Subject: CVE-2021-47189: btrfs: fix memory ordering between normal and ordered work functions

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

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

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

btrfs: fix memory ordering between normal and ordered work functions

Ordered work functions aren't guaranteed to be handled by the same thread
which executed the normal work functions. The only way execution between
normal/ordered functions is synchronized is via the WORK_DONE_BIT,
unfortunately the used bitops don't guarantee any ordering whatsoever.

This manifested as seemingly inexplicable crashes on ARM64, where
async_chunk::inode is seen as non-null in async_cow_submit which causes
submit_compressed_extents to be called and crash occurs because
async_chunk::inode suddenly became NULL. The call trace was similar to:

    pc : submit_compressed_extents+0x38/0x3d0
    lr : async_cow_submit+0x50/0xd0
    sp : ffff800015d4bc20

    <registers omitted for brevity>

    Call trace:
     submit_compressed_extents+0x38/0x3d0
     async_cow_submit+0x50/0xd0
     run_ordered_work+0xc8/0x280
     btrfs_work_helper+0x98/0x250
     process_one_work+0x1f0/0x4ac
     worker_thread+0x188/0x504
     kthread+0x110/0x114
     ret_from_fork+0x10/0x18

Fix this by adding respective barrier calls which ensure that all
accesses preceding setting of WORK_DONE_BIT are strictly ordered before
setting the flag. At the same time add a read barrier after reading of
WORK_DONE_BIT in run_ordered_work which ensures all subsequent loads
would be strictly ordered after reading the bit. This in turn ensures
are all accesses before WORK_DONE_BIT are going to be strictly ordered
before any access that can occur in ordered_func.

The Linux kernel CVE team has assigned CVE-2021-47189 to this issue.

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

	Issue introduced in 3.15 with commit 08a9ff326418 and fixed in 4.4.293 with commit bd660a20fea3
	Issue introduced in 3.15 with commit 08a9ff326418 and fixed in 4.9.291 with commit 637d652d351f
	Issue introduced in 3.15 with commit 08a9ff326418 and fixed in 4.14.256 with commit 804a9d239ae9
	Issue introduced in 3.15 with commit 08a9ff326418 and fixed in 4.19.218 with commit ed058d735a70
	Issue introduced in 3.15 with commit 08a9ff326418 and fixed in 5.4.162 with commit 670f6b3867c8
	Issue introduced in 3.15 with commit 08a9ff326418 and fixed in 5.10.82 with commit 6adbc07ebcaf
	Issue introduced in 3.15 with commit 08a9ff326418 and fixed in 5.15.5 with commit 47e6f9f69153
	Issue introduced in 3.15 with commit 08a9ff326418 and fixed in 5.16 with commit 45da9c1767ac

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-2021-47189
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:
	fs/btrfs/async-thread.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/bd660a20fea3ec60a49709ef5360f145ec0fe779
	https://git.kernel.org/stable/c/637d652d351fd4f263ef302dc52f3971d314e500
	https://git.kernel.org/stable/c/804a9d239ae9cbe88e861a7cd62319cc6ec7b136
	https://git.kernel.org/stable/c/ed058d735a70f4b063323f1a7bb33cda0f987513
	https://git.kernel.org/stable/c/670f6b3867c8f0f11e5097f353b164cecfec6179
	https://git.kernel.org/stable/c/6adbc07ebcaf8bead08b21687d49e0fc94400987
	https://git.kernel.org/stable/c/47e6f9f69153247109042010f3a77579e9dc61ff
	https://git.kernel.org/stable/c/45da9c1767ac31857df572f0a909fbe88fd5a7e9