Description

In the Linux kernel, the following vulnerability has been resolved: erofs: handle overlapped pclusters out of crafted images properly syzbot reported a task hang issue due to a deadlock case where it is waiting for the folio lock of a cached folio that will be used for cache I/Os. After looking into the crafted fuzzed image, I found it's formed with several overlapped big pclusters as below: Ext: logical offset | length : physical offset | length 0: 0.. 16384 | 16384 : 151552.. 167936 | 16384 1: 16384.. 32768 | 16384 : 155648.. 172032 | 16384 2: 32768.. 49152 | 16384 : 537223168.. 537239552 | 16384 ... Here, extent 0/1 are physically overlapped although it's entirely _impossible_ for normal filesystem images generated by mkfs. First, managed folios containing compressed data will be marked as up-to-date and then unlocked immediately (unlike in-place folios) when compressed I/Os are complete. If physical blocks are not submitted in the incremental order, there should be separate BIOs to avoid dependency issues. However, the current code mis-arranges z_erofs_fill_bio_vec() and BIO submission which causes unexpected BIO waits. Second, managed folios will be connected to their own pclusters for efficient inter-queries. However, this is somewhat hard to implement easily if overlapped big pclusters exist. Again, these only appear in fuzzed images so let's simply fall back to temporary short-lived pages for correctness. Additionally, it justifies that referenced managed folios cannot be truncated for now and reverts part of commit 2080ca1ed3e4 ("erofs: tidy up `struct z_erofs_bvec`") for simplicity although it shouldn't be any difference.

Weaknesses 1

Affected Products 2

References 5

• git.kernel.org https://git.kernel.org/stable/c/1bf7e414cac303c9aec1be67872e19be8b64980c
• git.kernel.org https://git.kernel.org/stable/c/9cfa199bcbbbba31cbf97b2786f44f4464f3f29a
  Patch
• git.kernel.org https://git.kernel.org/stable/c/9e2f9d34dd12e6e5b244ec488bcebd0c2d566c50
  Patch
• git.kernel.org https://git.kernel.org/stable/c/b9b30af0e86ffb485301ecd83b9129c9dfb7ebf8
  Patch
• git.kernel.org https://git.kernel.org/stable/c/c1172e65aad4b115392ea4c6e61e56e5b9b69df4

Remediation

• git.kernel.org https://git.kernel.org/stable/c/9cfa199bcbbbba31cbf97b2786f44f4464f3f29a
  Patch
• git.kernel.org https://git.kernel.org/stable/c/9e2f9d34dd12e6e5b244ec488bcebd0c2d566c50
  Patch
• git.kernel.org https://git.kernel.org/stable/c/b9b30af0e86ffb485301ecd83b9129c9dfb7ebf8
  Patch