🛡️ CVE-2024-46704 — kernel

workqueue: Fix spruious data race in __flush_work()

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.