CVE-2026-43067

Summary

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

ext4: handle wraparound when searching for blocks for indirect mapped blocks

Commit 4865c768b563 ("ext4: always allocate blocks only from groups inode can use") restricts what blocks will be allocated for indirect block based files to block numbers that fit within 32-bit block numbers.

However, when using a review bot running on the latest Gemini LLM to check this commit when backporting into an LTS based kernel, it raised this concern:

If ac->ac_g_ex.fe_group is >= ngroups (for instance, if the goal group was populated via stream allocation from s_mb_last_groups), then start will be >= ngroups.

Does this allow allocating blocks beyond the 32-bit limit for indirect block mapped files? The commit message mentions that ext4_mb_scan_groups_linear() takes care to not select unsupported groups. However, its loop uses group = *start, and the very first iteration will call ext4_mb_scan_group() with this unsupported group because next_linear_group() is only called at the end of the iteration.

After reviewing the code paths involved and considering the LLM review, I determined that this can happen when there is a file system where some files/directories are extent-mapped and others are indirect-block mapped. To address this, add a safety clamp in ext4_mb_scan_groups().

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/f89bba144938921a2249237ad04a0183ff3f8930
• https://git.kernel.org/stable/c/83170a05908b6cf2fb3235d3065bf613ff866f3c
• https://git.kernel.org/stable/c/4bec4a498ce86314d470ae6144120461f2138c29
• https://git.kernel.org/stable/c/12624c5b724a81e14e532972b40d863b0de3b7d1
• https://git.kernel.org/stable/c/2a368ccddfc492a0aa951e2caef2985f20e96503
• https://git.kernel.org/stable/c/bb81702370fad22c06ca12b6e1648754dbc37e0f

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