# CVE-2024-47736: erofs: handle overlapped pclusters out of crafted images properly

CVE-2024-47736 is fixed in linux on 1 platform, first by DEBIAN-CVE-2024-47736, released October 21, 2024. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2024-47736
- Priority: Routine
- CVSS: 5.5 (Moderate)
- EPSS: 0.0018
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Oct 21, 2024
- Weakness: Improper Locking
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2024-47736

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 6.1.170-1 | [DEBIAN-CVE-2024-47736](https://patchblox.com/security/data/fix/linux/debian-cve-2024-47736.md) | Oct 21, 2024 |
| Linux | linux Debian 13.5 (trixie) | 6.11.2-1 | [DEBIAN-CVE-2024-47736](https://patchblox.com/security/data/fix/linux/debian-cve-2024-47736.md) | Oct 21, 2024 |
| Linux | linux Debian 14 | 6.11.2-1 | [DEBIAN-CVE-2024-47736](https://patchblox.com/security/data/fix/linux/debian-cve-2024-47736.md) | Oct 21, 2024 |

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