KVM: x86: Add a helper to handle filtering of unpermitted XCR0 features

Add a helper, kvm_get_filtered_xcr0(), to dedup code that needs to account
for XCR0 features that require explicit opt-in on a per-process basis.  In
addition to documenting when KVM should/shouldn't consult
xstate_get_guest_group_perm(), the helper will also allow sanitizing the
filtered XCR0 to avoid enumerating architecturally illegal XCR0 values,
e.g. XTILE_CFG without XTILE_DATA.

No functional changes intended.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
[sean: rename helper, move to x86.h, massage changelog]
Reviewed-by: Aaron Lewis <aaronlewis@google.com>
Tested-by: Aaron Lewis <aaronlewis@google.com>
Link: https://lore.kernel.org/r/20230405004520.421768-2-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 9583a110cf5f26b0b3fed60ca7a5321f9ad53d14..1daf0df6a9b8a697bfd35e23865883835eb56556 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1002,7 +1002,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
                entry->eax = entry->ebx = entry->ecx = 0;
                break;
        case 0xd: {
-               u64 permitted_xcr0 = kvm_caps.supported_xcr0 & xstate_get_guest_group_perm();
+               u64 permitted_xcr0 = kvm_get_filtered_xcr0();
                u64 permitted_xss = kvm_caps.supported_xss;
 
                entry->eax &= permitted_xcr0;

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 237c483b1230168e8bc4cb16e9bb1de7f392c9af..181f155933cfcf39f19f1401bfd2ddf6bbd025a2 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4531,9 +4531,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
                        r = 0;
                break;
        case KVM_CAP_XSAVE2: {
-               u64 guest_perm = xstate_get_guest_group_perm();
-
-               r = xstate_required_size(kvm_caps.supported_xcr0 & guest_perm, false);
+               r = xstate_required_size(kvm_get_filtered_xcr0(), false);
                if (r < sizeof(struct kvm_xsave))
                        r = sizeof(struct kvm_xsave);
                break;

diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index a8167b47b8c829c6f1ff97bc932944ccccc42f43..9589c86812eb77017ba14a89e64ceb5a9be7ca17 100644 (file)
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -315,6 +315,19 @@ extern struct kvm_caps kvm_caps;
 
 extern bool enable_pmu;
 
+/*
+ * Get a filtered version of KVM's supported XCR0 that strips out dynamic
+ * features for which the current process doesn't (yet) have permission to use.
+ * This is intended to be used only when enumerating support to userspace,
+ * e.g. in KVM_GET_SUPPORTED_CPUID and KVM_CAP_XSAVE2, it does NOT need to be
+ * used to check/restrict guest behavior as KVM rejects KVM_SET_CPUID{2} if
+ * userspace attempts to enable unpermitted features.
+ */
+static inline u64 kvm_get_filtered_xcr0(void)
+{
+       return kvm_caps.supported_xcr0 & xstate_get_guest_group_perm();
+}
+
 static inline bool kvm_mpx_supported(void)
 {
        return (kvm_caps.supported_xcr0 & (XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR))