CVE-2026-45858

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

ext4: dont zero the entire extent if EXT4_EXT_DATA_PARTIAL_VALID1

When allocating initialized blocks from a large unwritten extent, or when splitting an unwritten extent during end I/O and converting it to initialized, there is currently a potential issue of stale data if the extent needs to be split in the middle.

   0  A      B  N
   [UUUUUUUUUUUU]    U: unwritten extent
   [--DDDDDDDD--]    D: valid data
      |<-  ->| ----> this range needs to be initialized

ext4_split_extent() first try to split this extent at B with EXT4_EXT_DATA_ENTIRE_VALID1 and EXT4_EXT_MAY_ZEROOUT flag set, but ext4_split_extent_at() failed to split this extent due to temporary lack of space. It zeroout B to N and mark the entire extent from 0 to N as written.

   0  A      B  N
   [WWWWWWWWWWWW]    W: written extent
   [SSDDDDDDDDZZ]    Z: zeroed, S: stale data

ext4_split_extent() then try to split this extent at A with EXT4_EXT_DATA_VALID2 flag set. This time, it split successfully and left a stale written extent from 0 to A.

   0  A      B   N
   [WW|WWWWWWWWWW]
   [SS|DDDDDDDDZZ]

Fix this by pass EXT4_EXT_DATA_PARTIAL_VALID1 to ext4_split_extent_at() when splitting at B, dont convert the entire extent to written and left it as unwritten after zeroing out B to N. The remaining work is just like the standard two-part split. ext4_split_extent() will pass the EXT4_EXT_DATA_VALID2 flag when it calls ext4_split_extent_at() for the second time, allowing it to properly handle the split. If the split is successful, it will keep extent from 0 to A as unwritten.

Affected Software

References

• https://git.kernel.org/stable/c/1bf6974822d1dba86cf11b5f05498581cf3488a2
• https://git.kernel.org/stable/c/58ddae5d77b1db3a27b891c75a8fa120239ac092
• https://git.kernel.org/stable/c/7015fcf473796e1d2d876f241bd9e0c36f3d4eef
• https://git.kernel.org/stable/c/d17857b4fb9ba5745b59be0ef38fd532991fccbf
• https://git.kernel.org/stable/c/d67c8ecf3d8fda9b8ef80e6f665d84b6d6ac9d88