[PATCH v6 02/12] libx86: introduce helper to fetch cpuid leaf

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

Introduce a helper based on the current Xen guest_cpuid code in order
to fetch a cpuid leaf from a policy. The newly introduced function in
cpuid.c should not be directly called and instead the provided
x86_cpuid_get_leaf macro should be used that will properly deal with
const and non-const inputs.

Also add a test to check that the introduced helper doesn't go over
the bounds of the policy.

Note the code in x86_cpuid_copy_from_buffer is not switched to use the
new function because of the boundary checks against the max fields of
the policy, which might not be properly set at the point where
x86_cpuid_copy_from_buffer get called, for example when filling an
empty policy from scratch.

Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Changes since v4:
 - Rename _x86_cpuid_get_leaf to x86_cpuid_get_leaf_const.

Changes since v3:
 - New in this version.
---
Regarding safety of the usage of array_access_nospec to obtain a
pointer to an element of an array, there are already other instances
of this usage, for example in viridian_time_wrmsr, so I would assume
this is fine.
---
 tools/tests/cpu-policy/test-cpu-policy.c | 75 ++++++++++++++++++++++++
 xen/arch/x86/cpuid.c                     | 55 +++--------------
 xen/include/xen/lib/x86/cpuid.h          | 19 ++++++
 xen/lib/x86/cpuid.c                      | 52 ++++++++++++++++
 4 files changed, 153 insertions(+), 48 deletions(-)

diff --git a/tools/tests/cpu-policy/test-cpu-policy.c 
b/tools/tests/cpu-policy/test-cpu-policy.c
index ed450a0997..3f777fc1fc 100644
--- a/tools/tests/cpu-policy/test-cpu-policy.c
+++ b/tools/tests/cpu-policy/test-cpu-policy.c
@@ -570,6 +570,80 @@ static void test_cpuid_out_of_range_clearing(void)
     }
 }
 
+static void test_cpuid_get_leaf_failure(void)
+{
+    static const struct test {
+        struct cpuid_policy p;
+        const char *name;
+        uint32_t leaf, subleaf;
+    } tests[] = {
+        /* Bound checking logic. */
+        {
+            .name = "Basic max leaf >= array size",
+            .p = {
+                .basic.max_leaf = CPUID_GUEST_NR_BASIC,
+            },
+        },
+        {
+            .name = "Feature max leaf >= array size",
+            .p = {
+                .basic.max_leaf = CPUID_GUEST_NR_BASIC - 1,
+                .feat.max_subleaf = CPUID_GUEST_NR_FEAT,
+            },
+            .leaf = 0x00000007,
+        },
+        {
+            .name = "Extended max leaf >= array size",
+            .p = {
+                .extd.max_leaf = 0x80000000 + CPUID_GUEST_NR_EXTD,
+            },
+            .leaf = 0x80000000,
+        },
+
+        {
+            .name = "Basic leaf >= max leaf",
+            .p = {
+                .basic.max_leaf = CPUID_GUEST_NR_BASIC - 1,
+            },
+            .leaf = CPUID_GUEST_NR_BASIC,
+        },
+        {
+            .name = "Feature leaf >= max leaf",
+            .p = {
+                .basic.max_leaf = CPUID_GUEST_NR_BASIC - 1,
+                .feat.max_subleaf = CPUID_GUEST_NR_FEAT - 1,
+            },
+            .leaf = 0x00000007,
+            .subleaf = CPUID_GUEST_NR_FEAT,
+        },
+        {
+            .name = "Extended leaf >= max leaf",
+            .p = {
+                .extd.max_leaf = 0x80000000 + CPUID_GUEST_NR_EXTD - 1,
+            },
+            .leaf = 0x80000000 + CPUID_GUEST_NR_EXTD,
+        },
+    };
+    const struct cpuid_policy pc;
+    const struct cpuid_leaf *lc;
+    struct cpuid_policy p;
+    struct cpuid_leaf *l;
+
+    /* Constness build test. */
+    lc = x86_cpuid_get_leaf(&pc, 0, 0);
+    l = x86_cpuid_get_leaf(&p, 0, 0);
+
+    printf("Testing CPUID get leaf bound checking:\n");
+
+    for ( size_t i = 0; i < ARRAY_SIZE(tests); ++i )
+    {
+        const struct test *t = &tests[i];
+
+        if ( x86_cpuid_get_leaf(&t->p, t->leaf, t->subleaf) )
+            fail("  Test %s get leaf fail\n", t->name);
+    }
+}
+
 static void test_is_compatible_success(void)
 {
     static struct test {
@@ -685,6 +759,7 @@ int main(int argc, char **argv)
     test_cpuid_serialise_success();
     test_cpuid_deserialise_failure();
     test_cpuid_out_of_range_clearing();
+    test_cpuid_get_leaf_failure();
 
