[PATCH] Fix page corruption caused by racy check in __free_pages

[David Chen]

When we upgraded our kernel, we started seeing some page corruption like
the following consistently:

 BUG: Bad page state in process ganesha.nfsd  pfn:1304ca
 page:0000000022261c55 refcount:0 mapcount:-128 mapping:0000000000000000 index:0x0 pfn:0x1304ca
 flags: 0x17ffffc0000000()
 raw: 0017ffffc0000000 ffff8a513ffd4c98 ffffeee24b35ec08 0000000000000000
 raw: 0000000000000000 0000000000000001 00000000ffffff7f 0000000000000000
 page dumped because: nonzero mapcount
 CPU: 0 PID: 15567 Comm: ganesha.nfsd Kdump: loaded Tainted: P    B      O      5.10.158-1.nutanix.20221209.el7.x86_64 #1
 Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/05/2016
 Call Trace:
  dump_stack+0x74/0x96
  bad_page.cold+0x63/0x94
  check_new_page_bad+0x6d/0x80
  rmqueue+0x46e/0x970
  get_page_from_freelist+0xcb/0x3f0
  ? _cond_resched+0x19/0x40
  __alloc_pages_nodemask+0x164/0x300
  alloc_pages_current+0x87/0xf0
  skb_page_frag_refill+0x84/0x110
  ...

Sometimes, it would also show up as corruption in the free list pointer and
cause crashes.

After bisecting the issue, we found the issue started from e320d3012d25:

	if (put_page_testzero(page))
		free_the_page(page, order);
	else if (!PageHead(page))
		while (order-- > 0)
			free_the_page(page + (1 << order), order);

So the problem is the check PageHead is racy because at this point we
already dropped our reference to the page. So even if we came in with
compound page, the page can already be freed and PageHead can return
false and we will end up freeing all the tail pages causing double free.

Fixes: e320d3012d25 ("mm/page_alloc.c: fix freeing non-compound pages")
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Chunwei Chen <david.chen@xxxxxxxxxxx>
---
 mm/page_alloc.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0745aedebb37..3bb3484563ed 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5631,9 +5631,12 @@ EXPORT_SYMBOL(get_zeroed_page);
  */
 void __free_pages(struct page *page, unsigned int order)
 {
+	/* get PageHead before we drop reference */
+	int head = PageHead(page);
+
 	if (put_page_testzero(page))
 		free_the_page(page, order);
-	else if (!PageHead(page))
+	else if (!head)
 		while (order-- > 0)
 			free_the_page(page + (1 << order), order);
 }
-- 
2.22.3