CVE-2026-45912: ext4: don't cache extent during splitting extent

In the Linux kernel, the following vulnerability has been resolved:

ext4: don't cache extent during splitting extent

Caching extents during the splitting process is risky, as it may result in stale extents remaining in the status tree. Moreover, in most cases, the corresponding extent block entries are likely already cached before the split happens, making caching here not particularly useful.

Assume we have an unwritten extent, and then DIO writes the first half.

[UUUUUUUUUUUUUUUU] on-disk extent U: unwritten extent [UUUUUUUUUUUUUUUU] extent status tree |<- ->| ----> dio write this range

First, when ext4splitextentat() splits this extent, it truncates the existing extent and then inserts a new one. During this process, this extent status entry may be shrunk, and calls to ext4findextent() and ext4cacheextents() may occur, which could potentially insert the truncated range as a hole into the extent status tree. After the split is completed, this hole is not replaced with the correct status.

[UUUUUUU|UUUUUUUU] on-disk extent U: unwritten extent [UUUUUUU|HHHHHHHH] extent status tree H: hole

Then, the outer calling functions will not correct this remaining hole extent either. Finally, if we perform a delayed buffer write on this latter part, it will re-insert the delayed extent and cause an error in space accounting.

In adition, if the unwritten extent cache is not shrunk during the splitting, ext4cacheextents() also conflicts with existing extents when caching extents. In the future, we will add checks when caching extents, which will trigger a warning. Therefore, Do not cache extents that are being split.