A vulnerability was found in dosfstools. In FAT12 two 12 bit entries are combined to a 24 bit value (three bytes). Therefore, when an even numbered FAT entry is set in FAT12, it must be be combined with the following entry. To prevent accessing beyond the end of the FAT array, it must be checked that the cluster is not the last one.

Previously, the check tested that the requested cluster was equal to fs->clusters - 1. However, fs->clusters is the number of data clusters not including the two reserved FAT entries at the start so the test triggered two clusters early.

If the third to last entry was written on a FAT12 filesystem with an odd number of clusters, the second to last entry would be corrupted. This corruption may also lead to invalid memory accesses when the corrupted entry becomes out of bounds and is used later.

External references:

https://github.com/dosfstools/dosfstools/issues/12
https://blog.fuzzing-project.org/44-dosfstools-fsck.vfat-Several-invalid-memory-accesses.html


Upstream fix:

https://github.com/dosfstools/dosfstools/commit/07908124838afcc99c577d1d3e84cef2dbd39cb7