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 WORKDONEBIT, unfortunately the used bitops don't guarantee any ordering whatsoever.
This manifested as seemingly inexplicable crashes on ARM64, where asyncchunk::inode is seen as non-null in asynccowsubmit which causes submitcompressedextents to be called and crash occurs because asyncchunk::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 WORKDONEBIT are strictly ordered before setting the flag. At the same time add a read barrier after reading of WORKDONEBIT in runorderedwork which ensures all subsequent loads would be strictly ordered after reading the bit. This in turn ensures are all accesses before WORKDONEBIT are going to be strictly ordered before any access that can occur in ordered_func.