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 ext4_split_extent_at() 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 ext4_find_extent() and
ext4_cache_extents() 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, ext4_cache_extents() 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.