linux-kernel - Re: [PATCH v2 1/2] KVM: nVMX: Disallow access to vmcs12 fields that aren't supported by "hardware"

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] [day] [month] [year] [list]

Message-Id: <1D1B6FEC-1D50-4A0D-92D1-33B522576670@zytor.com>
Date: Wed, 31 Dec 2025 07:38:18 -0800
From: Xin Li <xin@...or.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Chao Gao <chao.gao@...el.com>,
        Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: Re: [PATCH v2 1/2] KVM: nVMX: Disallow access to vmcs12 fields that
 aren't supported by "hardware"


> On Dec 30, 2025, at 2:02 PM, Sean Christopherson <seanjc@...gle.com> wrote:
> 
> Disallow access (VMREAD/VMWRITE) to fields that the loaded incarnation of
> KVM doesn't support, e.g. due to lack of hardware support, as a middle
> ground between allowing access to any vmcs12 field defined by KVM (current
> behavior) and gating access based on the userspace-defined vCPU model (the
> most correct, but costly, implementation).
> 
> Disallowing access to unsupported fields helps a tiny bit in terms of
> closing the virtualization hole (see below), but the main motivation is to
> avoid having to weed out unsupported fields when synchronizing between
> vmcs12 and a shadow VMCS.  Because shadow VMCS accesses are done via
> VMREAD and VMWRITE, KVM _must_ filter out unsupported fields (or eat
> VMREAD/VMWRITE failures), and filtering out just shadow VMCS fields is
> about the same amount of effort, and arguably much more confusing.
> 
> As a bonus, this also fixes a KVM-Unit-Test failure bug when running on
> _hardware_ without support for TSC Scaling, which fails with the same
> signature as the bug fixed by commit ba1f82456ba8 ("KVM: nVMX: Dynamically
> compute max VMCS index for vmcs12"):
> 
>  FAIL: VMX_VMCS_ENUM.MAX_INDEX expected: 19, actual: 17
> 
> Dynamically computing the max VMCS index only resolved the issue where KVM
> was hardcoding max index, but for CPUs with TSC Scaling, that was "good
> enough".
> 
> Cc: Xin Li <xin@...or.com>
> Cc: Chao Gao <chao.gao@...el.com>
> Cc: Yosry Ahmed <yosry.ahmed@...ux.dev>
> Link: https://lore.kernel.org/all/20251026201911.505204-22-xin@zytor.com
> Link: https://lore.kernel.org/all/YR2Tf9WPNEzrE7Xg@google.com
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Xin Li <xin@...or.com <mailto:xin@...or.com>>