x86/CPUID: shrink max_{,sub}leaf fields according to actual leaf cont…

…ents

Zapping leaf data for out of range leaves is just one half of it: To
avoid guests (bogusly or worse) inferring information from mere leaf
presence, also shrink maximum indicators such that the respective
trailing entry is not all blank (unless of course it's the initial
subleaf of a leaf that's not the final one).

This is also in preparation of bumping the maximum basic leaf we
support, to ensure guests not getting exposed related features won't
observe a change in behavior.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

tools/tests/cpu-policy/test-cpu-policy.c

@@ -8,10 +8,13 @@
 #include <err.h>
 
 #include <xen-tools/libs.h>
+#include <xen/asm/x86-defns.h>
 #include <xen/asm/x86-vendors.h>
 #include <xen/lib/x86/cpu-policy.h>
 #include <xen/domctl.h>
 
+#define XSTATE_FP_SSE  (X86_XCR0_FP | X86_XCR0_SSE)
+
 static unsigned int nr_failures;
 #define fail(fmt, ...)                          \
 ({                                              \
@@ -553,6 +556,103 @@ static void test_cpuid_out_of_range_clearing(void)
     }
 }
 
+static void test_cpuid_maximum_leaf_shrinking(void)
+{
+    static const struct test {
+        const char *name;
+        struct cpuid_policy p;
+    } tests[] = {
+        {
+            .name = "basic",
+            .p = {
+                /* Very basic information only. */
+                .basic.max_leaf = 1,
+                .basic.raw_fms = 0xc2,
+            },
+        },
+        {
+            .name = "cache",
+            .p = {
+                /* Cache subleaves present. */
+                .basic.max_leaf = 4,
+                .cache.subleaf[0].type = 1,
+            },
+        },
+        {
+            .name = "feat#0",
+            .p = {
+                /* Subleaf 0 only with some valid bit. */
+                .basic.max_leaf = 7,
+                .feat.max_subleaf = 0,
+                .feat.fsgsbase = 1,
+            },
+        },
+        {
+            .name = "feat#1",
+            .p = {
+                /* Subleaf 1 only with some valid bit. */
+                .basic.max_leaf = 7,
+                .feat.max_subleaf = 1,
+                .feat.avx_vnni = 1,
+            },
+        },
+        {
+            .name = "topo",
+            .p = {
+                /* Topology subleaves present. */
+                .basic.max_leaf = 0xb,
+                .topo.subleaf[0].type = 1,
+            },
+        },
+        {
+            .name = "xstate",
+            .p = {
+                /* First subleaf always valid (and then non-zero). */
+                .basic.max_leaf = 0xd,
+                .xstate.xcr0_low = XSTATE_FP_SSE,
+            },
+        },
+        {
+            .name = "extd",
+            .p = {
+                /* Commonly available information only. */
+                .extd.max_leaf = 0x80000008,
+                .extd.maxphysaddr = 0x28,
+                .extd.maxlinaddr = 0x30,
+            },
+        },
+    };
+
+    printf("Testing CPUID maximum leaf shrinking:\n");
+
+    for ( size_t i = 0; i < ARRAY_SIZE(tests); ++i )
+    {
+        const struct test *t = &tests[i];
+        struct cpuid_policy *p = memdup(&t->p);
+
+        p->basic.max_leaf = ARRAY_SIZE(p->basic.raw) - 1;
+        p->feat.max_subleaf = ARRAY_SIZE(p->feat.raw) - 1;
+        p->extd.max_leaf = 0x80000000 | (ARRAY_SIZE(p->extd.raw) - 1);
+
+        x86_cpuid_policy_shrink_max_leaves(p);
+
+        /* Check the the resulting max (sub)leaf values against expecations. */
+        if ( p->basic.max_leaf != t->p.basic.max_leaf )
+             fail("  Test %s basic fail - expected %#x, got %#x\n",
+                  t->name, t->p.basic.max_leaf, p->basic.max_leaf);
+
+        if ( p->extd.max_leaf != t->p.extd.max_leaf )
+             fail("  Test %s extd fail - expected %#x, got %#x\n",
+                  t->name, t->p.extd.max_leaf, p->extd.max_leaf);
+
+        if ( p->feat.max_subleaf != t->p.feat.max_subleaf )
+             fail("  Test %s feat fail - expected %#x, got %#x\n",
+                  t->name, t->p.feat.max_subleaf, p->feat.max_subleaf);
+
+        free(p);
+    }
+}
+
 static void test_is_compatible_success(void)
 {
     static struct test {
@@ -668,6 +768,7 @@ int main(int argc, char **argv)
     test_cpuid_serialise_success();
     test_cpuid_deserialise_failure();
     test_cpuid_out_of_range_clearing();
+    test_cpuid_maximum_leaf_shrinking();
 
