CVE-2026-45920

Description

In the Linux kernel, the following vulnerability has been resolved: ext4: fix dirtyclusters double decrement on fs shutdown fstests test generic/388 occasionally reproduces a warning in ext4_put_super() associated with the dirty clusters count: WARNING: CPU: 7 PID: 76064 at fs/ext4/super.c:1324 ext4_put_super+0x48c/0x590 [ext4] Tracing the failure shows that the warning fires due to an s_dirtyclusters_counter value of -1. IOW, this appears to be a spurious decrement as opposed to some sort of leak. Further tracing of the dirty cluster count deltas and an LLM scan of the resulting output identified the cause as a double decrement in the error path between ext4_mb_mark_diskspace_used() and the caller ext4_mb_new_blocks(). First, note that generic/388 is a shutdown vs. fsstress test and so produces a random set of operations and shutdown injections. In the problematic case, the shutdown triggers an error return from the ext4_handle_dirty_metadata() call(s) made from ext4_mb_mark_context(). The changed value is non-zero at this point, so ext4_mb_mark_diskspace_used() does not exit after the error bubbles up from ext4_mb_mark_context(). Instead, the former decrements both cluster counters and returns the error up to ext4_mb_new_blocks(). The latter falls into the !ar->len out path which decrements the dirty clusters counter a second time, creating the inconsistency. To avoid this problem and simplify ownership of the cluster reservation in this codepath, lift the counter reduction to a single place in the caller. This makes it more clear that ext4_mb_new_blocks() is responsible for acquiring cluster reservation (via ext4_claim_free_clusters()) in the !delalloc case as well as releasing it, regardless of whether it ends up consumed or returned due to failure.

Predictions

Heuristic predictions, AS-IS, for prioritization only.

Mitigations

References

• https://git.kernel.org/stable/c/523d5a4df3c649fa305c89efb552ec62a1ce9d3d
• https://git.kernel.org/stable/c/ca408af08544d96769c93a3d81a7f63f61129e95
• https://git.kernel.org/stable/c/55576fa14771d33994c29a9ae960e07bb3f56c20
• https://git.kernel.org/stable/c/dbc4e10619ed87a50e637b96f2e574df36a7a769
• https://git.kernel.org/stable/c/61e372122b6d95aec940fdaea0a16f988f359897
• https://git.kernel.org/stable/c/3924aea2c33df3864929c1acd178bfc29d8f005f
• https://git.kernel.org/stable/c/81982a11406c5da6c6e2b188028e7056e16b7128
• https://git.kernel.org/stable/c/94a8cea54cd935c54fa2fba70354757c0fc245e3
• https://security-tracker.debian.org/tracker/CVE-2026-45920
• https://www.suse.com/security/cve/CVE-2026-45920.html