[PATCH 2/2] mm, memory_hotplug: test_pages_in_a_zone do not pass the end of zone

[Michal Hocko]

From: Mikhail Zaslonko <zaslonko@xxxxxxxxxxxxx>

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 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

Fix this by checking whether the pfn to check is within the zone.

[mhocko@xxxxxxxx: separated this change from
http://lkml.kernel.org/r/20181105150401.97287-2-zaslonko@xxxxxxxxxxxxx]
Signed-off-by: Mikhail Zaslonko <zaslonko@xxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/memory_hotplug.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 07872789d778..7711d0e327b6 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1274,6 +1274,9 @@ 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;
-- 
2.20.1