[PATCH v2] x86/microcode/AMD: Fix out-of-bounds on systems with CPU-less NUMA nodes

[Florent Revest <revest@chromium.org>]

Currently, load_microcode_amd() iterates over all NUMA nodes, retrieves
their CPU masks and unconditonally 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+0xdb/0x143
 __ubsan_handle_out_of_bounds+0xf5/0x120
 load_microcode_amd+0x58f/0x6b0
 request_microcode_amd+0x17c/0x250
 reload_store+0x174/0x2b0
 kernfs_fop_write_iter+0x227/0x2d0
 vfs_write+0x322/0x510
 ksys_write+0xb5/0x160
 do_syscall_64+0x6b/0xa0
 entry_SYSCALL_64_after_hwframe+0x67/0xd1

This changes the iteration to only loop on NUMA nodes which have CPUs
before attempting to update their microcodes.

Fixes: 7ff6edf4fef3 ("x86/microcode/AMD: Fix mixed steppings support")
Signed-off-by: Florent Revest <revest@chromium.org>
Cc: stable@vger.kernel.org
---
 arch/x86/kernel/cpu/microcode/amd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/microcode/amd.c b/arch/x86/kernel/cpu/microcode/amd.c
index 95ac1c6a84fbe..d1e74bfe130f8 100644
--- a/arch/x86/kernel/cpu/microcode/amd.c
+++ b/arch/x86/kernel/cpu/microcode/amd.c
@@ -1068,7 +1068,7 @@ 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);
 
-- 
2.49.0.rc0.332.g42c0ae87b1-goog