CVE-2025-21991x86/microcode/AMD: Fix out-of-bounds on systems with CPU-less NUMA nodes

Official description

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. ]

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/d509c4731090ebd9bbdb72c70a2d70003ae81f4f
• https://git.kernel.org/stable/c/985a536e04bbfffb1770df43c6470f635a6b1073
• https://git.kernel.org/stable/c/18b5d857c6496b78ead2fd10001b81ae32d30cac
• https://git.kernel.org/stable/c/ec52240622c4d218d0240079b7c1d3ec2328a9f4
• https://git.kernel.org/stable/c/e686349cc19e800dac8971929089ba5ff59abfb0
• https://git.kernel.org/stable/c/488ffc0cac38f203979f83634236ee53251ce593
• https://git.kernel.org/stable/c/5ac295dfccb5b015493f86694fa13a0dde4d3665
• https://git.kernel.org/stable/c/e3e89178a9f4a80092578af3ff3c8478f9187d59
• https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html
• https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html