Loader: Implement RandomUtils (for ASLR)

stratosphere/loader/source/ldr_nso.cpp

@@ -2,6 +2,7 @@
 #include <algorithm>
 #include <cstdio>
 #include "ldr_nso.hpp"
+#include "ldr_random.hpp"
 
 static NsoUtils::NsoHeader g_nso_headers[NSO_NUM_MAX] = {0};
 static bool g_nso_present[NSO_NUM_MAX] = {0};
@@ -118,7 +119,7 @@ Result NsoUtils::CalculateNsoLoadExtents(u32 addspace_type, u32 args_size, NsoLo
     }
     
     /* Calculate ASLR extents for address space type. */
-    u64 addspace_start, addspace_size, addspace_end;
+    u64 addspace_start, addspace_size;
     if (kernelAbove200()) {
         switch (addspace_type & 0xE) {
             case 0:
@@ -147,14 +148,13 @@ Result NsoUtils::CalculateNsoLoadExtents(u32 addspace_type, u32 args_size, NsoLo
             addspace_size = 0x3FE00000ULL;
         }
     }
-    addspace_end = addspace_start + addspace_size;
-    if (addspace_start + extents->total_size > addspace_end) {
+    if (extents->total_size > addspace_size) {
         return 0xD001;
     }
     
     u64 aslr_slide = 0;
     if (addspace_type & 0x20) {
-        /* TODO: Apply a random ASLR slide. */
+        aslr_slide = RandomUtils::GetRandomU64((addspace_size - extents->total_size) >> 21) << 21;
     }
     
     extents->base_address = addspace_start + aslr_slide;

stratosphere/loader/source/ldr_random.cpp

@@ -0,0 +1,49 @@
+#include <switch.h>
+
+#include "ldr_random.hpp"
+
+/* Official HOS uses TinyMT. This is high effort. Let's just use XorShift. */
+/* https://en.wikipedia.org/wiki/Xorshift */
+
+static u32 g_random_state[4] = {0};
+static bool g_has_initialized = false;
+
+static void EnsureRandomState() {
+    if (g_has_initialized) {
+        return;
+    }
+    
+    /* Retrieve process entropy with svcGetInfo. */
+    u64 val = 0;
+    for (unsigned int i = 0; i < 4; i++) {
+        if (R_FAILED(svcGetInfo(&val, 0xB, 0, i))) {
+            /* TODO: Panic? */
+        }
+        g_random_state[i] = val & 0xFFFFFFFF;
+    }
+    
+    g_has_initialized = true;
+}
+
+u32 RandomUtils::GetNext() {
+    EnsureRandomState();
+    u32 s, t = g_random_state[3];
+    t ^= t << 11;
+    t ^= t >> 8;
+    g_random_state[3] = g_random_state[2]; g_random_state[2] = g_random_state[1]; g_random_state[1] = (s = g_random_state[0]);
+    t ^= s;
+    t ^= s >> 19;
+    g_random_state[0] = t;
+    return t;
+}
+
+/* These are slightly biased, but I think that's totally okay. */
+u32 RandomUtils::GetRandomU32(u32 max) {
+    return GetNext() % max;
+}
+
+u32 RandomUtils::GetRandomU64(u64 max) {
+    u64 val = GetNext();
+    val |= ((u64)GetNext()) << 32;
+    return val % max;
+}

stratosphere/loader/source/ldr_random.hpp

@@ -0,0 +1,9 @@
+#pragma once
+#include <switch.h>
+
+class RandomUtils {
+    public:
+        static u32 GetNext();
+        static u32 GetRandomU32(u32 max);
+        static u32 GetRandomU64(u64 max);
+};