[PATCH v6 12/12] x86/CPUID: shrink max_{,sub}leaf fields according to actual leaf contents

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

From: Jan Beulich <jbeulich@xxxxxxxx>

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.

Note that such shrinking is only done when creating a policy for a
domain from scratch. Migrated in domains keep their previous policy if
present untouched, and for migrated in domains not having CPUID data
the crafted Xen pre-4.13 policy is not trimmed to keep a behavior
compatible with those older Xen versions.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v4:
 - New in this version, picked up from 540d911c2813.
 - Only shrink policies for newly created domains.
---
 tools/include/xenguest.h                 |   3 +
 tools/libs/guest/xg_cpuid_x86.c          |   5 ++
 tools/libs/light/libxl_cpuid.c           |   7 ++
 tools/tests/cpu-policy/test-cpu-policy.c | 101 +++++++++++++++++++++++
 xen/include/xen/lib/x86/cpuid.h          |   7 ++
 xen/lib/x86/cpuid.c                      |  39 +++++++++
 6 files changed, 162 insertions(+)

diff --git a/tools/include/xenguest.h b/tools/include/xenguest.h
index 3462d27516..e8b0d3ff16 100644
--- a/tools/include/xenguest.h
+++ b/tools/include/xenguest.h
@@ -830,6 +830,9 @@ int xc_cpu_policy_apply_featureset(xc_interface *xch, 
xc_cpu_policy_t *policy,
                                    const uint32_t *featureset,
                                    unsigned int nr_features);
 
+/* Sanitize a policy: can change the contents of the passed policy. */
+void xc_cpu_policy_sanitize(xc_interface *xch, xc_cpu_policy_t *policy);
+
 int xc_get_cpu_levelling_caps(xc_interface *xch, uint32_t *caps);
 int xc_get_cpu_featureset(xc_interface *xch, uint32_t index,
                           uint32_t *nr_features, uint32_t *featureset);
diff --git a/tools/libs/guest/xg_cpuid_x86.c b/tools/libs/guest/xg_cpuid_x86.c
index 8bc7bd7224..c630dd8a73 100644
--- a/tools/libs/guest/xg_cpuid_x86.c
+++ b/tools/libs/guest/xg_cpuid_x86.c
@@ -688,3 +688,8 @@ int xc_cpu_policy_apply_featureset(xc_interface *xch, 
xc_cpu_policy_t *policy,
 
     return 0;
 }
+
+void xc_cpu_policy_sanitize(xc_interface *xch, xc_cpu_policy_t *policy)
+{
+    x86_cpuid_policy_shrink_max_leaves(&policy->cpuid);
+}
diff --git a/tools/libs/light/libxl_cpuid.c b/tools/libs/light/libxl_cpuid.c
index 556e8f41a7..c7294680d4 100644
--- a/tools/libs/light/libxl_cpuid.c
+++ b/tools/libs/light/libxl_cpuid.c
@@ -689,6 +689,13 @@ int libxl__cpuid_legacy(libxl_ctx *ctx, uint32_t domid, 
bool restore,
         goto out;
     }
 
+    /*
+     * Do not attempt any modifications if creating a policy that aims to be
+     * compatible with pre-4.13 Xen versions.
+     */
+    if (!restore)
+        xc_cpu_policy_sanitize(ctx->xch, policy);
+
     r = xc_cpu_policy_set_domain(ctx->xch, domid, policy);
     if (r) {
         LOGED(ERROR, domid, "Failed to set domain CPUID policy");
diff --git a/tools/tests/cpu-policy/test-cpu-policy.c 
b/tools/tests/cpu-policy/test-cpu-policy.c
index 686d7a886c..20419a6108 100644
--- a/tools/tests/cpu-policy/test-cpu-policy.c
+++ b/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, ...)                          \
 ({                                              \
@@ -671,6 +674,103 @@ static void test_msr_get_entry(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 {
@@ -787,6 +887,7 @@ int main(int argc, char **argv)
     test_cpuid_deserialise_failure();
     test_cpuid_out_of_range_clearing();
     test_cpuid_get_leaf_failure();
+    test_cpuid_maximum_leaf_shrinking();
 
     test_msr_serialise_success();
     test_msr_deserialise_failure();
diff --git a/xen/include/xen/lib/x86/cpuid.h b/xen/include/xen/lib/x86/cpuid.h
index 050cd4f9d1..86cda38986 100644
--- a/xen/include/xen/lib/x86/cpuid.h
+++ b/xen/include/xen/lib/x86/cpuid.h
@@ -386,6 +386,13 @@ 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;
diff --git a/xen/lib/x86/cpuid.c b/xen/lib/x86/cpuid.c
index 924f882fc4..6a943cd91b 100644
--- a/xen/lib/x86/cpuid.c
+++ b/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;
-- 
2.34.1