Subject: CVE-2021-47492: mm, thp: bail out early in collapse_file for writeback page

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

mm, thp: bail out early in collapse_file for writeback page

Currently collapse_file does not explicitly check PG_writeback, instead,
page_has_private and try_to_release_page are used to filter writeback
pages.  This does not work for xfs with blocksize equal to or larger
than pagesize, because in such case xfs has no page->private.

This makes collapse_file bail out early for writeback page.  Otherwise,
xfs end_page_writeback will panic as follows.

  page:fffffe00201bcc80 refcount:0 mapcount:0 mapping:ffff0003f88c86a8 index:0x0 pfn:0x84ef32
  aops:xfs_address_space_operations [xfs] ino:30000b7 dentry name:"libtest.so"
  flags: 0x57fffe0000008027(locked|referenced|uptodate|active|writeback)
  raw: 57fffe0000008027 ffff80001b48bc28 ffff80001b48bc28 ffff0003f88c86a8
  raw: 0000000000000000 0000000000000000 00000000ffffffff ffff0000c3e9a000
  page dumped because: VM_BUG_ON_PAGE(((unsigned int) page_ref_count(page) + 127u <= 127u))
  page->mem_cgroup:ffff0000c3e9a000
  ------------[ cut here ]------------
  kernel BUG at include/linux/mm.h:1212!
  Internal error: Oops - BUG: 0 [#1] SMP
  Modules linked in:
  BUG: Bad page state in process khugepaged  pfn:84ef32
   xfs(E)
  page:fffffe00201bcc80 refcount:0 mapcount:0 mapping:0 index:0x0 pfn:0x84ef32
   libcrc32c(E) rfkill(E) aes_ce_blk(E) crypto_simd(E) ...
  CPU: 25 PID: 0 Comm: swapper/25 Kdump: loaded Tainted: ...
  pstate: 60400005 (nZCv daif +PAN -UAO -TCO BTYPE=--)
  Call trace:
    end_page_writeback+0x1c0/0x214
    iomap_finish_page_writeback+0x13c/0x204
    iomap_finish_ioend+0xe8/0x19c
    iomap_writepage_end_bio+0x38/0x50
    bio_endio+0x168/0x1ec
    blk_update_request+0x278/0x3f0
    blk_mq_end_request+0x34/0x15c
    virtblk_request_done+0x38/0x74 [virtio_blk]
    blk_done_softirq+0xc4/0x110
    __do_softirq+0x128/0x38c
    __irq_exit_rcu+0x118/0x150
    irq_exit+0x1c/0x30
    __handle_domain_irq+0x8c/0xf0
    gic_handle_irq+0x84/0x108
    el1_irq+0xcc/0x180
    arch_cpu_idle+0x18/0x40
    default_idle_call+0x4c/0x1a0
    cpuidle_idle_call+0x168/0x1e0
    do_idle+0xb4/0x104
    cpu_startup_entry+0x30/0x9c
    secondary_start_kernel+0x104/0x180
  Code: d4210000 b0006161 910c8021 94013f4d (d4210000)
  ---[ end trace 4a88c6a074082f8c ]---
  Kernel panic - not syncing: Oops - BUG: Fatal exception in interrupt

The Linux kernel CVE team has assigned CVE-2021-47492 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.4 with commit 99cb0dbd47a1 and fixed in 5.10.77 with commit 69a7fa5cb0de
	Issue introduced in 5.4 with commit 99cb0dbd47a1 and fixed in 5.14.16 with commit 5e669d8ab30a
	Issue introduced in 5.4 with commit 99cb0dbd47a1 and fixed in 5.15 with commit 74c42e1baacf

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-47492
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	mm/khugepaged.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/69a7fa5cb0de06c8956b040f19a7248c8c8308ca
	https://git.kernel.org/stable/c/5e669d8ab30ab61dec3c36e27b4711f07611e6fc
	https://git.kernel.org/stable/c/74c42e1baacf206338b1dd6b6199ac964512b5bb