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 wouldnt 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 Software

References

• https://git.kernel.org/stable/c/488ffc0cac38f203979f83634236ee53251ce593
• https://git.kernel.org/stable/c/5ac295dfccb5b015493f86694fa13a0dde4d3665
• https://git.kernel.org/stable/c/e3e89178a9f4a80092578af3ff3c8478f9187d59
• https://git.kernel.org/stable/c/e686349cc19e800dac8971929089ba5ff59abfb0
• https://git.kernel.org/stable/c/ec52240622c4d218d0240079b7c1d3ec2328a9f4