     test_msr_serialise_success();
     test_msr_deserialise_failure();

xen/arch/x86/cpuid.c

@@ -341,6 +341,8 @@ static void __init calculate_host_policy(void)
         p->extd.raw[0xa].d |= ((1u << SVM_FEATURE_VMCBCLEAN) |
                                (1u << SVM_FEATURE_TSCRATEMSR));
     }
+
+    x86_cpuid_policy_shrink_max_leaves(p);
 }
 
 static void __init guest_common_default_feature_adjustments(uint32_t *fs)
@@ -410,6 +412,8 @@ static void __init calculate_pv_max_policy(void)
     recalculate_xstate(p);
 
     p->extd.raw[0xa] = EMPTY_LEAF; /* No SVM for PV guests. */
+
+    x86_cpuid_policy_shrink_max_leaves(p);
 }
 
 static void __init calculate_pv_def_policy(void)
@@ -430,6 +434,8 @@ static void __init calculate_pv_def_policy(void)
     sanitise_featureset(pv_featureset);
     cpuid_featureset_to_policy(pv_featureset, p);
     recalculate_xstate(p);
+
+    x86_cpuid_policy_shrink_max_leaves(p);
 }
 
 static void __init calculate_hvm_max_policy(void)
@@ -495,6 +501,8 @@ static void __init calculate_hvm_max_policy(void)
     sanitise_featureset(hvm_featureset);
     cpuid_featureset_to_policy(hvm_featureset, p);
     recalculate_xstate(p);
+
+    x86_cpuid_policy_shrink_max_leaves(p);
 }
 
 static void __init calculate_hvm_def_policy(void)
@@ -519,6 +527,8 @@ static void __init calculate_hvm_def_policy(void)
     sanitise_featureset(hvm_featureset);
     cpuid_featureset_to_policy(hvm_featureset, p);
     recalculate_xstate(p);
+
+    x86_cpuid_policy_shrink_max_leaves(p);
 }
 
 void __init init_guest_cpuid(void)
@@ -699,6 +709,8 @@ void recalculate_cpuid_policy(struct domain *d)
 
     if ( !p->extd.page1gb )
         p->extd.raw[0x19] = EMPTY_LEAF;
+
+    x86_cpuid_policy_shrink_max_leaves(p);
 }
 
 int init_domain_cpuid_policy(struct domain *d)

xen/arch/x86/traps.c

@@ -964,13 +964,15 @@ void cpuid_hypervisor_leaves(const struct vcpu *v, uint32_t leaf,
     uint32_t base = is_viridian_domain(d) ? 0x40000100 : 0x40000000;
     uint32_t idx  = leaf - base;
     unsigned int limit = is_viridian_domain(d) ? p->hv2_limit : p->hv_limit;
+    unsigned int dflt = is_pv_domain(d) ? XEN_CPUID_MAX_PV_NUM_LEAVES
+                                        : XEN_CPUID_MAX_HVM_NUM_LEAVES;
 
     if ( limit == 0 )
         /* Default number of leaves */
-        limit = XEN_CPUID_MAX_NUM_LEAVES;
+        limit = dflt;
     else
         /* Clamp toolstack value between 2 and MAX_NUM_LEAVES. */
-        limit = min(max(limit, 2u), XEN_CPUID_MAX_NUM_LEAVES + 0u);
+        limit = min(max(limit, 2u), dflt);
 
     if ( idx > limit )
         return;

xen/include/public/arch-x86/cpuid.h

@@ -113,6 +113,10 @@
 /* Max. address width in bits taking memory hotplug into account. */
 #define XEN_CPUID_MACHINE_ADDRESS_WIDTH_MASK (0xffu << 0)
 
