[PATCH 5.17] mm: validate buddy page before using

[Xianting Tian]

Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its migratetype.")
fixes a bug in 1dd214b8f21c and there is a similar bug in d9dddbf55667 that
can be fixed in a similar way too.

In unset_migratetype_isolate(), we also need the fix, so move page_is_buddy()
from mm/page_alloc.c to mm/internal.h

In addition, for RISC-V arch the first 2MB RAM could be reserved for opensbi,
so it would have pfn_base=512 and mem_map began with 512th PFN when
CONFIG_FLATMEM=y.
But __find_buddy_pfn algorithm thinks the start pfn 0, it could get 0 pfn or
less than the pfn_base value. We need page_is_buddy() to verify the buddy to
prevent accessing an invalid buddy.

Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: zjb194813@xxxxxxxxxxxxxxx
Reported-by: tianhu.hh@xxxxxxxxxxxxxxx
Signed-off-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
---
 mm/internal.h       | 34 ++++++++++++++++++++++++++++++++++
 mm/page_alloc.c     | 37 +++----------------------------------
 mm/page_isolation.c |  3 ++-
 3 files changed, 39 insertions(+), 35 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index d80300392a19..dfa80bdfe5c6 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -386,6 +386,40 @@ static inline bool is_data_mapping(vm_flags_t flags)
 	return (flags & (VM_WRITE | VM_SHARED | VM_STACK)) == VM_WRITE;
 }
 
+/*
+ * This function checks whether a page is free && is the buddy
+ * we can coalesce a page and its buddy if
+ * (a) the buddy is not in a hole (check before calling!) &&
+ * (b) the buddy is in the buddy system &&
+ * (c) a page and its buddy have the same order &&
+ * (d) a page and its buddy are in the same zone.
+ *
+ * For recording whether a page is in the buddy system, we set PageBuddy.
+ * Setting, clearing, and testing PageBuddy is serialized by zone->lock.
+ *
+ * For recording page's order, we use page_private(page).
+ */
+static inline bool page_is_buddy(struct page *page, struct page *buddy,
+							unsigned int order)
+{
+	if (!page_is_guard(buddy) && !PageBuddy(buddy))
+		return false;
+
+	if (buddy_order(buddy) != order)
+		return false;
+
+	/*
+	 * zone check is done late to avoid uselessly calculating
+	 * zone/node ids for pages that could never merge.
+	 */
+	if (page_zone_id(page) != page_zone_id(buddy))
+		return false;
+
+	VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
+
+	return true;
+}
+
 /* mm/util.c */
 void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma,
 		struct vm_area_struct *prev);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b1caa1c6c887..78ada8dedefb 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -886,40 +886,6 @@ static inline void set_buddy_order(struct page *page, unsigned int order)
 	__SetPageBuddy(page);
 }
 
-/*
- * This function checks whether a page is free && is the buddy
- * we can coalesce a page and its buddy if
- * (a) the buddy is not in a hole (check before calling!) &&
- * (b) the buddy is in the buddy system &&
- * (c) a page and its buddy have the same order &&
- * (d) a page and its buddy are in the same zone.
- *
- * For recording whether a page is in the buddy system, we set PageBuddy.
- * Setting, clearing, and testing PageBuddy is serialized by zone->lock.
- *
- * For recording page's order, we use page_private(page).
- */
-static inline bool page_is_buddy(struct page *page, struct page *buddy,
-							unsigned int order)
-{
-	if (!page_is_guard(buddy) && !PageBuddy(buddy))
-		return false;
-
-	if (buddy_order(buddy) != order)
-		return false;
-
-	/*
-	 * zone check is done late to avoid uselessly calculating
-	 * zone/node ids for pages that could never merge.
-	 */
-	if (page_zone_id(page) != page_zone_id(buddy))
-		return false;
-
-	VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
-
-	return true;
-}
-
 #ifdef CONFIG_COMPACTION
 static inline struct capture_control *task_capc(struct zone *zone)
 {
@@ -1129,6 +1095,9 @@ static inline void __free_one_page(struct page *page,
 
 			buddy_pfn = __find_buddy_pfn(pfn, order);
 			buddy = page + (buddy_pfn - pfn);
+
+			if (!page_is_buddy(page, buddy, order))
+				goto done_merging;
 			buddy_mt = get_pageblock_migratetype(buddy);
 
 			if (migratetype != buddy_mt
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index f67c4c70f17f..5d14cef812ee 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -93,7 +93,8 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
 			buddy_pfn = __find_buddy_pfn(pfn, order);
 			buddy = page + (buddy_pfn - pfn);
 
-			if (!is_migrate_isolate_page(buddy)) {
+			if (page_is_buddy(page, buddy, order) &&
+			    !is_migrate_isolate_page(buddy)) {
 				isolated_page = !!__isolate_free_page(page, order);
 				/*
 				 * Isolating a free page in an isolated pageblock
-- 
2.17.1