1 files changed, 160 insertions, 132 deletions

diff --git a/src/crypto/rand/urandom.c b/src/crypto/rand/urandom.c
index 1cc5260..788a979 100644
--- a/src/crypto/rand/urandom.c
+++ b/src/crypto/rand/urandom.c
@@ -30,126 +30,92 @@
 
 
 /* This file implements a PRNG by reading from /dev/urandom, optionally with a
- * buffer, which is unsafe across |fork|. */
-
-#define BUF_SIZE 4096
-
-/* rand_buffer contains unused, random bytes, some of which may have been
- * consumed already. */
+ * fork-safe buffer.
+ *
+ * If buffering is enabled then it maintains a global, linked list of buffers.
+ * Threads which need random bytes grab a buffer from the list under a lock and
+ * copy out the bytes that they need. In the rare case that the buffer is
+ * empty, it's refilled from /dev/urandom outside of the lock.
+ *
+ * Large requests are always serviced from /dev/urandom directly.
+ *
+ * Each buffer contains the PID of the process that created it and it's tested
+ * against the current PID each time. Thus processes that fork will discard all
+ * the buffers filled by the parent process. There are two problems with this:
+ *
+ * 1) glibc maintains a cache of the current PID+PPID and, if this cache isn't
+ *    correctly invalidated, the getpid() will continue to believe that
+ *    it's the old process. Glibc depends on the glibc wrappers for fork,
+ *    vfork and clone being used in order to invalidate the getpid() cache.
+ *
+ * 2) If a process forks, dies and then its child forks, it's possible that
+ *    the third process will end up with the same PID as the original process.
+ *    If the second process never used any random values then this will mean
+ *    that the third process has stale, cached values and won't notice.
+ */
+
+/* BUF_SIZE is intended to be a 4K allocation with malloc overhead. struct
+ * rand_buffer also fits in this space and the remainder is entropy. */
+#define BUF_SIZE (4096 - 16)
+
+/* rand_buffer contains unused, random bytes. These structures form a linked
+ * list via the |next| pointer, which is NULL in the final element. */
 struct rand_buffer {
-  size_t used;
-  uint8_t rand[BUF_SIZE];
+  size_t used; /* used contains the number of bytes of |rand| that have
+                  been consumed. */
+  struct rand_buffer *next;
+  pid_t pid; /* pid contains the pid at the time that the buffer was
+                created so that data is not duplicated after a fork. */
+  pid_t ppid; /* ppid contains the parent pid in order to try and reduce
+                 the possibility of duplicated PID confusing the
+                 detection of a fork. */
+  uint8_t rand[];
 };
 
-/* requested_lock is used to protect the |*_requested| variables. */
-static struct CRYPTO_STATIC_MUTEX requested_lock = CRYPTO_STATIC_MUTEX_INIT;
+/* rand_bytes_per_buf is the number of actual entropy bytes in a buffer. */
+static const size_t rand_bytes_per_buf = BUF_SIZE - sizeof(struct rand_buffer);
 
-/* urandom_fd_requested is set by |RAND_set_urandom_fd|.  It's protected by
- * |requested_lock|. */
-static int urandom_fd_requested = -2;
+static struct CRYPTO_STATIC_MUTEX global_lock = CRYPTO_STATIC_MUTEX_INIT;
 
-/* urandom_fd is a file descriptor to /dev/urandom. It's protected by |once|. */
-static int urandom_fd = -2;
+/* list_head is the start of a global, linked-list of rand_buffer objects. It's
+ * protected by |global_lock|. */
+static struct rand_buffer *list_head;
 
-/* urandom_buffering_requested is set by |RAND_enable_fork_unsafe_buffering|.
- * It's protected by |requested_lock|. */
-static int urandom_buffering_requested = 0;
+/* urandom_fd is a file descriptor to /dev/urandom. It's protected by
+ * |global_lock|. */
+static int urandom_fd = -2;
 
 /* urandom_buffering controls whether buffering is enabled (1) or not (0). This
- * is protected by |once|. */
+ * is protected by |global_lock|. */
 static int urandom_buffering = 0;
 
