CVE-2026-45920

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.

Weakness

The product calls free() twice on the same memory address.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/3924aea2c33df3864929c1acd178bfc29d8f005f
• https://git.kernel.org/stable/c/523d5a4df3c649fa305c89efb552ec62a1ce9d3d
• https://git.kernel.org/stable/c/55576fa14771d33994c29a9ae960e07bb3f56c20
• https://git.kernel.org/stable/c/61e372122b6d95aec940fdaea0a16f988f359897
• https://git.kernel.org/stable/c/81982a11406c5da6c6e2b188028e7056e16b7128
• https://git.kernel.org/stable/c/94a8cea54cd935c54fa2fba70354757c0fc245e3
• https://git.kernel.org/stable/c/ca408af08544d96769c93a3d81a7f63f61129e95
• https://git.kernel.org/stable/c/dbc4e10619ed87a50e637b96f2e574df36a7a769