Description

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

KVM: x86: Reject Hyper-V's SEND_IPI hypercalls if local APIC isn't in-kernel

Advertise support for Hyper-V's 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. don't 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
can't 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.

Classification

CVE ID: CVE-2025-21779

Affected Products

Vendor: Linux, Linux

Product: Linux, Linux

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.02% (probability of being exploited)

EPSS Percentile: 1.76% (scored less or equal to compared to others)

EPSS Date: 2025-03-27 (when was this score calculated)

References

https://nvd.nist.gov/vuln/detail/CVE-2025-21779
https://git.kernel.org/stable/c/5393cf22312418262679eaadb130d608c75fe690
https://git.kernel.org/stable/c/874ff13c73c45ecb38cb82191e8c1d523f0dc81b
https://git.kernel.org/stable/c/aca8be4403fb90db7adaf63830e27ebe787a76e8
https://git.kernel.org/stable/c/ca29f58ca374c40a0e69c5306fc5c940a0069074
https://git.kernel.org/stable/c/a8de7f100bb5989d9c3627d3a223ee1c863f3b69

Timeline