-static CRYPTO_once_t once = CRYPTO_ONCE_INIT;
-
-/* init_once initializes the state of this module to values previously
- * requested. This is the only function that modifies |urandom_fd| and
- * |urandom_buffering|, whose values may be read safely after calling the
- * once. */
-static void init_once(void) {
-  CRYPTO_STATIC_MUTEX_lock_read(&requested_lock);
-  urandom_buffering = urandom_buffering_requested;
-  int fd = urandom_fd_requested;
-  CRYPTO_STATIC_MUTEX_unlock(&requested_lock);
-
-  if (fd == -2) {
-    do {
-      fd = open("/dev/urandom", O_RDONLY);
-    } while (fd == -1 && errno == EINTR);
+/* urandom_get_fd_locked returns a file descriptor to /dev/urandom. The caller
+ * of this function must hold |global_lock|. */
+static int urandom_get_fd_locked(void) {
+  if (urandom_fd != -2) {
+    return urandom_fd;
   }
 
-  if (fd < 0) {
-    abort();
-  }
-
-  int flags = fcntl(fd, F_GETFD);
-  if (flags == -1) {
-    abort();
-  }
-  flags |= FD_CLOEXEC;
-  if (fcntl(fd, F_SETFD, flags) == -1) {
-    abort();
-  }
-  urandom_fd = fd;
+  urandom_fd = open("/dev/urandom", O_RDONLY);
+  return urandom_fd;
 }
 
-void RAND_cleanup(void) {}
+/* RAND_cleanup frees all buffers, closes any cached file descriptor
+ * and resets the global state. */
+void RAND_cleanup(void) {
+  struct rand_buffer *cur;
 
-void RAND_set_urandom_fd(int fd) {
-  fd = dup(fd);
-  if (fd < 0) {
-    abort();
+  CRYPTO_STATIC_MUTEX_lock_write(&global_lock);
+  while ((cur = list_head)) {
+    list_head = cur->next;
+    OPENSSL_free(cur);
   }
-
-  CRYPTO_STATIC_MUTEX_lock_write(&requested_lock);
-  urandom_fd_requested = fd;
-  CRYPTO_STATIC_MUTEX_unlock(&requested_lock);
-
-  CRYPTO_once(&once, init_once);
-  if (urandom_fd != fd) {
-    abort();  // Already initialized.
+  if (urandom_fd >= 0) {
+    close(urandom_fd);
   }
-}
-
-void RAND_enable_fork_unsafe_buffering(int fd) {
-  if (fd >= 0) {
-    fd = dup(fd);
-    if (fd < 0) {
-      abort();
-    }
-  } else {
-    fd = -2;
-  }
-
-  CRYPTO_STATIC_MUTEX_lock_write(&requested_lock);
-  urandom_buffering_requested = 1;
-  urandom_fd_requested = fd;
-  CRYPTO_STATIC_MUTEX_unlock(&requested_lock);
-
-  CRYPTO_once(&once, init_once);
-  if (urandom_buffering != 1 || (fd >= 0 && urandom_fd != fd)) {
-    abort();  // Already initialized.
-  }
-}
-
-static struct rand_buffer *get_thread_local_buffer(void) {
-  struct rand_buffer *buf =
-      CRYPTO_get_thread_local(OPENSSL_THREAD_LOCAL_URANDOM_BUF);
-  if (buf != NULL) {
-    return buf;
-  }
-
-  buf = OPENSSL_malloc(sizeof(struct rand_buffer));
-  if (buf == NULL) {
-    return NULL;
-  }
-  buf->used = BUF_SIZE;  /* To trigger a |read_full| on first use. */
-  if (!CRYPTO_set_thread_local(OPENSSL_THREAD_LOCAL_URANDOM_BUF, buf,
-                               OPENSSL_free)) {
-    OPENSSL_free(buf);
-    return NULL;
-  }
-
-  return buf;
+  urandom_fd = -2;
+  list_head = NULL;
+  CRYPTO_STATIC_MUTEX_unlock(&global_lock);
 }
 
 /* read_full reads exactly |len| bytes from |fd| into |out| and returns 1. In
@@ -172,48 +138,110 @@ static char read_full(int fd, uint8_t *out, size_t len) {
   return 1;
 }
 
