[PATCH 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. If the page being
unmapped has HPAGE_RESV_OWNER set, and needs a reservation, set the
restore_reserve flag, which will move the page from free to reserved.

Suggested-by: Rik van Riel <riel@xxxxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 mm/hugetlb.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ed1581b670d4..fa2c17767e44 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5677,6 +5677,16 @@ 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));
 
+		if (is_vma_resv_set(vma, HPAGE_RESV_OWNER) &&
+		    vma_needs_reservation(h, vma, start)) {
+			/*
+			 * Restore the reservation if needed, otherwise the
+			 * backing page could be stolen by someone.
+			 */
+			folio_set_hugetlb_restore_reserve(page_folio(page));
+			vma_add_reservation(h, vma, address);
+		}
+
 		spin_unlock(ptl);
 		tlb_remove_page_size(tlb, page, huge_page_size(h));
 		/*
-- 
2.34.1