Set mmap_rnd_bits to maximum value.

Also make sure it is above a minimum threshold, else abort.

Bug: 27681085
Change-Id: Id03ede491e5e013a2a7a090abb306d00cb74f901

init/init.cpp

@@ -18,6 +18,7 @@
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <fstream>
 #include <libgen.h>
 #include <paths.h>
 #include <signal.h>
@@ -290,6 +291,95 @@ ret:
     return result;
 }
 
+static void security_failure() {
+    ERROR("Security failure; rebooting into recovery mode...\n");
+    android_reboot(ANDROID_RB_RESTART2, 0, "recovery");
+    while (true) { pause(); }  // never reached
+}
+
+#define MMAP_RND_PATH "/proc/sys/vm/mmap_rnd_bits"
+#define MMAP_RND_COMPAT_PATH "/proc/sys/vm/mmap_rnd_compat_bits"
+
+/* __attribute__((unused)) due to lack of mips support: see mips block
+ * in set_mmap_rnd_bits_action */
+static bool __attribute__((unused)) set_mmap_rnd_bits_min(int start, int min, bool compat) {
+    std::string path;
+    if (compat) {
+        path = MMAP_RND_COMPAT_PATH;
+    } else {
+        path = MMAP_RND_PATH;
+    }
+    std::ifstream inf(path, std::fstream::in);
+    if (!inf) {
+        return false;
+    }
+    while (start >= min) {
+        // try to write out new value
+        std::string str_val = std::to_string(start);
+        std::ofstream of(path, std::fstream::out);
+        if (!of) {
+            return false;
+        }
+        of << str_val << std::endl;
+        of.close();
+
+        // check to make sure it was recorded
+        inf.seekg(0);
+        std::string str_rec;
+        inf >> str_rec;
+        if (str_val.compare(str_rec) == 0) {
+            break;
+        }
+        start--;
+    }
+    inf.close();
+    return (start >= min);
+}
+
+/*
+ * Set /proc/sys/vm/mmap_rnd_bits and potentially
+ * /proc/sys/vm/mmap_rnd_compat_bits to the maximum supported values.
+ * Returns -1 if unable to set these to an acceptable value.  Apply
+ * upstream patch-sets https://lkml.org/lkml/2015/12/21/337 and
+ * https://lkml.org/lkml/2016/2/4/831 to enable this.
+ */
+static int set_mmap_rnd_bits_action(const std::vector<std::string>& args)
+{
+    int ret = -1;
+
+    /* values are arch-dependent */
+#if defined(__aarch64__)
+    /* arm64 supports 18 - 33 bits depending on pagesize and VA_SIZE */
+    if (set_mmap_rnd_bits_min(33, 24, false)
+            && set_mmap_rnd_bits_min(16, 16, true)) {
+        ret = 0;
+    }
+#elif defined(__x86__64__)
+    /* x86_64 supports 28 - 32 bits */
+    if (set_mmap_rnd_bits_min(32, 32, false)
+            && set_mmap_rnd_bits_min(16, 16, true)) {
+        ret = 0;
+    }
+#elif defined(__arm__) || defined(__i386__)
+    /* check to see if we're running on 64-bit kernel */
+    bool h64 = !access(MMAP_RND_COMPAT_PATH, F_OK);
+    /* supported 32-bit architecture must have 16 bits set */
+    if (set_mmap_rnd_bits_min(16, 16, h64)) {
+        ret = 0;
+    }
+#elif defined(__mips__) || defined(__mips64__)
+    // TODO: add mips support b/27788820
+    ret = 0;
+#else
+    ERROR("Unknown architecture\n");
+#endif
+    if (ret == -1) {
+        ERROR("Unable to set adequate mmap entropy value!\n");
+        security_failure();
+    }
+    return ret;
+}
+
 static int keychord_init_action(const std::vector<std::string>& args)
 {
     keychord_init();
@@ -492,12 +582,6 @@ static int audit_callback(void *data, security_class_t /*cls*/, char *buf, size_
     return 0;
 }
 
-static void security_failure() {
-    ERROR("Security failure; rebooting into recovery mode...\n");
-    android_reboot(ANDROID_RB_RESTART2, 0, "recovery");
-    while (true) { pause(); }  // never reached
-}
-
 static void selinux_initialize(bool in_kernel_domain) {
     Timer t;
 
@@ -646,6 +730,7 @@ int main(int argc, char** argv) {
     am.QueueBuiltinAction(wait_for_coldboot_done_action, "wait_for_coldboot_done");
     // ... so that we can start queuing up actions that require stuff from /dev.
     am.QueueBuiltinAction(mix_hwrng_into_linux_rng_action, "mix_hwrng_into_linux_rng");
+    am.QueueBuiltinAction(set_mmap_rnd_bits_action, "set_mmap_rnd_bits");
     am.QueueBuiltinAction(keychord_init_action, "keychord_init");
     am.QueueBuiltinAction(console_init_action, "console_init");