-/* read_from_buffer reads |requested| random bytes from the buffer into |out|,
- * refilling it if necessary to satisfy the request. */
-static void read_from_buffer(struct rand_buffer *buf,
-                             uint8_t *out, size_t requested) {
-  size_t remaining = BUF_SIZE - buf->used;
+/* CRYPTO_sysrand puts |num| random bytes into |out|. */
+void CRYPTO_sysrand(uint8_t *out, size_t requested) {
+  int fd;
+  struct rand_buffer *buf;
+  size_t todo;
+  pid_t pid, ppid;
+
+  if (requested == 0) {
+    return;
+  }
 
-  while (requested > remaining) {
-    memcpy(out, &buf->rand[buf->used], remaining);
-    buf->used += remaining;
-    out += remaining;
-    requested -= remaining;
+  CRYPTO_STATIC_MUTEX_lock_write(&global_lock);
+  fd = urandom_get_fd_locked();
 
-    if (!read_full(urandom_fd, buf->rand, BUF_SIZE)) {
+  if (fd < 0) {
+    CRYPTO_STATIC_MUTEX_unlock(&global_lock);
+    abort();
+    return;
+  }
+
+  /* If buffering is not enabled, or if the request is large, then the
+   * result comes directly from urandom. */
+  if (!urandom_buffering || requested > BUF_SIZE / 2) {
+    CRYPTO_STATIC_MUTEX_unlock(&global_lock);
+    if (!read_full(fd, out, requested)) {
       abort();
-      return;
     }
-    buf->used = 0;
-    remaining = BUF_SIZE;
+    return;
   }
 
-  memcpy(out, &buf->rand[buf->used], requested);
-  buf->used += requested;
-}
+  pid = getpid();
+  ppid = getppid();
 
-/* CRYPTO_sysrand puts |requested| random bytes into |out|. */
-void CRYPTO_sysrand(uint8_t *out, size_t requested) {
-  if (requested == 0) {
-    return;
+  for (;;) {
+    buf = list_head;
+    if (buf && buf->pid == pid && buf->ppid == ppid &&
+        rand_bytes_per_buf - buf->used >= requested) {
+      memcpy(out, &buf->rand[buf->used], requested);
+      buf->used += requested;
+      CRYPTO_STATIC_MUTEX_unlock(&global_lock);
+      return;
+    }
+
+    /* If we don't immediately have enough entropy with the correct
+     * PID, remove the buffer from the list in order to gain
+     * exclusive access and unlock. */
+    if (buf) {
+      list_head = buf->next;
+    }
+    CRYPTO_STATIC_MUTEX_unlock(&global_lock);
+
+    if (!buf) {
+      buf = (struct rand_buffer *)OPENSSL_malloc(BUF_SIZE);
+      if (!buf) {
+        abort();
+        return;
+      }
+      /* The buffer doesn't contain any random bytes yet
+       * so we mark it as fully used so that it will be
+       * filled below. */
+      buf->used = rand_bytes_per_buf;
+      buf->next = NULL;
+      buf->pid = pid;
+      buf->ppid = ppid;
+    }
+
+    if (buf->pid == pid && buf->ppid == ppid) {
+      break;
+    }
+
+    /* We have forked and so cannot use these bytes as they
+     * may have been used in another process. */
+    OPENSSL_free(buf);
+    CRYPTO_STATIC_MUTEX_lock_write(&global_lock);
   }
 
-  CRYPTO_once(&once, init_once);
-  if (urandom_buffering && requested < BUF_SIZE) {
-    struct rand_buffer *buf = get_thread_local_buffer();
-    if (buf != NULL) {
-      read_from_buffer(buf, out, requested);
+  while (requested > 0) {
+    todo = rand_bytes_per_buf - buf->used;
+    if (todo > requested) {
+      todo = requested;
+    }
+    memcpy(out, &buf->rand[buf->used], todo);
+    requested -= todo;
+    out += todo;
+    buf->used += todo;
+
+    if (buf->used < rand_bytes_per_buf) {
+      break;
+    }
+
+    if (!read_full(fd, buf->rand, rand_bytes_per_buf)) {
+      OPENSSL_free(buf);
+      abort();
       return;
     }
-  }
 
-  if (!read_full(urandom_fd, out, requested)) {
-    abort();
+    buf->used = 0;
   }
+
+  CRYPTO_STATIC_MUTEX_lock_write(&global_lock);
+  assert(list_head != buf);
+  buf->next = list_head;
+  list_head = buf;
+  CRYPTO_STATIC_MUTEX_unlock(&global_lock);
 }
 
 #endif  /* !OPENSSL_WINDOWS */