CVE-2026-45858: ext4: don't zero the entire extent if EXT4_EXT_DATA_PARTIAL_VALID1

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-05-27 · updated 2026-06-25

Description

In the Linux kernel, the following vulnerability has been resolved: ext4: don't 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, don't 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 products

Linux kernel: from 3.2.33 before 3.3; from 3.4.16 before 3.5; from 3.6.4 before 3.7; from 3.7.1 before 6.6.130; from 6.7 before 6.12.75; from 6.13 before 6.18.14 — fixed in 3.3, 3.5, 3.7, 6.12.75, 6.18.14, 6.19.4

How to fix

Upgrade to a fixed version: Linux kernel 3.3, 3.5, 3.7, 6.12.75. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2026-45858 or the next vulnerability affects your stack