Check fd status before using urandom

tests/unit/s2n_handshake_test.c

@@ -19,6 +19,7 @@
 #include <fcntl.h>
 #include <stdint.h>
 #include <stdlib.h>
+#include <sys/wait.h>
 #include <unistd.h>
 
 #include "api/s2n.h"
@@ -434,6 +435,41 @@ int main(int argc, char **argv)
         }
     }
 
+    /* Ensure that a handshake can be performed after all file descriptors are closed */
+    {
+        /* A fork is created to ensure that closing file descriptors (like stdout) won't impact
+         * other tests.
+         */
+        pid_t pid = fork();
+        if (pid == 0) {
+            long max_file_descriptors = sysconf(_SC_OPEN_MAX);
+            for (long fd = 0; fd < max_file_descriptors; fd++) {
+                EXPECT_TRUE(fd <= INT_MAX);
+                close((int) fd);
+            }
+
+            DEFER_CLEANUP(struct s2n_cert_chain_and_key *chain_and_key = NULL, s2n_cert_chain_and_key_ptr_free);
+            EXPECT_SUCCESS(s2n_test_cert_chain_and_key_new(&chain_and_key,
+                    S2N_DEFAULT_TEST_CERT_CHAIN, S2N_DEFAULT_TEST_PRIVATE_KEY));
+            EXPECT_NOT_NULL(chain_and_key);
+
+            DEFER_CLEANUP(struct s2n_config *config = s2n_config_new(), s2n_config_ptr_free);
+            EXPECT_NOT_NULL(config);
+            EXPECT_SUCCESS(s2n_config_set_cipher_preferences(config, "default"));
+            EXPECT_SUCCESS(s2n_config_add_cert_chain_and_key_to_store(config, chain_and_key));
+            EXPECT_SUCCESS(s2n_config_set_verification_ca_location(config, S2N_DEFAULT_TEST_CERT_CHAIN, NULL));
+            EXPECT_SUCCESS(s2n_config_disable_x509_verification(config));
+
+            EXPECT_SUCCESS(test_cipher_preferences(config, config, chain_and_key, S2N_SIGNATURE_RSA));
+
+            exit(EXIT_SUCCESS);
+        }
+
+        int status = 0;
+        EXPECT_EQUAL(waitpid(pid, &status, 0), pid);
+        EXPECT_EQUAL(status, EXIT_SUCCESS);
+    }
+
     END_TEST();
     return 0;
 }

tests/unit/s2n_random_test.c

@@ -48,6 +48,10 @@
 #define NUMBER_OF_RANGE_FUNCTION_CALLS 200
 #define MAX_REPEATED_OUTPUT            4
 
+S2N_RESULT s2n_rand_device_validate(struct s2n_rand_device *device);
+S2N_RESULT s2n_rand_get_urandom_for_test(struct s2n_rand_device **device);
+S2N_RESULT s2n_rand_set_urandom_for_test();
+
 struct random_test_case {
     const char *test_case_label;
     int (*test_case_cb)(struct random_test_case *test_case);
@@ -788,6 +792,63 @@ static int s2n_random_rand_bytes_after_cleanup_cb(struct random_test_case *test_
     return S2N_SUCCESS;
 }
 
