sysrng-linux: re-open /dev/urandom every time

Prompted by the following comment of Daiki Ueno:
> I also wonder why we keep the fd open for such a long time in the first
> place. Both OpenSSL and NSS have a similar fallback to /dev/urandom
> if getrandom is not available, but opens the device in one-shot,
> when reseeding is needed (and that's pretty rare).
https://gitlab.com/gnutls/gnutls/-/merge_requests/1383#note_521749519

Signed-off-by: Alexander Sosedkin <asosedkin@redhat.com>

diff --git a/lib/nettle/sysrng-linux.c b/lib/nettle/sysrng-linux.c
index 2e17ecdd0c00a95567ce0401543d0051b5906f4e..62024b121f784ca3c658fc04b8eb226a1dd96e71 100644 (file)
--- a/lib/nettle/sysrng-linux.c
+++ b/lib/nettle/sysrng-linux.c
@@ -46,10 +46,6 @@
 #include <sys/time.h>
 #include <fcntl.h>
 
-static int _gnutls_urandom_fd = -1;
-static ino_t _gnutls_urandom_fd_ino = 0;
-static dev_t _gnutls_urandom_fd_rdev = 0;
-
 get_entropy_func _rnd_get_system_entropy = NULL;
 
 #if defined(__linux__)
@@ -127,11 +123,18 @@ static int _rnd_get_system_entropy_urandom(void* _rnd, size_t size)
 {
        uint8_t* rnd = _rnd;
        uint32_t done;
+       int urandom_fd;
+
+       urandom_fd = open("/dev/urandom", O_RDONLY);
+       if (urandom_fd < 0) {
+               _gnutls_debug_log("Cannot open /dev/urandom!\n");
+               return GNUTLS_E_RANDOM_DEVICE_ERROR;
+       }
 
        for (done = 0; done < size;) {
                int res;
                do {
-                       res = read(_gnutls_urandom_fd, rnd + done, size - done);
+                       res = read(urandom_fd, rnd + done, size - done);
                } while (res < 0 && errno == EINTR);
 
                if (res <= 0) {
@@ -145,74 +148,57 @@ static int _rnd_get_system_entropy_urandom(void* _rnd, size_t size)
                                        ("Failed to read /dev/urandom: end of file\n");
                        }
 
+                       close(urandom_fd);
                        return GNUTLS_E_RANDOM_DEVICE_ERROR;
                }
 
                done += res;
        }
 
+       close(urandom_fd);
        return 0;
 }
 
-/* This is called when gnutls_global_init() is called for second time.
- * It must check whether any resources are still available.
- * The particular problem it solves is to verify that the urandom fd is still
- * open (for applications that for some reason closed all fds */
 int _rnd_system_entropy_check(void)
 {
-       int ret;
-       struct stat st;
-
-       if (_gnutls_urandom_fd == -1) /* not using urandom */
-               return 0;
-
-       ret = fstat(_gnutls_urandom_fd, &st);
-       if (ret < 0 || st.st_ino != _gnutls_urandom_fd_ino || st.st_rdev != _gnutls_urandom_fd_rdev) {
-               return _rnd_system_entropy_init();
-       }
+       /* A no-op now when we open and close /dev/urandom every time */
        return 0;
 }
 
 int _rnd_system_entropy_init(void)
 {
-       int old;
-       struct stat st;
+       int urandom_fd;
 
 #if defined(__linux__)
        /* Enable getrandom() usage if available */
        if (have_getrandom()) {
                _rnd_get_system_entropy = _rnd_get_system_entropy_getrandom;
-               _gnutls_debug_log("getrandom random generator was detected\n");
+               _gnutls_debug_log("getrandom random generator was selected\n");
                return 0;
+       } else {
+               _gnutls_debug_log("getrandom is not available\n");
        }
 #endif
 
-       /* First fallback: /dev/unrandom */
-       _gnutls_urandom_fd = open("/dev/urandom", O_RDONLY);
-       if (_gnutls_urandom_fd < 0) {
-               _gnutls_debug_log("Cannot open urandom!\n");
-               return gnutls_assert_val(GNUTLS_E_RANDOM_DEVICE_ERROR);
-       }
+       /* Fallback: /dev/urandom */
 
-       old = fcntl(_gnutls_urandom_fd, F_GETFD);
-       if (old != -1)
-               fcntl(_gnutls_urandom_fd, F_SETFD, old | FD_CLOEXEC);
-
-       if (fstat(_gnutls_urandom_fd, &st) >= 0) {
-               _gnutls_urandom_fd_ino = st.st_ino;
-               _gnutls_urandom_fd_rdev = st.st_rdev;
+       /* Check that we can open it */
+       urandom_fd = open("/dev/urandom", O_RDONLY);
+       if (urandom_fd < 0) {
+               _gnutls_debug_log("Cannot open /dev/urandom during initialization!\n");
+               return gnutls_assert_val(GNUTLS_E_RANDOM_DEVICE_ERROR);
        }
+       close(urandom_fd);
 
        _rnd_get_system_entropy = _rnd_get_system_entropy_urandom;
+       _gnutls_debug_log("/dev/urandom random generator was selected\n");
 
        return 0;
 }
 
 void _rnd_system_entropy_deinit(void)
 {
-       if (_gnutls_urandom_fd >= 0) {
-               close(_gnutls_urandom_fd);
-               _gnutls_urandom_fd = -1;
-       }
+       /* A no-op now when we open and close /dev/urandom every time */
+       return;
 }