[PATCH 4/5] mm: hwpoison: fix thp split handling in memory_failure()

[Andrea Righi]

From: Naoya Horiguchi <n-horiguchi at ah.jp.nec.com>

When memory_failure() runs on a thp tail page after pmd is split, we
trigger the following VM_BUG_ON_PAGE():

   page:ffffd7cd819b0040 count:0 mapcount:0 mapping:         (null) index:0x1
   flags: 0x1fffc000400000(hwpoison)
   page dumped because: VM_BUG_ON_PAGE(!page_count(p))
   ------------[ cut here ]------------
   kernel BUG at /src/linux-dev/mm/memory-failure.c:1132!

memory_failure() passed refcount and page lock from tail page to head
page, which is not needed because we can pass any subpage to
split_huge_page().

Fixes: 61f5d698cc97 ("mm: re-enable THP")
Link: http://lkml.kernel.org/r/1477961577-7183-1-git-send-email-n-horiguchi@ah.jp.nec.com
Signed-off-by: Naoya Horiguchi <n-horiguchi at ah.jp.nec.com>
Cc: <stable at vger.kernel.org>	[4.5+]
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

CVE-2019-10124

(backported from commit c3901e722b2975666f42748340df798114742d6d)
Signed-off-by: Andrea Righi <andrea.righi at canonical.com>
---
 mm/memory-failure.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 3730845e349a..2d7355cf70bf 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1156,10 +1156,10 @@ int memory_failure(unsigned long pfn, int trapno, int flags)
 	}
 
 	if (!PageHuge(p) && PageTransHuge(hpage)) {
-		lock_page(hpage);
-		if (!PageAnon(hpage) || unlikely(split_huge_page(hpage))) {
-			unlock_page(hpage);
-			if (!PageAnon(hpage))
+		lock_page(p);
+		if (!PageAnon(p) || unlikely(split_huge_page(p))) {
+			unlock_page(p);
+			if (!PageAnon(p))
 				pr_err("MCE: %#lx: non anonymous thp\n", pfn);
 			else
 				pr_err("MCE: %#lx: thp split failed\n", pfn);
@@ -1168,7 +1168,7 @@ int memory_failure(unsigned long pfn, int trapno, int flags)
 			put_hwpoison_page(p);
 			return -EBUSY;
 		}
-		unlock_page(hpage);
+		unlock_page(p);
 		VM_BUG_ON_PAGE(!page_count(p), p);
 		hpage = compound_head(p);
 	}
-- 
2.19.1