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Linux KernelOperating system · Linux

CVE-2025-21991

HIGH · 7.8 CVSS v3.1 Published 2025-04-02
Fix available
A fix is available. Upgrade to 4.15 / 4.20 or later.
See remediation →
80/100
Remediation priority · High
Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
In the Linux kernel, the following vulnerability has been resolved: x86/microcode/AMD: Fix out-of-bounds on systems with CPU-less NUMA nodes Currently, load_microcode_amd() iterates over all NUMA nodes, retrieves their CPU masks and unconditionally accesses per-CPU data for the first CPU of each mask. According to Documentation/admin-guide/mm/numaperf.rst: "Some memory may share the same node as a CPU, and others are provided as memory only nodes." Therefore, some node CPU masks may be empty and wouldn't have a "first CPU". On a machine with far memory (and therefore CPU-less NUMA nodes): - cpumask_of_node(nid) is 0 - cpumask_first(0) is CONFIG_NR_CPUS - cpu_data(CONFIG_NR_CPUS) accesses the cpu_info per-CPU array at an index that is 1 out of bounds This does not have any security implications since flashing microcode is a privileged operation but I believe this has reliability implications by potentially corrupting memory while flashing a microcode update. When booting with CONFIG_UBSAN_BOUNDS=y on an AMD machine that flashes a microcode update. I get the following splat: UBSAN: array-index-out-of-bounds in arch/x86/kernel/cpu/microcode/amd.c:X:Y index 512 is out of range for type 'unsigned long[512]' [...] Call Trace: dump_stack __ubsan_handle_out_of_bounds load_microcode_amd request_microcode_amd reload_store kernfs_fop_write_iter vfs_write ksys_write do_syscall_64 entry_SYSCALL_64_after_hwframe Change the loop to go over only NUMA nodes which have CPUs before determining whether the first CPU on the respective node needs microcode update. [ bp: Massage commit message, fix typo. ]

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis

A detailed technical summary for this CVE is being prepared.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Linux KernelOperating system
Affected:>= 4.14.308, < 4.15>= 4.19.276, < 4.20>= 5.4.235, < 5.5>= 5.10.173, < 5.11>= 5.15.99, < 5.16>= 6.1.16, < 6.1.132>= 6.2.3, < 6.6.84>= 6.7, < 6.12.20>= 6.13, < 6.13.8= 6.14

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 4.15 / 4.20 / 5.5 or later
Fixed in 4.154.205.5
Vendor patch git.kernel.org →
Recommended fix High confidence

Kernel 4.15, 4.20, 5.5, or 5.11 (depending on which major version branch you are on)

  1. Identify the currently running kernel version using `uname -r`
  2. For systems running 4.14.x (>=4.14.308): upgrade to kernel 4.15 or later
  3. For systems running 4.19.x (>=4.19.276): upgrade to kernel 4.20 or later
  4. For systems running 5.4.x (>=5.4.235): upgrade to kernel 5.5 or later
  5. For systems running 5.10.x (>=5.10.173): upgrade to kernel 5.11 or later
  6. Alternatively, apply the vendor patch commit 18b5d857c6496b78ead2fd10001b81ae32d30cac to the affected kernel source and rebuild
  7. After upgrade/patch, verify the fix by booting with CONFIG_UBSAN_BOUNDS=y on an AMD system with CPU-less NUMA nodes and loading microcode
Caveat The fix is a targeted code change with no expected breaking changes; however, kernel upgrades may introduce other changes - review release notes for your distribution

Generated from the published advisory — verify against the referenced sources before acting.

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $1,950. Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

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