os.urandom() doesn't block on Linux anymore

Issue #26839: On Linux, os.urandom() now calls getrandom() with GRND_NONBLOCK
to fall back on reading /dev/urandom if the urandom entropy pool is not
initialized yet. Patch written by Colm Buckley.

1 files changed, 21 insertions, 3 deletions

diff --git a/Python/random.c b/Python/random.c
index 79157b8b6f..ecfd44bbda 100644
--- a/Python/random.c
+++ b/Python/random.c
@@ -6,6 +6,9 @@
 #  ifdef HAVE_SYS_STAT_H
 #    include <sys/stat.h>
 #  endif
+#  ifdef HAVE_LINUX_RANDOM_H
+#    include <linux/random.h>
+#  endif
 #  ifdef HAVE_GETRANDOM
 #    include <sys/random.h>
 #  elif defined(HAVE_GETRANDOM_SYSCALL)
@@ -122,9 +125,13 @@ py_getrandom(void *buffer, Py_ssize_t size, int raise)
     /* Is getrandom() supported by the running kernel?
      * Need Linux kernel 3.17 or newer, or Solaris 11.3 or newer */
     static int getrandom_works = 1;
-    /* Use non-blocking /dev/urandom device. On Linux at boot, the getrandom()
-     * syscall blocks until /dev/urandom is initialized with enough entropy. */
-    const int flags = 0;
+
+    /* getrandom() on Linux will block if called before the kernel has
+     * initialized the urandom entropy pool. This will cause Python
+     * to hang on startup if called very early in the boot process -
+     * see https://bugs.python.org/issue26839. To avoid this, use the
+     * GRND_NONBLOCK flag. */
+    const int flags = GRND_NONBLOCK;
     int n;
 
     if (!getrandom_works)
@@ -168,6 +175,17 @@ py_getrandom(void *buffer, Py_ssize_t size, int raise)
                 getrandom_works = 0;
                 return 0;
             }
+            if (errno == EAGAIN) {
+                /* If we failed with EAGAIN, the entropy pool was
+                 * uninitialized. In this case, we return failure to fall
+                 * back to reading from /dev/urandom.
+                 *
+                 * Note: In this case the data read will not be random so
+                 * should not be used for cryptographic purposes. Retaining
+                 * the existing semantics for practical purposes. */
+                getrandom_works = 0;
+                return 0;
+            }
 
             if (errno == EINTR) {
                 if (PyErr_CheckSignals()) {