KVM: x86: Disable support for Shadow Stacks if TDP is disabled

Make TDP a hard requirement for Shadow Stacks, as there are no plans to
add Shadow Stack support to the Shadow MMU.  E.g. KVM hasn't been taught
to understand the magic Writable=0,Dirty=1 combination that is required
for Shadow Stack accesses, and so enabling Shadow Stacks when using
shadow paging will put the guest into an infinite #PF loop (KVM thinks the
shadow page tables have a valid mapping, hardware says otherwise).

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-27-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 32fde9e80c2863c38cc588cfdf4f7bbe631020bb..499c86bd457e15b4a1308d162b00d619d3bb3e47 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -955,6 +955,14 @@ void kvm_set_cpu_caps(void)
        if (!tdp_enabled || !boot_cpu_has(X86_FEATURE_OSPKE))
                kvm_cpu_cap_clear(X86_FEATURE_PKU);
 
+       /*
+        * Shadow Stacks aren't implemented in the Shadow MMU.  Shadow Stack
+        * accesses require "magic" Writable=0,Dirty=1 protection, which KVM
+        * doesn't know how to emulate or map.
+        */
+       if (!tdp_enabled)
+               kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
+
        kvm_cpu_cap_init(CPUID_7_EDX,
                F(AVX512_4VNNIW),
                F(AVX512_4FMAPS),