CVE-2021-46982

In the Linux kernel, the following vulnerability has been resolved: f2fs:
compress: fix race condition of overwrite vs truncate pos_fsstress testcase
complains a panic as belew: ------------[ cut here ]------------ kernel BUG
at fs/f2fs/compress.c:1082! invalid opcode: 0000 [#1] SMP PTI CPU: 4 PID:
2753477 Comm: kworker/u16:2 Tainted: G OE 5.12.0-rc1-custom #1 Hardware
name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
Workqueue: writeback wb_workfn (flush-252:16) RIP:
0010:prepare_compress_overwrite+0x4c0/0x760 [f2fs] Call Trace:
f2fs_prepare_compress_overwrite+0x5f/0x80 [f2fs]
f2fs_write_cache_pages+0x468/0x8a0 [f2fs] f2fs_write_data_pages+0x2a4/0x2f0
[f2fs] do_writepages+0x38/0xc0 __writeback_single_inode+0x44/0x2a0
writeback_sb_inodes+0x223/0x4d0 __writeback_inodes_wb+0x56/0xf0
wb_writeback+0x1dd/0x290 wb_workfn+0x309/0x500 process_one_work+0x220/0x3c0
worker_thread+0x53/0x420 kthread+0x12f/0x150 ret_from_fork+0x22/0x30 The
root cause is truncate() may race with overwrite as below, so that one
reference count left in page can not guarantee the page attaching in
mapping tree all the time, after truncation, later find_lock_page() may
return NULL pointer. - prepare_compress_overwrite - f2fs_pagecache_get_page

• unlock_page - f2fs_setattr - truncate_setsize - truncate_inode_page -
  delete_from_page_cache - find_lock_page Fix this by avoiding referencing
  updated page.