UBUNTU-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. ]
02 / AFFECTED SOFTWARE
Affected packages
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03 / CONNECTIONS
Connected vulnerabilities
04 / EVIDENCE
Source records
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. ]
05 / REFERENCES
Further evidence
- https://git.kernel.org/linus/e3e89178a9f4a80092578af3ff3c8478f9187d59
- 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
- https://ubuntu.com/security/CVE-2025-21991
- https://ubuntu.com/security/notices/USN-7585-1
- https://ubuntu.com/security/notices/USN-7585-2
- https://ubuntu.com/security/notices/USN-7585-3
- https://ubuntu.com/security/notices/USN-7585-4
- https://ubuntu.com/security/notices/USN-7585-5
- https://ubuntu.com/security/notices/USN-7585-6
- https://ubuntu.com/security/notices/USN-7585-7
- https://ubuntu.com/security/notices/USN-7591-1
- https://ubuntu.com/security/notices/USN-7591-2
- https://ubuntu.com/security/notices/USN-7591-3
- https://ubuntu.com/security/notices/USN-7591-4
- https://ubuntu.com/security/notices/USN-7591-5
- https://ubuntu.com/security/notices/USN-7591-6
- https://ubuntu.com/security/notices/USN-7592-1
- https://ubuntu.com/security/notices/USN-7593-1
- https://ubuntu.com/security/notices/USN-7597-1
- https://ubuntu.com/security/notices/USN-7597-2
- https://ubuntu.com/security/notices/USN-7598-1
- https://ubuntu.com/security/notices/USN-7602-1
- https://ubuntu.com/security/notices/USN-7605-1
- https://ubuntu.com/security/notices/USN-7605-2
- https://ubuntu.com/security/notices/USN-7606-1
- https://ubuntu.com/security/notices/USN-7628-1
- https://ubuntu.com/security/notices/USN-7640-1
- https://ubuntu.com/security/notices/USN-7655-1
- https://ubuntu.com/security/notices/USN-7764-1
- https://ubuntu.com/security/notices/USN-7764-2
- https://ubuntu.com/security/notices/USN-7765-1
- https://ubuntu.com/security/notices/USN-7766-1
- https://ubuntu.com/security/notices/USN-7767-1
- https://ubuntu.com/security/notices/USN-7767-2
- https://ubuntu.com/security/notices/USN-7779-1
- https://ubuntu.com/security/notices/USN-7790-1
- https://ubuntu.com/security/notices/USN-7800-1
- https://ubuntu.com/security/notices/USN-7801-1
- https://ubuntu.com/security/notices/USN-7801-2
- https://ubuntu.com/security/notices/USN-7801-3
- https://ubuntu.com/security/notices/USN-7802-1
- https://ubuntu.com/security/notices/USN-7809-1
- https://www.cve.org/CVERecord?id=CVE-2025-21991