Vulnerability CVE-2021-46989
Published: 2024-02-28

Description:
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.

 References:
https://git.kernel.org/stable/c/52dde855663e5db824af51db39b5757d2ef3e28a
https://git.kernel.org/stable/c/c451a6bafb5f422197d31536f82116aed132b72c
https://git.kernel.org/stable/c/adbd8a2a8cc05d9e501f93e5c95c59307874cc99
https://git.kernel.org/stable/c/c477f62db1a0c0ecaa60a29713006ceeeb04b685
https://git.kernel.org/stable/c/97314e45aa1223a42d60256a62c5d9af54baf446
https://git.kernel.org/stable/c/c3187cf32216313fb316084efac4dab3a8459b1d
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