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

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Message-ID: <68eee932c6ef_2f89910045@dwillia2-mobl4.notmuch>
Date: Tue, 14 Oct 2025 17:22:10 -0700
From: <dan.j.williams@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, Sean Christopherson
	<seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Kai Huang
	<kai.huang@...el.com>, Xiaoyao Li <xiaoyao.li@...el.com>, Rick Edgecombe
	<rick.p.edgecombe@...el.com>
Subject: Re: [PATCH] KVM: VMX: Inject #UD if guest tries to execute SEAMCALL
 or TDCALL

Sean Christopherson wrote:
> Add VMX exit handlers for SEAMCALL and TDCALL, and a SEAMCALL handler for
> TDX, to inject a #UD if a non-TD guest attempts to execute SEAMCALL or
> TDCALL, or if a TD guest attempst to execute SEAMCALL.  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 unprivilege 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.

Filed an errata for this.

> Note #3!  The aforementioned Trust Domain spec uses confusing pseudocde
> 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".

This one I am not following. Is this mixing the #UD and exit cases? The
long form says inSEAM generates #UD and that is consistent with the
"64-Bit Mode Exceptions" table.

For exit it says: "When invoked in SEAM VMX non-root operation or legacy
VMX non-root operation, this instruction can cause a VM exit".