[PATCH v2 1/2] mm/hugetlb: Restore the reservation if needed

[Breno Leitao]

Currently there is a bug that a huge page could be stolen, and when the
original owner tries to fault in it, it causes a page fault.

You can achieve that by:
  1) Creating a single page
	echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages

  2) mmap() the page above with MAP_HUGETLB into (void *ptr1).
	* This will mark the page as reserved
  3) touch the page, which causes a page fault and allocates the page
	* This will move the page out of the free list.
	* It will also unreserved the page, since there is no more free
	  page
  4) madvise(MADV_DONTNEED) the page
	* This will free the page, but not mark it as reserved.
  5) Allocate a secondary page with mmap(MAP_HUGETLB) into (void *ptr2).
	* it should fail, but, since there is no more available page.
	* But, since the page above is not reserved, this mmap() succeed.
  6) Faulting at ptr1 will cause a SIGBUS
	* it will try to allocate a huge page, but there is none
	  available

A full reproducer is in selftest. See
https://lore.kernel.org/all/20240105155419.1939484-1-leitao@xxxxxxxxxx/

Fix this by restoring the reserved page if necessary.

These are the condition for the page restore:

 * The system is not using surplus pages. The goal is to reduce the
   surplus usage for this case.
 * If the VMA has the HPAGE_RESV_OWNER flag set, and is PRIVATE. This is
   safely checked using __vma_private_lock()
 * The page is anonymous

Once this is scenario is found, set the `hugetlb_restore_reserve` bit in
the folio. Then check if the resv reservations need to be adjusted
later, done later, after the spinlock, since the vma_xxxx_reservation()
might touch the file system lock.

Suggested-by: Rik van Riel <riel@xxxxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 mm/hugetlb.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ed1581b670d4..44f1e6366d04 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5585,6 +5585,7 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	struct page *page;
 	struct hstate *h = hstate_vma(vma);
 	unsigned long sz = huge_page_size(h);
+	bool adjust_reservation = false;
 	unsigned long last_addr_mask;
 	bool force_flush = false;
 
@@ -5677,7 +5678,31 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		hugetlb_count_sub(pages_per_huge_page(h), mm);
 		hugetlb_remove_rmap(page_folio(page));
 
+		/*
+		 * Restore the reservation for anonymous page, otherwise the
+		 * backing page could be stolen by someone.
+		 * If there we are freeing a surplus, do not set the restore
+		 * reservation bit.
+		 */
+		if (!h->surplus_huge_pages && __vma_private_lock(vma) &&
+		    folio_test_anon(page_folio(page))) {
+			folio_set_hugetlb_restore_reserve(page_folio(page));
+			/* Reservation to be adjusted after the spin lock */
+			adjust_reservation = true;
+		}
+
 		spin_unlock(ptl);
+
+		/*
+		 * Adjust the reservation for the region that will have the
+		 * reserve restored. Keep in mind that vma_needs_reservation() changes
+		 * resv->adds_in_progress if it succeeds. If this is not done,
+		 * do_exit() will not see it, and will keep the reservation
+		 * forever.
+		 */
+		if (adjust_reservation && vma_needs_reservation(h, vma, address))
+			vma_add_reservation(h, vma, address);
+
 		tlb_remove_page_size(tlb, page, huge_page_size(h));
 		/*
 		 * Bail out after unmapping reference page if supplied
-- 
2.34.1