From: Paolo Bonzini Date: Wed, 9 Jul 2025 17:45:14 +0000 (-0400) Subject: KVM: x86: avoid underflow when scaling TSC frequency X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=4578a747f3c7950be3feb93c2db32eb597a3e55b;p=linux-block.git KVM: x86: avoid underflow when scaling TSC frequency In function kvm_guest_time_update(), __scale_tsc() is used to calculate a TSC *frequency* rather than a TSC value. With low-enough ratios, a TSC value that is less than 1 would underflow to 0 and to an infinite while loop in kvm_get_time_scale(): kvm_guest_time_update(struct kvm_vcpu *v) if (kvm_caps.has_tsc_control) tgt_tsc_khz = kvm_scale_tsc(tgt_tsc_khz, v->arch.l1_tsc_scaling_ratio); __scale_tsc(u64 ratio, u64 tsc) ratio=122380531, tsc=2299998, N=48 ratio*tsc >> N = 0.999... -> 0 Later in the function: Call Trace: kvm_get_time_scale arch/x86/kvm/x86.c:2458 [inline] kvm_guest_time_update+0x926/0xb00 arch/x86/kvm/x86.c:3268 vcpu_enter_guest.constprop.0+0x1e70/0x3cf0 arch/x86/kvm/x86.c:10678 vcpu_run+0x129/0x8d0 arch/x86/kvm/x86.c:11126 kvm_arch_vcpu_ioctl_run+0x37a/0x13d0 arch/x86/kvm/x86.c:11352 kvm_vcpu_ioctl+0x56b/0xe60 virt/kvm/kvm_main.c:4188 vfs_ioctl fs/ioctl.c:51 [inline] __do_sys_ioctl fs/ioctl.c:871 [inline] __se_sys_ioctl+0x12d/0x190 fs/ioctl.c:857 do_syscall_x64 arch/x86/entry/common.c:51 [inline] do_syscall_64+0x59/0x110 arch/x86/entry/common.c:81 entry_SYSCALL_64_after_hwframe+0x78/0xe2 This can really happen only when fuzzing, since the TSC frequency would have to be nonsensically low. Fixes: 35181e86df97 ("KVM: x86: Add a common TSC scaling function") Reported-by: Yuntao Liu Suggested-by: Sean Christopherson Signed-off-by: Paolo Bonzini --- diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index b58a74c1722d..de51dbd85a58 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3258,9 +3258,11 @@ int kvm_guest_time_update(struct kvm_vcpu *v) /* With all the info we got, fill in the values */ - if (kvm_caps.has_tsc_control) + if (kvm_caps.has_tsc_control) { tgt_tsc_khz = kvm_scale_tsc(tgt_tsc_khz, v->arch.l1_tsc_scaling_ratio); + tgt_tsc_khz = tgt_tsc_khz ? : 1; + } if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) { kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL,