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CVE-2025-21991 Affecting kernel-rt package, versions *

Severity

 Recommended
0.0
medium
0
10

Based on Red Hat Enterprise Linux security rating.

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  • Snyk IDSNYK-RHEL7-KERNELRT-9641219
  • published3 Apr 2025
  • disclosed2 Apr 2025
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Introduced: 2 Apr 2025

NewCVE-2025-21991  (opens in a new tab)

How to fix?

There is no fixed version for RHEL:7 kernel-rt.

NVD Description

Note: Versions mentioned in the description apply only to the upstream kernel-rt package and not the kernel-rt package as distributed by RHEL. See How to fix? for RHEL:7 relevant fixed versions and status.

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. ]

CVSS Base Scores

version 3.1