CVE-2025-21991

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. ]