     test_msr_serialise_success();
     test_msr_deserialise_failure();
diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index b5af48324a..0407a54626 100644
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -825,48 +825,16 @@ void guest_cpuid(const struct vcpu *v, uint32_t leaf,
     switch ( leaf )
     {
     case 0 ... CPUID_GUEST_NR_BASIC - 1:
-        ASSERT(p->basic.max_leaf < ARRAY_SIZE(p->basic.raw));
-        if ( leaf > min_t(uint32_t, p->basic.max_leaf,
-                          ARRAY_SIZE(p->basic.raw) - 1) )
-            return;
-
-        switch ( leaf )
-        {
-        case 0x4:
-            if ( subleaf >= ARRAY_SIZE(p->cache.raw) )
-                return;
-
-            *res = array_access_nospec(p->cache.raw, subleaf);
-            break;
-
-        case 0x7:
-            ASSERT(p->feat.max_subleaf < ARRAY_SIZE(p->feat.raw));
-            if ( subleaf > min_t(uint32_t, p->feat.max_subleaf,
-                                 ARRAY_SIZE(p->feat.raw) - 1) )
-                return;
-
-            *res = array_access_nospec(p->feat.raw, subleaf);
-            break;
-
-        case 0xb:
-            if ( subleaf >= ARRAY_SIZE(p->topo.raw) )
-                return;
-
-            *res = array_access_nospec(p->topo.raw, subleaf);
-            break;
-
-        case XSTATE_CPUID:
-            if ( !p->basic.xsave || subleaf >= ARRAY_SIZE(p->xstate.raw) )
-                return;
+    case 0x80000000 ... 0x80000000 + CPUID_GUEST_NR_EXTD - 1:
+    {
+        const struct cpuid_leaf *tmp = x86_cpuid_get_leaf(p, leaf, subleaf);
 
-            *res = array_access_nospec(p->xstate.raw, subleaf);
-            break;
+        if ( !tmp )
+            return;
 
-        default:
-            *res = array_access_nospec(p->basic.raw, leaf);
-            break;
-        }
+        *res = *tmp;
         break;
+    }
 
     case 0x40000000 ... 0x400000ff:
         if ( is_viridian_domain(d) )
@@ -881,15 +849,6 @@ void guest_cpuid(const struct vcpu *v, uint32_t leaf,
     case 0x40000100 ... 0x400001ff:
         return cpuid_hypervisor_leaves(v, leaf, subleaf, res);
 
