Reproducibility of MRSEAM for Intel TDX #5
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ahunter6
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Nov 21, 2024
Intel TDX protects guest VM's from malicious host and certain physical attacks. TDX introduces a new operation mode, Secure Arbitration Mode (SEAM) to isolate and protect guest VM's. A TDX guest VM runs in SEAM and, unlike VMX, direct control and interaction with the guest by the host VMM is not possible. Instead, Intel TDX Module, which also runs in SEAM, provides a SEAMCALL API. The SEAMCALL that provides the ability to enter a guest is TDH.VP.ENTER. The TDX Module processes TDH.VP.ENTER, and enters the guest via VMX VMLAUNCH/VMRESUME instructions. When a guest VM-exit requires host VMM interaction, the TDH.VP.ENTER SEAMCALL returns to the host VMM (KVM). Add tdh_vp_enter() to wrap the SEAMCALL invocation of TDH.VP.ENTER. TDH.VP.ENTER is different from other SEAMCALLS in several ways: - it may take some time to return as the guest executes - it uses more arguments - after it returns some host state may need to be restored TDH.VP.ENTER arguments are passed through General Purpose Registers (GPRs). For the special case of the TD guest invoking TDG.VP.VMCALL, nearly any GPR can be used, as well as XMM0 to XMM15. Notably, RBP is not used, and Linux mandates the TDX Module feature NO_RBP_MOD, which is enforced elsewhere. Additionally, XMM registers are not required for the existing Guest Hypervisor Communication Interface and are handled by existing KVM code should they be modified by the guest. There are 2 input formats and 5 output formats for TDH.VP.ENTER arguments. Input #1 : Initial entry or following a previous async. TD Exit Input #2 : Following a previous TDCALL(TDG.VP.VMCALL) Output #1 : On Error (No TD Entry) Output #2 : Async. Exits with a VMX Architectural Exit Reason Output #3 : Async. Exits with a non-VMX TD Exit Status Output #4 : Async. Exits with Cross-TD Exit Details Output #5 : On TDCALL(TDG.VP.VMCALL) Currently, to keep things simple, the wrapper function does not attempt to support different formats, and just passes all the GPRs that could be used. The GPR values are held by KVM in the area set aside for guest GPRs. KVM code uses the guest GPR area (vcpu->arch.regs[]) to set up for or process results of tdh_vp_enter(). Therefore changing tdh_vp_enter() to use more complex argument formats would also alter the way KVM code interacts with tdh_vp_enter(). Signed-off-by: Kai Huang <[email protected]> --- -Kai: - This is not nice, but for now have no clue on improvement.
rpedgeco
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Nov 30, 2024
Intel TDX protects guest VM's from malicious host and certain physical attacks. TDX introduces a new operation mode, Secure Arbitration Mode (SEAM) to isolate and protect guest VM's. A TDX guest VM runs in SEAM and, unlike VMX, direct control and interaction with the guest by the host VMM is not possible. Instead, Intel TDX Module, which also runs in SEAM, provides a SEAMCALL API. The SEAMCALL that provides the ability to enter a guest is TDH.VP.ENTER. The TDX Module processes TDH.VP.ENTER, and enters the guest via VMX VMLAUNCH/VMRESUME instructions. When a guest VM-exit requires host VMM interaction, the TDH.VP.ENTER SEAMCALL returns to the host VMM (KVM). Add tdh_vp_enter() to wrap the SEAMCALL invocation of TDH.VP.ENTER. TDH.VP.ENTER is different from other SEAMCALLS in several ways: - it may take some time to return as the guest executes - it uses more arguments - after it returns some host state may need to be restored TDH.VP.ENTER arguments are passed through General Purpose Registers (GPRs). For the special case of the TD guest invoking TDG.VP.VMCALL, nearly any GPR can be used, as well as XMM0 to XMM15. Notably, RBP is not used, and Linux mandates the TDX Module feature NO_RBP_MOD, which is enforced elsewhere. Additionally, XMM registers are not required for the existing Guest Hypervisor Communication Interface and are handled by existing KVM code should they be modified by the guest. There