Subject: CVE-2024-46704: workqueue: Fix spruious data race in __flush_work()

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

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

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

workqueue: Fix spruious data race in __flush_work()

When flushing a work item for cancellation, __flush_work() knows that it
exclusively owns the work item through its PENDING bit. 134874e2eee9
("workqueue: Allow cancel_work_sync() and disable_work() from atomic
contexts on BH work items") added a read of @work->data to determine whether
to use busy wait for BH work items that are being canceled. While the read
is safe when @from_cancel, @work->data was read before testing @from_cancel
to simplify code structure:

	data = *work_data_bits(work);
	if (from_cancel &&
	    !WARN_ON_ONCE(data & WORK_STRUCT_PWQ) && (data & WORK_OFFQ_BH)) {

While the read data was never used if !@from_cancel, this could trigger
KCSAN data race detection spuriously:

  ==================================================================
  BUG: KCSAN: data-race in __flush_work / __flush_work

  write to 0xffff8881223aa3e8 of 8 bytes by task 3998 on cpu 0:
   instrument_write include/linux/instrumented.h:41 [inline]
   ___set_bit include/asm-generic/bitops/instrumented-non-atomic.h:28 [inline]
   insert_wq_barrier kernel/workqueue.c:3790 [inline]
   start_flush_work kernel/workqueue.c:4142 [inline]
   __flush_work+0x30b/0x570 kernel/workqueue.c:4178
   flush_work kernel/workqueue.c:4229 [inline]
   ...

  read to 0xffff8881223aa3e8 of 8 bytes by task 50 on cpu 1:
   __flush_work+0x42a/0x570 kernel/workqueue.c:4188
   flush_work kernel/workqueue.c:4229 [inline]
   flush_delayed_work+0x66/0x70 kernel/workqueue.c:4251
   ...

  value changed: 0x0000000000400000 -> 0xffff88810006c00d

Reorganize the code so that @from_cancel is tested before @work->data is
accessed. The only problem is triggering KCSAN detection spuriously. This
shouldn't need READ_ONCE() or other access qualifiers.

No functional changes.

The Linux kernel CVE team has assigned CVE-2024-46704 to this issue.

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

	Issue introduced in 6.10 with commit 134874e2eee9 and fixed in 6.10.7 with commit 91d09642127a
	Issue introduced in 6.10 with commit 134874e2eee9 and fixed in 6.11 with commit 8bc35475ef1a

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-2024-46704
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:
	kernel/workqueue.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/91d09642127a32fde231face2ff489af70eef316
	https://git.kernel.org/stable/c/8bc35475ef1a23b0e224f3242eb11c76cab0ea88