CVE-2023-54021

Vulnerability

In the Linux kernel's ext4 filesystem, the function ext4_mb_normalize_request was mis-setting ac_f_ex instead of ac_g_ex [1]. This caused the block-allocation goal start to be improperly initialized, and additionally a missing size check against ar->pright could lead to an underflow: ar->pright - size would become a very large value, which when passed to ext4_get_group_no_and_offset() triggers a BUG_ON [2].

Exploitation

An attacker who can mount or interact with a specially corrupted ext4 filesystem, or trigger specific write allocation patterns that reach the flawed code path, could cause a denial of service (crash) of the kernel [1][2]. No special privileges are required beyond the ability to cause ext4 block allocation (e.g., by writing files).

Impact

The primary impact is a kernel panic (BUG_ON), which results in a denial of service for the system. There is no indication of privilege escalation or data corruption beyond the crash itself.

Mitigation

The fix was applied to the Linux kernel stable branches. Users should update to a kernel version containing the commit that corrects the goal start and adds the size check [1][2].