The Zephyr ext2 file system validates the on-disk superblock in ext2verifydisksuperblock() (subsys/fs/ext2/ext2impl.c) before completing a mount. The validator checked the magic number, block size, revision and feature flags, but did not verify that the on-disk fields sblockspergroup and sinodespergroup are non-zero. Both fields are read directly from the image and are later used as divisors during mount-time initialization.

During mount, getngroups() divides and modulos sblockscount by sblockspergroup (reached via ext2fetchblockgroup() from ext2initfs()), and getitableentry() divides (ino - 1) by sinodespergroup when fetching the root inode (both in subsys/fs/ext2/ext2diskops.c). A superblock with either field set to zero therefore causes an integer division by zero during the mount sequence.

An attacker who can present a crafted ext2 image to a device that mounts ext2 — removable media such as an SD card or a USB mass-storage device — can trigger this. On ARMv7-M / ARMv8-M-mainline Cortex-M targets, divide-by-zero trapping is enabled (SCBCCRDIV0TRP), so the division raises a UsageFault that Zephyr treats as a fatal error, producing a denial of service. The impact is limited to availability; the malformed value is consumed only as a divisor.

The fix rejects a zero sblockspergroup or sinodespergroup in the superblock validator, returning -EINVAL so the mount fails before any block-group or inode I/O occurs.