CVE-2021-47189

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.