CVE-2025-21779

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

KVM: x86: Reject Hyper-Vs SEND_IPI hypercalls if local APIC isnt in-kernel

Advertise support for Hyper-Vs SEND_IPI and SEND_IPI_EX hypercalls if and only if the local API is emulated/virtualized by KVM, and explicitly reject said hypercalls if the local APIC is emulated in userspace, i.e. dont rely on userspace to opt-in to KVM_CAP_HYPERV_ENFORCE_CPUID.

Rejecting SEND_IPI and SEND_IPI_EX fixes a NULL-pointer dereference if Hyper-V enlightenments are exposed to the guest without an in-kernel local APIC:

dump_stack+0xbe/0xfd __kasan_report.cold+0x34/0x84 kasan_report+0x3a/0x50 __apic_accept_irq+0x3a/0x5c0 kvm_hv_send_ipi.isra.0+0x34e/0x820 kvm_hv_hypercall+0x8d9/0x9d0 kvm_emulate_hypercall+0x506/0x7e0 __vmx_handle_exit+0x283/0xb60 vmx_handle_exit+0x1d/0xd0 vcpu_enter_guest+0x16b0/0x24c0 vcpu_run+0xc0/0x550 kvm_arch_vcpu_ioctl_run+0x170/0x6d0 kvm_vcpu_ioctl+0x413/0xb20 __se_sys_ioctl+0x111/0x160 do_syscal1_64+0x30/0x40 entry_SYSCALL_64_after_hwframe+0x67/0xd1

Note, checking the sending vCPU is sufficient, as the per-VM irqchip_mode cant be modified after vCPUs are created, i.e. if one vCPU has an in-kernel local APIC, then all vCPUs have an in-kernel local APIC.

Weakness

A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/45fa526b0f5a34492ed0536c3cdf88b78380e4de
• https://git.kernel.org/stable/c/5393cf22312418262679eaadb130d608c75fe690
• https://git.kernel.org/stable/c/874ff13c73c45ecb38cb82191e8c1d523f0dc81b
• https://git.kernel.org/stable/c/a8de7f100bb5989d9c3627d3a223ee1c863f3b69
• https://git.kernel.org/stable/c/aca8be4403fb90db7adaf63830e27ebe787a76e8
• https://git.kernel.org/stable/c/ca29f58ca374c40a0e69c5306fc5c940a0069074