[PATCH 5.15] mm/filemap: fix UAF in find_lock_entries

[Liu Shixin <liushixin2@xxxxxxxxxx>]

Release refcount after xas_set to fix UAF which may cause panic like this:

 page:ffffea000491fa40 refcount:1 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x1247e9
 head:ffffea000491fa00 order:3 compound_mapcount:0 compound_pincount:0
 memcg:ffff888104f91091
 flags: 0x2fffff80010200(slab|head|node=0|zone=2|lastcpupid=0x1fffff)
...
page dumped because: VM_BUG_ON_PAGE(PageTail(page))
 ------------[ cut here ]------------
 kernel BUG at include/linux/page-flags.h:632!
 invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN
 CPU: 1 PID: 7642 Comm: sh Not tainted 5.15.51-dirty #26
...
 Call Trace:
  <TASK>
  __invalidate_mapping_pages+0xe7/0x540
  drop_pagecache_sb+0x159/0x320
  iterate_supers+0x120/0x240
  drop_caches_sysctl_handler+0xaa/0xe0
  proc_sys_call_handler+0x2b4/0x480
  new_sync_write+0x3d6/0x5c0
  vfs_write+0x446/0x7a0
  ksys_write+0x105/0x210
  do_syscall_64+0x35/0x80
  entry_SYSCALL_64_after_hwframe+0x44/0xae
 RIP: 0033:0x7f52b5733130
...

This problem has been fixed on mainline by patch 6b24ca4a1a8d ("mm: Use
multi-index entries in the page cache") since it deletes the related code.

Fixes: 5c211ba29deb ("mm: add and use find_lock_entries")
Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
---
 mm/filemap.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 00e391e75880..24b5d7ebdc29 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2087,16 +2087,18 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
 {
 	XA_STATE(xas, &mapping->i_pages, start);
 	struct page *page;
+	bool putpage;
 
 	rcu_read_lock();
 	while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
+		putpage = true;
 		if (!xa_is_value(page)) {
 			if (page->index < start)
-				goto put;
+				goto next;
 			if (page->index + thp_nr_pages(page) - 1 > end)
-				goto put;
+				goto next;
 			if (!trylock_page(page))
-				goto put;
+				goto next;
 			if (page->mapping != mapping || PageWriteback(page))
 				goto unlock;
 			VM_BUG_ON_PAGE(!thp_contains(page, xas.xa_index),
@@ -2105,20 +2107,24 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
 		indices[pvec->nr] = xas.xa_index;
 		if (!pagevec_add(pvec, page))
 			break;
+		putpage = false;
 		goto next;
 unlock:
 		unlock_page(page);
-put:
-		put_page(page);
 next:
 		if (!xa_is_value(page) && PageTransHuge(page)) {
 			unsigned int nr_pages = thp_nr_pages(page);
 
 			/* Final THP may cross MAX_LFS_FILESIZE on 32-bit */
 			xas_set(&xas, page->index + nr_pages);
-			if (xas.xa_index < nr_pages)
+			if (xas.xa_index < nr_pages) {
+				if (putpage)
+					put_page(page);
 				break;
+			}
 		}
+		if (putpage)
+			put_page(page);
 	}
 	rcu_read_unlock();
 
-- 
2.25.1