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CVE-2026-93798: btrfs: fix reloc root cleanup in merge_reloc_roots()

• Severity: high (CVSS 7.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.1% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-24 · updated 2026-10-03

Description

In the Linux kernel, the following vulnerability has been resolved: btrfs: fix reloc root cleanup in merge_reloc_roots() If the root we got has zero root refs in its root item, we are resetting the root's ->reloc_root without using barriers like we do everywhere else. Sashiko complained about this while reviewing another patch, and it's correct (see the Link tag below). Also, we should not clear BTRFS_ROOT_DEAD_RELOC_TREE from the root unless the root points to the reloc root we have. Fix this by using clear_reloc_root(), which issues the memory barrier after setting the root's ->reloc_root to NULL and before clearing the bit BTRFS_ROOT_DEAD_RELOC_TREE from the root.

Affected products

• Linux kernel: from 5.4.54 before 5.5; from 5.7.11 before 5.8; 5.8 — fixed in 5.5, 5.8, 7.2

How to fix

Upgrade to a fixed version: Linux kernel 5.5, 5.8, 7.2. Check the vendor advisory for mitigations.

References

• https://git.kernel.org/stable/c/0bc25f0a2d1b3e0a691431c4d7aef36afcd527c5
• https://git.kernel.org/stable/c/29531470b1bf1e8bd2df4941fe913106edbca615
• https://git.kernel.org/stable/c/415f8665a969d3c906e4fd7c90fcbb4b357886f6
• https://git.kernel.org/stable/c/63d6b1f04cd91c825329712aa526215ffa5d11ca
• https://git.kernel.org/stable/c/ab48583aa9948205ff9a6470e23dbff74c565e24
• https://git.kernel.org/stable/c/b78fe9563e2d5ae47805f1e5dc722c91fd30e1f8
• https://git.kernel.org/stable/c/ee695a968aa73b1e67ccbf2133dc0e54381d32b2

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