-#define XEN_CPUID_MAX_NUM_LEAVES 5
+#define XEN_CPUID_MAX_PV_NUM_LEAVES 5
+#define XEN_CPUID_MAX_HVM_NUM_LEAVES 4
+#define XEN_CPUID_MAX_NUM_LEAVES \
+    (XEN_CPUID_MAX_PV_NUM_LEAVES > XEN_CPUID_MAX_HVM_NUM_LEAVES ? \
+     XEN_CPUID_MAX_PV_NUM_LEAVES : XEN_CPUID_MAX_HVM_NUM_LEAVES)
 
 #endif /* __XEN_PUBLIC_ARCH_X86_CPUID_H__ */

xen/include/xen/lib/x86/cpuid.h

@@ -351,6 +351,13 @@ void x86_cpuid_policy_fill_native(struct cpuid_policy *p);
  */
 void x86_cpuid_policy_clear_out_of_range_leaves(struct cpuid_policy *p);
 
+/**
+ * Shrink max leaf/subleaf values such that the last respective valid entry
+ * isn't all blank.  While permitted by the spec, such extraneous leaves may
+ * provide undue "hints" to guests.
+ */
+void x86_cpuid_policy_shrink_max_leaves(struct cpuid_policy *p);
+
 #ifdef __XEN__
 #include <public/arch-x86/xen.h>
 typedef XEN_GUEST_HANDLE_64(xen_cpuid_leaf_t) cpuid_leaf_buffer_t;

xen/lib/x86/cpuid.c

@@ -236,6 +236,45 @@ void x86_cpuid_policy_clear_out_of_range_leaves(struct cpuid_policy *p)
                 ARRAY_SIZE(p->extd.raw) - 1);
 }
 
+void x86_cpuid_policy_shrink_max_leaves(struct cpuid_policy *p)
+{
+    unsigned int i;
+
+    p->basic.raw[0x4] = p->cache.raw[0];
+
+    for ( i = p->feat.max_subleaf; i; --i )
+        if ( p->feat.raw[i].a | p->feat.raw[i].b |
+             p->feat.raw[i].c | p->feat.raw[i].d )
+            break;
+    p->feat.max_subleaf = i;
+    p->basic.raw[0x7] = p->feat.raw[i];
+
+    p->basic.raw[0xb] = p->topo.raw[0];
+
+    /*
+     * Due to the way xstate gets handled in the hypervisor (see
+     * recalculate_xstate()) there is (for now at least) no need to fiddle
+     * with the xstate subleaves (IOW we assume they're already in consistent
+     * shape, for coming from either hardware or recalculate_xstate()).
+     */
+    p->basic.raw[0xd] = p->xstate.raw[0];
+
+    for ( i = p->basic.max_leaf; i; --i )
+        if ( p->basic.raw[i].a | p->basic.raw[i].b |
+             p->basic.raw[i].c | p->basic.raw[i].d )
+            break;
+    p->basic.max_leaf = i;
+
+    for ( i = p->extd.max_leaf & 0xffff; i; --i )
+        if ( p->extd.raw[i].a | p->extd.raw[i].b |
+             p->extd.raw[i].c | p->extd.raw[i].d )
+            break;
+    if ( i | p->extd.raw[0].b | p->extd.raw[0].c | p->extd.raw[0].d )
+        p->extd.max_leaf = 0x80000000 | i;
+    else
+        p->extd.max_leaf = 0;
+}
+
 const uint32_t *x86_cpuid_lookup_deep_deps(uint32_t feature)
 {
     static const uint32_t deep_features[] = INIT_DEEP_FEATURES;