gcc-plugins: latent_entropy: use /dev/urandom

While the latent entropy plugin mostly doesn't derive entropy from
get_random_const() for measuring the call graph, when __latent_entropy is
applied to a constant, then it's initialized statically to output from
get_random_const(). In that case, this data is derived from a 64-bit
seed, which means a buffer of 512 bits doesn't really have that amount
of compile-time entropy.

This patch fixes that shortcoming by just buffering chunks of
/dev/urandom output and doling it out as requested.

At the same time, it's important that we don't break the use of
-frandom-seed, for people who want the runtime benefits of the latent
entropy plugin, while still having compile-time determinism. In that
case, we detect whether gcc's set_random_seed() has been called by
making a call to get_random_seed(noinit=true) in the plugin init
function, which is called after set_random_seed() is called but before
anything that calls get_random_seed(noinit=false), and seeing if it's
zero or not. If it's not zero, we're in deterministic mode, and so we
just generate numbers with a basic xorshift prng.

Note that we don't detect if -frandom-seed is being used using the
documented local_tick variable, because it's assigned via:
   local_tick = (unsigned) tv.tv_sec * 1000 + tv.tv_usec / 1000;
which may well overflow and become -1 on its own, and so isn't
reliable: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105171

[kees: The 256 byte rnd_buf size was chosen based on average (250),
 median (64), and std deviation (575) bytes of used entropy for a
 defconfig x86_64 build]

Fixes: 38addce8b600 ("gcc-plugins: Add latent_entropy plugin")
Cc: stable@vger.kernel.org
Cc: PaX Team <pageexec@freemail.hu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220405222815.21155-1-Jason@zx2c4.com

diff --git a/scripts/gcc-plugins/latent_entropy_plugin.c b/scripts/gcc-plugins/latent_entropy_plugin.c
index 589454bce93013e5fb0802873f0589b00721b20b..8425da41de0dab49c33c2f93e4eb7f1ccf095d79 100644 (file)
--- a/scripts/gcc-plugins/latent_entropy_plugin.c
+++ b/scripts/gcc-plugins/latent_entropy_plugin.c
@@ -86,25 +86,31 @@ static struct plugin_info latent_entropy_plugin_info = {
        .help           = "disable\tturn off latent entropy instrumentation\n",
 };
 
-static unsigned HOST_WIDE_INT seed;
-/*
- * get_random_seed() (this is a GCC function) generates the seed.
- * This is a simple random generator without any cryptographic security because
- * the entropy doesn't come from here.
- */
+static unsigned HOST_WIDE_INT deterministic_seed;
+static unsigned HOST_WIDE_INT rnd_buf[32];
+static size_t rnd_idx = ARRAY_SIZE(rnd_buf);
+static int urandom_fd = -1;
+
 static unsigned HOST_WIDE_INT get_random_const(void)
 {
-       unsigned int i;
-       unsigned HOST_WIDE_INT ret = 0;
-
-       for (i = 0; i < 8 * sizeof(ret); i++) {
-               ret = (ret << 1) | (seed & 1);
-               seed >>= 1;
-               if (ret & 1)
-                       seed ^= 0xD800000000000000ULL;
+       if (deterministic_seed) {
+               unsigned HOST_WIDE_INT w = deterministic_seed;
+               w ^= w << 13;
+               w ^= w >> 7;
+               w ^= w << 17;
+               deterministic_seed = w;
+               return deterministic_seed;
        }
 
-       return ret;
+       if (urandom_fd < 0) {
+               urandom_fd = open("/dev/urandom", O_RDONLY);
+               gcc_assert(urandom_fd >= 0);
+       }
+       if (rnd_idx >= ARRAY_SIZE(rnd_buf)) {
+               gcc_assert(read(urandom_fd, rnd_buf, sizeof(rnd_buf)) == sizeof(rnd_buf));
+               rnd_idx = 0;
+       }
+       return rnd_buf[rnd_idx++];
 }
 
 static tree tree_get_random_const(tree type)
@@ -537,8 +543,6 @@ static void latent_entropy_start_unit(void *gcc_data __unused,
        tree type, id;
        int quals;
 
-       seed = get_random_seed(false);
-
        if (in_lto_p)
                return;
 
@@ -573,6 +577,12 @@ __visible int plugin_init(struct plugin_name_args *plugin_info,
        const struct plugin_argument * const argv = plugin_info->argv;
        int i;
 
+       /*
+        * Call get_random_seed() with noinit=true, so that this returns
+        * 0 in the case where no seed has been passed via -frandom-seed.
+        */
+       deterministic_seed = get_random_seed(true);
+
        static const struct ggc_root_tab gt_ggc_r_gt_latent_entropy[] = {
                {
                        .base = &latent_entropy_decl,