+static int s2n_random_invalid_urandom_fd_cb(struct random_test_case *test_case)
+{
+    EXPECT_SUCCESS(s2n_disable_atexit());
+
+    struct s2n_rand_device *dev_urandom = NULL;
+    EXPECT_OK(s2n_rand_get_urandom_for_test(&dev_urandom));
+    EXPECT_NOT_NULL(dev_urandom);
+
+    for (size_t test = 0; test <= 1; test++) {
+        EXPECT_EQUAL(dev_urandom->fd, -1);
+
+        /* Validation should fail before initialization. */
+        EXPECT_ERROR(s2n_rand_device_validate(dev_urandom));
+
+        EXPECT_SUCCESS(s2n_init());
+
+        /* Validation should succeed after initialization. */
+        EXPECT_OK(s2n_rand_device_validate(dev_urandom));
+
+        /* Override the mix callback with urandom, in case support for rdrand is detected and enabled. */
+        EXPECT_OK(s2n_rand_set_urandom_for_test());
+
+        EXPECT_TRUE(dev_urandom->fd > STDERR_FILENO);
+        if (test == 0) {
+            /* Close the file descriptor. */
+            EXPECT_EQUAL(close(dev_urandom->fd), 0);
+        } else {
+            /* Make the file descriptor invalid by pointing it to STDERR. */
+            dev_urandom->fd = STDERR_FILENO;
+        }
+
+        /* Validation should fail when the file descriptor is invalid. */
+        EXPECT_ERROR(s2n_rand_device_validate(dev_urandom));
+
+        s2n_stack_blob(rand_data, 16, 16);
+        EXPECT_OK(s2n_get_public_random_data(&rand_data));
+
+        uint64_t public_bytes_used = 0;
+        EXPECT_OK(s2n_get_public_random_bytes_used(&public_bytes_used));
+
+        if (s2n_is_in_fips_mode()) {
+            /* The urandom implementation should not be in use when s2n-tls is in FIPS mode. */
+            EXPECT_EQUAL(public_bytes_used, 0);
+        } else {
+            /* When the urandom implementation is used, the file descriptor is re-opened and
+             * validation should succeed.
+             */
+            EXPECT_OK(s2n_rand_device_validate(dev_urandom));
+            EXPECT_TRUE(public_bytes_used > 0);
+        }
+
+        EXPECT_SUCCESS(s2n_cleanup());
+    }
+
+    return S2N_SUCCESS;
+}
+
 struct random_test_case random_test_cases[] = {
     { "Random API.", s2n_random_test_case_default_cb, CLONE_TEST_DETERMINE_AT_RUNTIME, EXIT_SUCCESS },
     { "Random API without prediction resistance.", s2n_random_test_case_without_pr_cb, CLONE_TEST_DETERMINE_AT_RUNTIME, EXIT_SUCCESS },
@@ -800,6 +861,7 @@ struct random_test_case random_test_cases[] = {
      */
     { "Test failure.", s2n_random_test_case_failure_cb, CLONE_TEST_DETERMINE_AT_RUNTIME, 1 },
     { "Test libcrypto's RAND engine is reset correctly after manual s2n_cleanup()", s2n_random_rand_bytes_after_cleanup_cb, CLONE_TEST_DETERMINE_AT_RUNTIME, EXIT_SUCCESS },
+    { "Test getting entropy with an invalid file descriptor", s2n_random_invalid_urandom_fd_cb, CLONE_TEST_DETERMINE_AT_RUNTIME, EXIT_SUCCESS },
 };
 
 int main(int argc, char **argv)

utils/s2n_random.c

@@ -67,6 +67,7 @@
 #include "crypto/s2n_drbg.h"
 #include "crypto/s2n_fips.h"
 #include "error/s2n_errno.h"
+#include "s2n_io.h"
 #include "stuffer/s2n_stuffer.h"
 #include "utils/s2n_fork_detection.h"
 #include "utils/s2n_init.h"
@@ -75,8 +76,6 @@
 #include "utils/s2n_result.h"
 #include "utils/s2n_safety.h"
 
-#define ENTROPY_SOURCE "/dev/urandom"
-
 #if defined(O_CLOEXEC)
     #define ENTROPY_FLAGS O_RDONLY | O_CLOEXEC
 #else
@@ -92,7 +91,10 @@
 /* Placeholder value for an uninitialized entropy file descriptor */
 #define UNINITIALIZED_ENTROPY_FD -1
 
-static int entropy_fd = UNINITIALIZED_ENTROPY_FD;
+static struct s2n_rand_device s2n_dev_urandom = {
+    .source = "/dev/urandom",
+    .fd = UNINITIALIZED_ENTROPY_FD,
+};
 
 struct s2n_rand_state {
     uint64_t cached_fork_generation_number;
@@ -115,15 +117,23 @@ static __thread struct s2n_rand_state s2n_per_thread_rand_state = {
     .drbgs_initialized = false
 };
 
-static int s2n_rand_init_impl(void);
-static int s2n_rand_cleanup_impl(void);
-static int s2n_rand_urandom_impl(void *ptr, uint32_t size);
-static int s2n_rand_rdrand_impl(void *ptr, uint32_t size);
+static int s2n_rand_init_cb_impl(void);
+static int s2n_rand_cleanup_cb_impl(void);
+static int s2n_rand_get_entropy_from_urandom(void *ptr, uint32_t size);
+static int s2n_rand_get_entropy_from_rdrand(void *ptr, uint32_t size);
+
+static s2n_rand_init_callback s2n_rand_init_cb = s2n_rand_init_cb_impl;
+static s2n_rand_cleanup_callback s2n_rand_cleanup_cb = s2n_rand_cleanup_cb_impl;
+static s2n_rand_seed_callback s2n_rand_seed_cb = s2n_rand_get_entropy_from_urandom;
+static s2n_rand_mix_callback s2n_rand_mix_cb = s2n_rand_get_entropy_from_urandom;
 
-static s2n_rand_init_callback s2n_rand_init_cb = s2n_rand_init_impl;
-static s2n_rand_cleanup_callback s2n_rand_cleanup_cb = s2n_rand_cleanup_impl;
-static s2n_rand_seed_callback s2n_rand_seed_cb = s2n_rand_urandom_impl;
-static s2n_rand_mix_callback s2n_rand_mix_cb = s2n_rand_urandom_impl;
+static int s2n_rand_entropy_fd_close_ptr(int *fd)
+{
+    if (fd && *fd != UNINITIALIZED_ENTROPY_FD) {
+        close(*fd);
+    }
+    return S2N_SUCCESS;
+}
 
