CVE-2024-40953

In the Linux kernel, the following vulnerability has been resolved:

KVM: Fix a data race on last_boosted_vcpu in kvm_vcpu_on_spin()

Use {READ,WRITE}_ONCE() to access kvm->last_boosted_vcpu to ensure the loads and stores are atomic. In the extremely unlikely scenario the compiler tears the stores, its theoretically possible for KVM to attempt to get a vCPU using an out-of-bounds index, e.g. if the write is split into multiple 8-bit stores, and is paired with a 32-bit load on a VM with 257 vCPUs:

CPU0 CPU1 last_boosted_vcpu = 0xff;

                                (last_boosted_vcpu = 0x100)
                                last_boosted_vcpu[15:8] = 0x01;

i = (last_boosted_vcpu = 0x1ff) last_boosted_vcpu[7:0] = 0x00;

vcpu = kvm->vcpu_array[0x1ff];

As detected by KCSAN:

BUG: KCSAN: data-race in kvm_vcpu_on_spin [kvm] / kvm_vcpu_on_spin [kvm]

write to 0xffffc90025a92344 of 4 bytes by task 4340 on cpu 16: kvm_vcpu_on_spin (arch/x86/kvm/../../../virt/kvm/kvm_main.c:4112) kvm handle_pause (arch/x86/kvm/vmx/vmx.c:5929) kvm_intel vmx_handle_exit (arch/x86/kvm/vmx/vmx.c:? arch/x86/kvm/vmx/vmx.c:6606) kvm_intel vcpu_run (arch/x86/kvm/x86.c:11107 arch/x86/kvm/x86.c:11211) kvm kvm_arch_vcpu_ioctl_run (arch/x86/kvm/x86.c:?) kvm kvm_vcpu_ioctl (arch/x86/kvm/../../../virt/kvm/kvm_main.c:?) kvm __se_sys_ioctl (fs/ioctl.c:52 fs/ioctl.c:904 fs/ioctl.c:890) __x64_sys_ioctl (fs/ioctl.c:890) x64_sys_call (arch/x86/entry/syscall_64.c:33) do_syscall_64 (arch/x86/entry/common.c:?) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

read to 0xffffc90025a92344 of 4 bytes by task 4342 on cpu 4: kvm_vcpu_on_spin (arch/x86/kvm/../../../virt/kvm/kvm_main.c:4069) kvm handle_pause (arch/x86/kvm/vmx/vmx.c:5929) kvm_intel vmx_handle_exit (arch/x86/kvm/vmx/vmx.c:? arch/x86/kvm/vmx/vmx.c:6606) kvm_intel vcpu_run (arch/x86/kvm/x86.c:11107 arch/x86/kvm/x86.c:11211) kvm kvm_arch_vcpu_ioctl_run (arch/x86/kvm/x86.c:?) kvm kvm_vcpu_ioctl (arch/x86/kvm/../../../virt/kvm/kvm_main.c:?) kvm __se_sys_ioctl (fs/ioctl.c:52 fs/ioctl.c:904 fs/ioctl.c:890) __x64_sys_ioctl (fs/ioctl.c:890) x64_sys_call (arch/x86/entry/syscall_64.c:33) do_syscall_64 (arch/x86/entry/common.c:?) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

value changed: 0x00000012 -> 0x00000000

Weakness

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/11a772d5376aa6d3e2e69b5b5c585f79b60c0e17
• https://git.kernel.org/stable/c/49f683b41f28918df3e51ddc0d928cb2e934ccdb
• https://git.kernel.org/stable/c/4c141136a28421b78f34969b25a4fa32e06e2180
• https://git.kernel.org/stable/c/71fbc3af3dacb26c3aa2f30bb3ab05c44d082c84
• https://git.kernel.org/stable/c/82bd728a06e55f5b5f93d10ce67f4fe7e689853a
• https://git.kernel.org/stable/c/92c77807d938145c7c3350c944ef9f39d7f6017c
• https://git.kernel.org/stable/c/95c8dd79f3a14df96b3820b35b8399bd91b2be60
• https://git.kernel.org/stable/c/a937ef951bba72f48d2402451419d725d70dba20
• https://git.kernel.org/stable/c/49f683b41f28918df3e51ddc0d928cb2e934ccdb
• https://git.kernel.org/stable/c/92c77807d938145c7c3350c944ef9f39d7f6017c
• https://git.kernel.org/stable/c/95c8dd79f3a14df96b3820b35b8399bd91b2be60
• https://git.kernel.org/stable/c/a937ef951bba72f48d2402451419d725d70dba20
• https://lists.debian.org/debian-lts-announce/2025/01/msg00001.html
• https://lists.debian.org/debian-lts-announce/2025/03/msg00002.html