Issue #18747: Re-seed OpenSSL's pseudo-random number generator after fork.

A pthread_atfork() child handler is used to seeded the PRNG with pid, time
and some stack data.

1 files changed, 72 insertions, 0 deletions

diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 95ed73d..cd6640f 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -18,6 +18,11 @@
 
 #ifdef WITH_THREAD
 #include "pythread.h"
+
+#ifdef HAVE_PTHREAD_ATFORK
+#  include <pthread.h>
+#endif
+
 #define PySSL_BEGIN_ALLOW_THREADS_S(save) \
     do { if (_ssl_locks_count>0) { (save) = PyEval_SaveThread(); } } while (0)
 #define PySSL_END_ALLOW_THREADS_S(save) \
@@ -2578,7 +2583,69 @@ Queries the entropy gather daemon (EGD) on the socket named by 'path'.\n\
 Returns number of bytes read.  Raises SSLError if connection to EGD\n\
 fails or if it does not provide enough data to seed PRNG.");
 
+/* Seed OpenSSL's PRNG at fork(), http://bugs.python.org/issue18747
+ *
+ * The child handler seeds the PRNG from pseudo-random data like pid, the
+ * current time (nanoseconds, miliseconds or seconds) and an uninitialized
+ * array. The array contains stack variables that are impossible to predict
+ * on most systems, e.g. function return address (subject to ASLR), the
+ * stack protection canary and automatic variables.
+ * The code is inspired by Apache's ssl_rand_seed() function.
+ *
+ * Note:
+ * The code uses pthread_atfork() until Python has a proper atfork API. The
+ * handlers are not removed from the child process.
+ */
+
+#if defined(HAVE_PTHREAD_ATFORK) && defined(WITH_THREAD)
+#define PYSSL_RAND_ATFORK 1
+
+static void
+PySSL_RAND_atfork_child(void)
+{
+    struct {
+        char stack[128];    /* uninitialized (!) stack data, 128 is an
+                               arbitrary number. */
+        pid_t pid;          /* current pid */
+        _PyTime_timeval tp; /* current time */
+    } seed;
+
+#ifdef WITH_VALGRIND
+    VALGRIND_MAKE_MEM_DEFINED(seed.stack, sizeof(seed.stack));
+#endif
+    seed.pid = getpid();
+    _PyTime_gettimeofday(&(seed.tp));
+
+#if 0
+    fprintf(stderr, "PySSL_RAND_atfork_child() seeds %i bytes in pid %i\n",
+            (int)sizeof(seed), seed.pid);
 #endif
+    RAND_add((unsigned char *)&seed, sizeof(seed), 0.0);
+}
+
+static int
+PySSL_RAND_atfork(void)
+{
+    static int registered = 0;
+    int retval;
+
+    if (registered)
+        return 0;
+
+    retval = pthread_atfork(NULL,                     /* prepare */
+                            NULL,                     /* parent */
+                            PySSL_RAND_atfork_child); /* child */
+    if (retval != 0) {
+        PyErr_SetFromErrno(PyExc_OSError);
+        return -1;
+    }
+    registered = 1;
+    return 0;
+}
+#endif /* HAVE_PTHREAD_ATFORK */
+
+#endif /* HAVE_OPENSSL_RAND */
+
 
 
 
@@ -2956,5 +3023,10 @@ PyInit__ssl(void)
     if (r == NULL || PyModule_AddObject(m, "_OPENSSL_API_VERSION", r))
         return NULL;
 
+#ifdef PYSSL_RAND_ATFORK
+    if (PySSL_RAND_atfork() == -1)
+        return NULL;
+#endif
+
     return m;
 }