Commit 9d7dfb95 authored by Sean Christopherson's avatar Sean Christopherson
Browse files

KVM: VMX: Inject #UD if guest tries to execute SEAMCALL or TDCALL 



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@vger.kernel.org
Cc: Kai Huang <kai.huang@intel.com>
Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: default avatarKai Huang <kai.huang@intel.com>
Reviewed-by: default avatarBinbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: default avatarXiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20251016182148.69085-2-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 4361f5aa

arch/x86/include/uapi/asm/vmx.h

+1 −0
Original line number Diff line number Diff line
@@ -93,6 +93,7 @@ 
#define EXIT_REASON_TPAUSE              68

#define EXIT_REASON_BUS_LOCK            74

#define EXIT_REASON_NOTIFY              75

#define EXIT_REASON_SEAMCALL            76

#define EXIT_REASON_TDCALL              77

#define EXIT_REASON_MSR_READ_IMM        84

#define EXIT_REASON_MSR_WRITE_IMM       85

arch/x86/kvm/vmx/nested.c

+8 −0
Original line number Diff line number Diff line
@@ -6728,6 +6728,14 @@ static bool nested_vmx_l1_wants_exit(struct kvm_vcpu *vcpu, 
	case EXIT_REASON_NOTIFY:

		/* Notify VM exit is not exposed to L1 */

		return false;

	case EXIT_REASON_SEAMCALL:

	case EXIT_REASON_TDCALL:

		/*

		 * SEAMCALL and TDCALL unconditionally VM-Exit, but aren't

		 * virtualized by KVM for L1 hypervisors, i.e. L1 should

		 * never want or expect such an exit.

		 */

		return false;

	default:

		return true;

	}

arch/x86/kvm/vmx/vmx.c

+8 −0
Original line number Diff line number Diff line
@@ -6032,6 +6032,12 @@ static int handle_vmx_instruction(struct kvm_vcpu *vcpu) 
	return 1;

}



static int handle_tdx_instruction(struct kvm_vcpu *vcpu)

{

	kvm_queue_exception(vcpu, UD_VECTOR);

	return 1;

}



#ifndef CONFIG_X86_SGX_KVM

static int handle_encls(struct kvm_vcpu *vcpu)

{
@@ -6157,6 +6163,8 @@ static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = { 
	[EXIT_REASON_ENCLS]		      = handle_encls,

	[EXIT_REASON_BUS_LOCK]                = handle_bus_lock_vmexit,

	[EXIT_REASON_NOTIFY]		      = handle_notify,

	[EXIT_REASON_SEAMCALL]		      = handle_tdx_instruction,

	[EXIT_REASON_TDCALL]		      = handle_tdx_instruction,

	[EXIT_REASON_MSR_READ_IMM]            = handle_rdmsr_imm,

	[EXIT_REASON_MSR_WRITE_IMM]           = handle_wrmsr_imm,

};