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: Kai Huang <kai.huang@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20251016182148.69085-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/include/uapi/asm/vmx.h b/arch/x86/include/uapi/asm/vmx.h
index 9792e329343e8715b3c6a722663f5c1d162e870d..1baa86dfe029329a3f576eaa098c2aaf3e7bcc98 100644 (file)
--- a/arch/x86/include/uapi/asm/vmx.h
+++ b/arch/x86/include/uapi/asm/vmx.h
@@ -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

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 76271962cb7083b475de6d7d24bf9cb918050650..bcea087b642fd04695d09a8d1f062e283018d9ab 100644 (file)
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -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;
        }

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index f87c216d976d7d344c924aa4cc18fe1bf8f9b731..91b6f2f3edc2ab19fb706efb9df2ad52b0ff4836 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -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,
 };