CVE-2026-45892

Summary

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

ext4: drop extent cache after doing PARTIAL_VALID1 zeroout

When splitting an unwritten extent in the middle and converting it to initialized in ext4_split_extent() with the EXT4_EXT_MAY_ZEROOUT and EXT4_EXT_DATA_VALID2 flags set, it could leave a stale unwritten extent.

Assume we have an unwritten file and buffered write in the middle of it without dioread_nolock enabled, it will allocate blocks as written extent.

   0  A      B  N
   [UUUUUUUUUUUU] on-disk extent      U: unwritten extent
   [UUUUUUUUUUUU] extent status tree
   [--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_PARTIAL_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 leave the entire extent as unwritten.

   0  A      B  N
   [UUUUUUUUUUUU] on-disk extent
   [UUUUUUUUUUUU] extent status tree
   [--DDDDDDDDZZ]                     Z: zeroed 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 leave an written extent from A to N.

   0  A      B  N
   [UUWWWWWWWWWW] on-disk extent      W: written extent
   [UUUUUUUUUUUU] extent status tree
   [--DDDDDDDDZZ]

Finally ext4_map_create_blocks() only insert extent A to B to the extent status tree, and leave an stale unwritten extent in the status tree.

   0  A      B  N
   [UUWWWWWWWWWW] on-disk extent      W: written extent
   [UUWWWWWWWWUU] extent status tree
   [--DDDDDDDDZZ]

Fix this issue by always cached extent status entry after zeroing out the second part.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/28db4bfc6f82fd20e2aadb7fc162244109a4eb31
• https://git.kernel.org/stable/c/f0931a5c17005a0c4fc35bd1a001245effc3354b
• https://git.kernel.org/stable/c/d8ee559fccdef713f058cfe5f2c03dc9b18be3b1
• https://git.kernel.org/stable/c/c2ee51d684adca7645e4aa74adca13f6750390bc
• https://git.kernel.org/stable/c/a1b962a821e7a52d48212ae269b45808b4411267
• https://git.kernel.org/stable/c/6d882ea3b0931b43530d44149b79fcd4ffc13030

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