Overview
About vulnerability
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. ]
Details
- Affected product:
- AlmaLinux 9.2 ESU , CentOS 8.4 ELS , CentOS 8.5 ELS , CentOS Stream 8 ELS , Oracle Linux 7 ELS , Ubuntu 16.04 ELS , Ubuntu 18.04 ELS , Ubuntu 20.04 ELS
- Affected packages:
- kernel @ 4.18.0 (+7 more)
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. ]