linux-kernel - Re: [PATCH v2 1/2] KVM: VMX: Inject #UD if guest tries to execute SEAMCALL or TDCALL

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Message-ID: <5929bfbf30d4ee97086d1ba6ec3e9904567f28a1.camel@intel.com>
Date: Fri, 17 Oct 2025 20:34:38 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>
CC: "Li, Xiaoyao" <xiaoyao.li@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "Williams, Dan J" <dan.j.williams@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Edgecombe,
 Rick P" <rick.p.edgecombe@...el.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>
Subject: Re: [PATCH v2 1/2] KVM: VMX: Inject #UD if guest tries to execute
 SEAMCALL or TDCALL

On Thu, 2025-10-16 at 11:21 -0700, Sean Christopherson wrote:
> Add VMX exit handlers for SEAMCALL and TDCALL to inject a #UD if a non-TD
> guest attempts to execute SEAMCALL or TDCALL.  Neither SEAMCALL nor TDCALL
> is gated by any software enablement other than VMXON, and so will generate
> a VM-Exit instead of e.g. a native #UD when executed from the guest kernel.
> 
> Note!  No unprivileged DoS of the L1 kernel is possible as TDCALL and
> SEAMCALL #GP at CPL > 0, and the CPL check is performed prior to the VMX
> non-root (VM-Exit) check, i.e. userspace can't crash the VM. And for a
> nested guest, KVM forwards unknown exits to L1, i.e. an L2 kernel can
> crash itself, but not L1.
> 
> Note #2!  The Intel® Trust Domain CPU Architectural Extensions spec's
> pseudocode shows the CPL > 0 check for SEAMCALL coming _after_ the VM-Exit,
> but that appears to be a documentation bug (likely because the CPL > 0
> check was incorrectly bundled with other lower-priority #GP checks).
> Testing on SPR and EMR shows that the CPL > 0 check is performed before
> the VMX non-root check, i.e. SEAMCALL #GPs when executed in usermode.
> 
> Note #3!  The aforementioned Trust Domain spec uses confusing pseudocode
> that says that SEAMCALL will #UD if executed "inSEAM", but "inSEAM"
> specifically means in SEAM Root Mode, i.e. in the TDX-Module.  The long-
> form description explicitly states that SEAMCALL generates an exit when
> executed in "SEAM VMX non-root operation".  But that's a moot point as the
> TDX-Module injects #UD if the guest attempts to execute SEAMCALL, as
> documented in the "Unconditionally Blocked Instructions" section of the
> TDX-Module base specification.
> 
> Cc: stable@...r.kernel.org
> Cc: Kai Huang <kai.huang@...el.com>
> Cc: Xiaoyao Li <xiaoyao.li@...el.com>
> Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Binbin Wu <binbin.wu@...ux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> 

Reviewed-by: Kai Huang <kai.huang@...el.com>