[PATCH] gcc-plugins: latent_entropy: use /dev/urandom

["Jason A. Donenfeld" <Jason@zx2c4.com>]

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.

Fixes: 38addce8b600 ("gcc-plugins: Add latent_entropy plugin")
Cc: stable@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
I'm not super familiar with this plugin or its conventions, so pointers
would be most welcome if something here looks amiss. The decision to
buffer 2k at a time is pretty arbitrary too; I haven't measured usage.

 scripts/gcc-plugins/latent_entropy_plugin.c | 34 +++++++++------------
 1 file changed, 15 insertions(+), 19 deletions(-)

diff --git a/scripts/gcc-plugins/latent_entropy_plugin.c b/scripts/gcc-plugins/latent_entropy_plugin.c
index 589454bce930..f238ba6726b8 100644
--- a/scripts/gcc-plugins/latent_entropy_plugin.c
+++ b/scripts/gcc-plugins/latent_entropy_plugin.c
@@ -82,29 +82,27 @@ __visible int plugin_is_GPL_compatible;
 static GTY(()) tree latent_entropy_decl;
 
 static struct plugin_info latent_entropy_plugin_info = {
-	.version	= "201606141920vanilla",
+	.version	= "202203311920vanilla",
 	.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 rnd_buf[256];
+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 (urandom_fd < 0) {
+		urandom_fd = open("/dev/urandom", O_RDONLY);
+		if (urandom_fd < 0)
+			abort();
 	}
-
-	return ret;
+	if (rnd_idx >= ARRAY_SIZE(rnd_buf)) {
+		if (read(urandom_fd, rnd_buf, sizeof(rnd_buf)) != sizeof(rnd_buf))
+			abort();
+		rnd_idx = 0;
+	}
+	return rnd_buf[rnd_idx++];
 }
 
 static tree tree_get_random_const(tree type)
@@ -537,8 +535,6 @@ 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;
 
-- 
2.35.1