CVE-2021-46989

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.

Gravité et risque d'exploitation

CVE-2021-46989 obtient un score CVSS de 7.1/10, soit une gravité élevée. Selon le modèle EPSS, la probabilité qu'elle soit exploitée dans les 30 prochains jours est de 0.24 %, ce qui la place devant 14.4 % des CVE référencées.

Que faire ?

• Vérifiez si Linux (versions <=5.11.*, 5.11.22, <adbd8a2a8cc05d9e501f93e5c95c59307874cc99, 0, 5.10.38, 31651c607151f1034cfb57e5a78678bea54c362b, <97314e45aa1223a42d60256a62c5d9af54baf446, 4.19, <=5.10.*, <52dde855663e5db824af51db39b5757d2ef3e28a, 5.4.120, <=5.4.*, 5.13, <=4.19.*, <=5.12.*, <c451a6bafb5f422197d31536f82116aed132b72c, <4.19, 4.19.191, <=*, <c477f62db1a0c0ecaa60a29713006ceeeb04b685, 5.12.5, <c3187cf32216313fb316084efac4dab3a8459b1d) est présent sur votre parc.
• Appliquez le correctif ou la version corrigée indiqués dans les références de l'éditeur.
• À défaut de correctif, limitez l'exposition (filtrage réseau, désactivation du composant).
• Automatisez la détection : les agents CVE Heimdall (gratuits, open source) corrèlent l'inventaire de vos serveurs avec la base CVE.

Références

• 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