, vmf.address, vmf.pte)) &&
+	if (huge_pte_uffd(huge_ptep_get(mm, vmf.address, vmf.pte)) &&
 	    (flags & FAULT_FLAG_WRITE) && !huge_pte_write(vmf.orig_pte)) {
-		if (!userfaultfd_wp_async(vma)) {
+		if (userfaultfd_wp(vma) && !userfaultfd_wp_async(vma)) {
 			spin_unlock(vmf.ptl);
 			hugetlb_vma_unlock_read(vma);
 			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
 			return handle_userfault(&vmf, VM_UFFD_WP);
 		}
 
+		/*
+		 * TODO: fix all cases where uffd PTE bits remain in VMAs that
+		 * are no longer registered with uffd.
+		 */
 		vmf.orig_pte = huge_pte_clear_uffd(vmf.orig_pte);
 		set_huge_pte_at(mm, vmf.address, vmf.pte, vmf.orig_pte,
 				huge_page_size(hstate_vma(vma)));

-- 
2.43.0