[PATCH v2 1/1] memory_hotplug: fix the panic when memory end is not on the section boundary

[Mikhail Zaslonko <zaslonko@linux.ibm.com>]

If memory end is not aligned with the sparse memory section boundary, the
mapping of such a section is only partly initialized. This may lead to
VM_BUG_ON due to uninitialized struct pages access from
is_mem_section_removable() or test_pages_in_a_zone() function triggered by
memory_hotplug sysfs handlers.

Here are the the panic examples:
 CONFIG_DEBUG_VM_PGFLAGS=y
 kernel parameter mem=2050M
 --------------------------
 page:000003d082008000 is uninitialized and poisoned
 page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
 Call Trace:
 ([<0000000000385b26>] test_pages_in_a_zone+0xde/0x160)
  [<00000000008f15c4>] show_valid_zones+0x5c/0x190
  [<00000000008cf9c4>] dev_attr_show+0x34/0x70
  [<0000000000463ad0>] sysfs_kf_seq_show+0xc8/0x148
  [<00000000003e4194>] seq_read+0x204/0x480
  [<00000000003b53ea>] __vfs_read+0x32/0x178
  [<00000000003b55b2>] vfs_read+0x82/0x138
  [<00000000003b5be2>] ksys_read+0x5a/0xb0
  [<0000000000b86ba0>] system_call+0xdc/0x2d8
 Last Breaking-Event-Address:
  [<0000000000385b26>] test_pages_in_a_zone+0xde/0x160
 Kernel panic - not syncing: Fatal exception: panic_on_oops

 CONFIG_DEBUG_VM_PGFLAGS=y
 kernel parameter mem=3075M
 --------------------------
 page:000003d08300c000 is uninitialized and poisoned
 page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
 Call Trace:
 ([<000000000038596c>] is_mem_section_removable+0xb4/0x190)
  [<00000000008f12fa>] show_mem_removable+0x9a/0xd8
  [<00000000008cf9c4>] dev_attr_show+0x34/0x70
  [<0000000000463ad0>] sysfs_kf_seq_show+0xc8/0x148
  [<00000000003e4194>] seq_read+0x204/0x480
  [<00000000003b53ea>] __vfs_read+0x32/0x178
  [<00000000003b55b2>] vfs_read+0x82/0x138
  [<00000000003b5be2>] ksys_read+0x5a/0xb0
  [<0000000000b86ba0>] system_call+0xdc/0x2d8
 Last Breaking-Event-Address:
  [<000000000038596c>] is_mem_section_removable+0xb4/0x190
 Kernel panic - not syncing: Fatal exception: panic_on_oops

This fix checks if the page lies within the zone boundaries before
accessing the struct page data. The check is added to both functions.

Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: <stable@vger.kernel.org>
---
 mm/memory_hotplug.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 38d94b703e9d..8402e70f74c2 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1229,9 +1229,8 @@ static struct page *next_active_pageblock(struct page *page)
 	return page + pageblock_nr_pages;
 }
 
-static bool is_pageblock_removable_nolock(struct page *page)
+static bool is_pageblock_removable_nolock(struct page *page, struct zone **zone)
 {
-	struct zone *zone;
 	unsigned long pfn;
 
 	/*
@@ -1241,15 +1240,14 @@ static bool is_pageblock_removable_nolock(struct page *page)
 	 * We have to take care about the node as well. If the node is offline
 	 * its NODE_DATA will be NULL - see page_zone.
 	 */
-	if (!node_online(page_to_nid(page)))
-		return false;
-
-	zone = page_zone(page);
 	pfn = page_to_pfn(page);
-	if (!zone_spans_pfn(zone, pfn))
+	if (*zone && !zone_spans_pfn(*zone, pfn))
 		return false;
+	if (!node_online(page_to_nid(page)))
+		return false;
+	*zone = page_zone(page);
 
-	return !has_unmovable_pages(zone, page, 0, MIGRATE_MOVABLE, true);
+	return !has_unmovable_pages(*zone, page, 0, MIGRATE_MOVABLE, true);
 }
 
 /* Checks if this range of memory is likely to be hot-removable. */
@@ -1257,10 +1255,11 @@ bool is_mem_section_removable(unsigned long start_pfn, unsigned long nr_pages)
 {
 	struct page *page = pfn_to_page(start_pfn);
 	struct page *end_page = page + nr_pages;
+	struct zone *zone = NULL;
 
 	/* Check the starting page of each pageblock within the range */
 	for (; page < end_page; page = next_active_pageblock(page)) {
-		if (!is_pageblock_removable_nolock(page))
+		if (!is_pageblock_removable_nolock(page, &zone))
 			return false;
 		cond_resched();
 	}
@@ -1296,6 +1295,9 @@ int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn,
 				i++;
 			if (i == MAX_ORDER_NR_PAGES || pfn + i >= end_pfn)
 				continue;
+			/* Check if we got outside of the zone */
+			if (zone && !zone_spans_pfn(zone, pfn + i))
+				return 0;
 			page = pfn_to_page(pfn + i);
 			if (zone && page_zone(page) != zone)
 				return 0;
-- 
2.16.4