CVE-2026-45858 ext4: don't zero the entire extent if EXT4_EXT_DATA_PARTIAL_VALID1
A flaw in the Linux kernel ext4 filesystem allows attackers to read stale data from disk when extent splitting fails due to space constraints.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-45858?
CVE-2026-45858 is a medium-severity vulnerability in Linux kernel, published on 27 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-45858?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-45858?
Apply the vendor's patch.
Is CVE-2026-45858 being exploited?
There is no known exploitation of CVE-2026-45858 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-88556, BELL-CVE-2026-45858, DEBIAN-CVE-2026-45858, GHSA-pqwg-4rrm-3fg7, ROOT-OS-DEBIAN-11-CVE-2026-45858, ROOT-OS-DEBIAN-12-CVE-2026-45858 and 3 more.