Re: [PATCH] mm/hugetlb: fix kernel NULL pointer dereference when replacing free hugetlb folios

[Oscar Salvador <osalvador@suse.de>]

On Thu, May 22, 2025 at 08:39:39PM +0800, Muchun Song wrote:
> But I think we could use "folio_order() > MAX_PAGE_ORDER" to replace the check
> of hstate_is_gigantic(), right? Then ee could remove the first parameter of hstate
> from alloc_and_dissolve_hugetlb_folio() and obtain hstate in it.

Yes, I think we can do that.
So something like the following (compily-tested only) maybe?

 From d7199339e905f83b54d22849e8f21f631916ce94 Mon Sep 17 00:00:00 2001
 From: Oscar Salvador <osalvador@suse.de>
 Date: Thu, 22 May 2025 19:51:04 +0200
 Subject: [PATCH] TMP
 
 ---
  mm/hugetlb.c | 38 +++++++++-----------------------------
  1 file changed, 9 insertions(+), 29 deletions(-)
 
 diff --git a/mm/hugetlb.c b/mm/hugetlb.c
 index bd8971388236..20f08de9e37d 100644
 --- a/mm/hugetlb.c
 +++ b/mm/hugetlb.c
 @@ -2787,15 +2787,13 @@ void restore_reserve_on_error(struct hstate *h, struct vm_area_struct *vma,
  /*
   * alloc_and_dissolve_hugetlb_folio - Allocate a new folio and dissolve
   * the old one
 - * @h: struct hstate old page belongs to
   * @old_folio: Old folio to dissolve
   * @list: List to isolate the page in case we need to
   * Returns 0 on success, otherwise negated error.
   */
 -static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
 -			struct folio *old_folio, struct list_head *list)
 +static int alloc_and_dissolve_hugetlb_folio(struct folio *old_folio, struct list_head *list)
  {
 -	gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
 +	struct hstate *h;
  	int nid = folio_nid(old_folio);
  	struct folio *new_folio = NULL;
  	int ret = 0;
 @@ -2829,7 +2827,11 @@ static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
  		cond_resched();
  		goto retry;
  	} else {
 +		h = folio_hstate(old_folio);
 +
  		if (!new_folio) {
 +			gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
 +
  			spin_unlock_irq(&hugetlb_lock);
  			new_folio = alloc_buddy_hugetlb_folio(h, gfp_mask, nid,
  							      NULL, NULL);
 @@ -2874,35 +2876,20 @@ static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
  
  int isolate_or_dissolve_huge_folio(struct folio *folio, struct list_head *list)
  {
 -	struct hstate *h;
  	int ret = -EBUSY;
  
 -	/*
 -	 * The page might have been dissolved from under our feet, so make sure
 -	 * to carefully check the state under the lock.
 -	 * Return success when racing as if we dissolved the page ourselves.
 -	 */
 -	spin_lock_irq(&hugetlb_lock);
 -	if (folio_test_hugetlb(folio)) {
 -		h = folio_hstate(folio);
 -	} else {
 -		spin_unlock_irq(&hugetlb_lock);
 -		return 0;
 -	}
 -	spin_unlock_irq(&hugetlb_lock);
 -
  	/*
  	 * Fence off gigantic pages as there is a cyclic dependency between
  	 * alloc_contig_range and them. Return -ENOMEM as this has the effect
  	 * of bailing out right away without further retrying.
  	 */
 -	if (hstate_is_gigantic(h))
 +	if (folio_order(folio) > MAX_PAGE_ORDER)
  		return -ENOMEM;
  
  	if (folio_ref_count(folio) && folio_isolate_hugetlb(folio, list))
  		ret = 0;
  	else if (!folio_ref_count(folio))
 -		ret = alloc_and_dissolve_hugetlb_folio(h, folio, list);
 +		ret = alloc_and_dissolve_hugetlb_folio(folio, list);
  
  	return ret;
  }
 @@ -2916,7 +2903,6 @@ int isolate_or_dissolve_huge_folio(struct folio *folio, struct list_head *list)
   */
  int replace_free_hugepage_folios(unsigned long start_pfn, unsigned long end_pfn)
  {
 -	struct hstate *h;
  	struct folio *folio;
  	int ret = 0;
  
 @@ -2924,15 +2910,9 @@ int replace_free_hugepage_folios(unsigned long start_pfn, unsigned long end_pfn)
  
  	while (start_pfn < end_pfn) {
  		folio = pfn_folio(start_pfn);
 -		if (folio_test_hugetlb(folio)) {
 -			h = folio_hstate(folio);
 -		} else {
 -			start_pfn++;
 -			continue;
 -		}
  
  		if (!folio_ref_count(folio)) {
 -			ret = alloc_and_dissolve_hugetlb_folio(h, folio,
 +			ret = alloc_and_dissolve_hugetlb_folio(folio,
  							       &isolate_list);
  			if (ret)
  				break;
 -- 
 2.49.0

 

-- 
Oscar Salvador
SUSE Labs