CVE-2021-46989

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

hfsplus: prevent corruption in shrinking truncate

I believe there are some issues introduced by commit 31651c607151

("hfsplus: avoid deadlock on file truncation")

HFS+ has extent records which always contains 8 extents. In case the

first extent record in catalog file gets full, new ones are allocated from

extents overflow file.

In case shrinking truncate happens to middle of an extent record which

locates in extents overflow file, the logic in hfsplus_file_truncate() was

changed so that call to hfs_brec_remove() is not guarded any more.

Right action would be just freeing the extents that exceed the new size

inside extent record by calling hfsplus_free_extents(), and then check if

the whole extent record should be removed. However since the guard

(blk_cnt > start) is now after the call to hfs_brec_remove(), this has

unfortunate effect that the last matching extent record is removed

unconditionally.

To reproduce this issue, create a file which has at least 10 extents, and

then perform shrinking truncate into middle of the last extent record, so

that the number of remaining extents is not under or divisible by 8. This

causes the last extent record (8 extents) to be removed totally instead of

truncating into middle of it. Thus this causes corruption, and lost data.

Fix for this is simply checking if the new truncated end is below the

start of this extent record, making it safe to remove the full extent

record. However call to hfs_brec_remove() can't be moved to it's previous

place since we're dropping ->tree_lock and it can cause a race condition

and the cached info being invalidated possibly corrupting the node data.

Another issue is related to this one. When entering into the block

(blk_cnt > start) we are not holding the ->tree_lock. We break out from

the loop not holding the lock, but hfs_find_exit() does unlock it. Not

sure if it's possible for someone else to take the lock under our feet,

but it can cause hard to debug errors and premature unlocking. Even if

there's no real risk of it, the locking should still always be kept in

balance. Thus taking the lock now just before the check.