-    case 0x80000000 ... 0x80000000 + CPUID_GUEST_NR_EXTD - 1:
-        ASSERT((p->extd.max_leaf & 0xffff) < ARRAY_SIZE(p->extd.raw));
-        if ( (leaf & 0xffff) > min_t(uint32_t, p->extd.max_leaf & 0xffff,
-                                     ARRAY_SIZE(p->extd.raw) - 1) )
-            return;
-
-        *res = array_access_nospec(p->extd.raw, leaf & 0xffff);
-        break;
-
     default:
         return;
     }
diff --git a/xen/include/xen/lib/x86/cpuid.h b/xen/include/xen/lib/x86/cpuid.h
index a4d254ea96..050cd4f9d1 100644
--- a/xen/include/xen/lib/x86/cpuid.h
+++ b/xen/include/xen/lib/x86/cpuid.h
@@ -431,6 +431,25 @@ int x86_cpuid_copy_from_buffer(struct cpuid_policy *policy,
                                uint32_t nr_entries, uint32_t *err_leaf,
                                uint32_t *err_subleaf);
 
+/**
+ * Get a cpuid leaf from a policy object.
+ *
+ * @param policy      The cpuid_policy object.
+ * @param leaf        The leaf index.
+ * @param subleaf     The subleaf index.
+ * @returns a pointer to the requested leaf or NULL in case of error.
+ *
+ * The function will perform out of bound checks. Do not call this function
+ * directly and instead use x86_cpuid_get_leaf that will deal with both const
+ * and non-const policies returning a pointer with constness matching that of
+ * the input.
+ */
+const struct cpuid_leaf *x86_cpuid_get_leaf_const(const struct cpuid_policy *p,
+                                                  uint32_t leaf,
+                                                  uint32_t subleaf);
+#define x86_cpuid_get_leaf(p, l, s) \
+    ((__typeof__(&(p)->basic.raw[0]))x86_cpuid_get_leaf_const(p, l, s))
+
 #endif /* !XEN_LIB_X86_CPUID_H */
 
 /*
diff --git a/xen/lib/x86/cpuid.c b/xen/lib/x86/cpuid.c
index 8eb88314f5..924f882fc4 100644
--- a/xen/lib/x86/cpuid.c
+++ b/xen/lib/x86/cpuid.c
@@ -493,6 +493,58 @@ int x86_cpuid_copy_from_buffer(struct cpuid_policy *p,
     return -ERANGE;
 }
 
+const struct cpuid_leaf *x86_cpuid_get_leaf_const(const struct cpuid_policy *p,
+                                                  uint32_t leaf,
+                                                  uint32_t subleaf)
+{
+    switch ( leaf )
+    {
+    case 0 ... CPUID_GUEST_NR_BASIC - 1:
+        if ( p->basic.max_leaf >= ARRAY_SIZE(p->basic.raw) ||
+             leaf > p->basic.max_leaf )
+            return NULL;
+
+        switch ( leaf )
+        {
+        case 0x4:
+            if ( subleaf >= ARRAY_SIZE(p->cache.raw) )
+                return NULL;
+
+            return &array_access_nospec(p->cache.raw, subleaf);
+
+        case 0x7:
+            if ( p->feat.max_subleaf >= ARRAY_SIZE(p->feat.raw) ||
+                 subleaf > p->feat.max_subleaf )
+                return NULL;
+
+            return &array_access_nospec(p->feat.raw, subleaf);
+
+        case 0xb:
+            if ( subleaf >= ARRAY_SIZE(p->topo.raw) )
+                return NULL;
+
+            return &array_access_nospec(p->topo.raw, subleaf);
+
+        case 0xd:
+            if ( !p->basic.xsave || subleaf >= ARRAY_SIZE(p->xstate.raw) )
+                return NULL;
+
+            return &array_access_nospec(p->xstate.raw, subleaf);
+        }
+
+        return &array_access_nospec(p->basic.raw, leaf);
+
+    case 0x80000000 ... 0x80000000 + CPUID_GUEST_NR_EXTD - 1:
+        if ( (p->extd.max_leaf & 0xffff) >= ARRAY_SIZE(p->extd.raw) ||
+             leaf > p->extd.max_leaf )
+            return NULL;
+
+        return &array_access_nospec(p->extd.raw, leaf & 0xffff);
+    }
+
+    return NULL;
+}
+
 /*
  * Local variables:
  * mode: C
-- 
2.34.1