This patch has a compilation problem, please ignore it.

On 2022/7/6 15:30, Liu Shixin wrote:

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@huawei.com

---

mm/filemap.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c index 00e391e75880..2c65dd314c49 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2090,7 +2090,11 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start, rcu_read_lock(); while ((page = find_get_entry(&xas, end, XA_PRESENT))) {

• unsigned long next_idx = xas.xa_index;
  

• if (!xa_is_value(page)) {

• 	if (PageTransHuge(page))
  

• 		next_idx = page->index + thp_nr_pages(page);
  if (page->index < start)
  	goto put;
  if (page->index + thp_nr_pages(page) - 1 > end)
  

@@ -2111,11 +2115,9 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start, put: put_page(page); next:

• if (!xa_is_value(page) && PageTransHuge(page)) {
  

• 	unsigned int nr_pages = thp_nr_pages(page);
  

• if (next_idx != xas.xa_index) {
  /* Final THP may cross MAX_LFS_FILESIZE on 32-bit */
  

• 	xas_set(&xas, page->index + nr_pages);
  

• 	xas_set(&xas, next_idx);
  if (xas.xa_index < nr_pages)
  	break;
  

  }