linux-kernel - Re: [PATCH 2/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPU has been created

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <6513254f-106e-4ed9-9622-8ed20909e1fa@amd.com>
Date: Wed, 9 Jul 2025 14:21:55 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Kai Huang <kai.huang@...el.com>, seanjc@...gle.com, pbonzini@...hat.com
Cc: kvm@...r.kernel.org, thomas.lendacky@....com, bp@...en8.de,
 isaku.yamahata@...el.com, xiaoyao.li@...el.com, rick.p.edgecombe@...el.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPU
 has been created



On 7/9/2025 11:08 AM, Kai Huang wrote:
> Reject the KVM_SET_TSC_KHZ VM ioctl when there's vCPU has already been
> created.

Probably the below is clear:

Reject KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created

> 
> The VM scope KVM_SET_TSC_KHZ ioctl is used to set up the default TSC
> frequency that all subsequent created vCPUs use.  It is only intended to
> be called before any vCPU is created.  Allowing it to be called after
> that only results in confusion but nothing good.
> 
> Note this is an ABI change.  But currently in Qemu (the de facto
> userspace VMM) only TDX uses this VM ioctl, and it is only called once
> before creating any vCPU, therefore the risk of breaking userspace is
> pretty low.
> 
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Kai Huang <kai.huang@...el.com>

Reviewed-by: Nikunj A Dadhania <nikunj@....com>

> ---
>  arch/x86/kvm/x86.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 699ca5e74bba..e5e55d549468 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7194,6 +7194,10 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
>  		u32 user_tsc_khz;
>  
>  		r = -EINVAL;
> +
> +		if (kvm->created_vcpus)
> +			goto out;
> +
>  		user_tsc_khz = (u32)arg;
>  
>  		if (kvm_caps.has_tsc_control &&