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

jira LE-3526
cve CVE-2025-21991
Rebuild_History Non-Buildable kernel-6.12.0-55.20.1.el10_0
commit-author Florent Revest <revest@chromium.org>
commit e3e8917

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. ]

Fixes: 7ff6edf ("x86/microcode/AMD: Fix mixed steppings support")
	Signed-off-by: Florent Revest <revest@chromium.org>
	Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
	Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250310144243.861978-1-revest@chromium.org
(cherry picked from commit e3e8917)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

arch/x86/kernel/cpu/microcode/amd.c

@@ -979,7 +979,7 @@ static enum ucode_state load_microcode_amd(u8 family, const u8 *data, size_t siz
 	if (ret != UCODE_OK)
 		return ret;
 
-	for_each_node(nid) {
+	for_each_node_with_cpus(nid) {
 		cpu = cpumask_first(cpumask_of_node(nid));
 		c = &cpu_data(cpu);