are 2 input formats and 5 output formats for TDH.VP.ENTER arguments. Input #1 : Initial entry or following a previous async. TD Exit Input #2 : Following a previous TDCALL(TDG.VP.VMCALL) Output #1 : On Error (No TD Entry) Output #2 : Async. Exits with a VMX Architectural Exit Reason Output #3 : Async. Exits with a non-VMX TD Exit Status Output #4 : Async. Exits with Cross-TD Exit Details Output #5 : On TDCALL(TDG.VP.VMCALL) Currently, to keep things simple, the wrapper function does not attempt to support different formats, and just passes all the GPRs that could be used. The GPR values are held by KVM in the area set aside for guest GPRs. KVM code uses the guest GPR area (vcpu->arch.regs[]) to set up for or process results of tdh_vp_enter(). Therefore changing tdh_vp_enter() to use more complex argument formats would also alter the way KVM code interacts with tdh_vp_enter(). Signed-off-by: Kai Huang <[email protected]> Signed-off-by: Adrian Hunter <[email protected]>
rpedgeco
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Dec 3, 2024
Intel TDX protects guest VM's from malicious host and certain physical attacks. TDX introduces a new operation mode, Secure Arbitration Mode (SEAM) to isolate and protect guest VM's. A TDX guest VM runs in SEAM and, unlike VMX, direct control and interaction with the guest by the host VMM is not possible. Instead, Intel TDX Module, which also runs in SEAM, provides a SEAMCALL API. The SEAMCALL that provides the ability to enter a guest is TDH.VP.ENTER. The TDX Module processes TDH.VP.ENTER, and enters the guest via VMX VMLAUNCH/VMRESUME instructions. When a guest VM-exit requires host VMM interaction, the TDH.VP.ENTER SEAMCALL returns to the host VMM (KVM). Add tdh_vp_enter() to wrap the SEAMCALL invocation of TDH.VP.ENTER. TDH.VP.ENTER is different from other SEAMCALLS in several ways: - it may take some time to return as the guest executes - it uses more arguments - after it returns some host state may need to be restored TDH.VP.ENTER arguments are passed through General Purpose Registers (GPRs). For the special case of the TD guest invoking TDG.VP.VMCALL, nearly any GPR can be used, as well as XMM0 to XMM15. Notably, RBP is not used, and Linux mandates the TDX Module feature NO_RBP_MOD, which is enforced elsewhere. Additionally, XMM registers are not required for the existing Guest Hypervisor Communication Interface and are handled by existing KVM code should they be modified by the guest. There are 2 input formats and 5 output formats for TDH.VP.ENTER arguments. Input #1 : Initial entry or following a previous async. TD Exit Input #2 : Following a previous TDCALL(TDG.VP.VMCALL) Output #1 : On Error (No TD Entry) Output #2 : Async. Exits with a VMX Architectural Exit Reason Output #3 : Async. Exits with a non-VMX TD Exit Status Output #4 : Async. Exits with Cross-TD Exit Details Output #5 : On TDCALL(TDG.VP.VMCALL) Currently, to keep things simple, the wrapper function does not attempt to support different formats, and just passes all the GPRs that could be used. The GPR values are held by KVM in the area set aside for guest GPRs. KVM code uses the guest GPR area (vcpu->arch.regs[]) to set up for or process results of tdh_vp_enter(). Therefore changing tdh_vp_enter() to use more complex argument formats would also alter the way KVM code interacts with tdh_vp_enter(). Signed-off-by: Kai Huang <[email protected]> --- -Kai: - This is not nice, but for now have no clue on improvement.
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Currently, the parameters BUILD_DATE, BUILD_NUM, and UPDATE_VERSION are not accessible to guest VMs according to the global_metadata.json file published by Intel (we have verified this lack of access using Intel ABI on a TDX guest VM).
However, these parameters are mandatory for reproducible builds of MRSEAM. The only certain way for users of CVMs to know these parameters is by obtaining them through a guest CVM.
So, why is it that these parameters are designed to be inaccessible by the guest VM?
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