 /* non-static for SAW proof */
 bool s2n_cpu_supports_rdrand()
@@ -332,9 +342,76 @@ S2N_RESULT s2n_get_private_random_bytes_used(uint64_t *bytes_used)
     return S2N_RESULT_OK;
 }
 
-static int s2n_rand_urandom_impl(void *ptr, uint32_t size)
+S2N_RESULT s2n_rand_get_urandom_for_test(struct s2n_rand_device **device)
+{
+    RESULT_ENSURE_REF(device);
+    RESULT_ENSURE(s2n_in_unit_test(), S2N_ERR_NOT_IN_UNIT_TEST);
+    *device = &s2n_dev_urandom;
+    return S2N_RESULT_OK;
+}
+
+static S2N_RESULT s2n_rand_device_open(struct s2n_rand_device *device)
+{
+    RESULT_ENSURE_REF(device);
+    RESULT_ENSURE_REF(device->source);
+
+    DEFER_CLEANUP(int fd = -1, s2n_rand_entropy_fd_close_ptr);
+    S2N_IO_RETRY_EINTR(fd, open(device->source, ENTROPY_FLAGS));
+    RESULT_ENSURE(fd >= 0, S2N_ERR_OPEN_RANDOM);
+
+    struct stat st = { 0 };
+    RESULT_ENSURE(fstat(fd, &st) == 0, S2N_ERR_OPEN_RANDOM);
+    device->dev = st.st_dev;
+    device->ino = st.st_ino;
+    device->mode = st.st_mode;
+    device->rdev = st.st_rdev;
+
+    device->fd = fd;
+
+    /* Disable closing the file descriptor with defer cleanup */
+    fd = UNINITIALIZED_ENTROPY_FD;
+
+    return S2N_RESULT_OK;
+}
+
+S2N_RESULT s2n_rand_device_validate(struct s2n_rand_device *device)
+{
+    RESULT_ENSURE_REF(device);
+    RESULT_ENSURE_NE(device->fd, UNINITIALIZED_ENTROPY_FD);
+
+    /* Ensure that the random device is still valid by comparing it to the current file descriptor
+     * status. From:
+     * https://github.com/openssl/openssl/blob/260d97229c467d17934ca3e2e0455b1b5c0994a6/providers/implementations/rands/seeding/rand_unix.c#L513
+     */
+    struct stat st = { 0 };
+    RESULT_ENSURE(fstat(device->fd, &st) == 0, S2N_ERR_OPEN_RANDOM);
+    RESULT_ENSURE_EQ(device->dev, st.st_dev);
+    RESULT_ENSURE_EQ(device->ino, st.st_ino);
+    RESULT_ENSURE_EQ(device->rdev, st.st_rdev);
+
+    /* Ensure that the mode is the same (equal to 0 when xor'd), but don't check the permission bits. */
+    mode_t permission_mask = ~(S_IRWXU | S_IRWXG | S_IRWXO);
+    RESULT_ENSURE_EQ((device->mode ^ st.st_mode) & permission_mask, 0);
+
+    return S2N_RESULT_OK;
+}
+
+static int s2n_rand_get_entropy_from_urandom(void *ptr, uint32_t size)
 {
-    POSIX_ENSURE(entropy_fd != UNINITIALIZED_ENTROPY_FD, S2N_ERR_NOT_INITIALIZED);
+    POSIX_ENSURE_REF(ptr);
+    POSIX_ENSURE(s2n_dev_urandom.fd != UNINITIALIZED_ENTROPY_FD, S2N_ERR_NOT_INITIALIZED);
+
+    /* It's possible that the file descriptor pointing to /dev/urandom was closed or changed from
+     * when it was last opened. Ensure that the file descriptor is still valid, and if it isn't,
+     * re-open it before getting entropy.
+     *
+     * If the file descriptor is invalid and the process doesn't have access to /dev/urandom (as is
+     * the case within a chroot tree), an error is raised here before attempting to indefinitely
+     * read.
+     */
+    if (s2n_result_is_error(s2n_rand_device_validate(&s2n_dev_urandom))) {
+        POSIX_GUARD_RESULT(s2n_rand_device_open(&s2n_dev_urandom));
+    }
 
     uint8_t *data = ptr;
     uint32_t n = size;
@@ -343,7 +420,7 @@ static int s2n_rand_urandom_impl(void *ptr, uint32_t size)
 
     while (n) {
         errno = 0;
-        int r = read(entropy_fd, data, n);
+        int r = read(s2n_dev_urandom.fd, data, n);
         if (r <= 0) {
             /*
              * A non-blocking read() on /dev/urandom should "never" fail,
@@ -450,19 +527,16 @@ RAND_METHOD s2n_openssl_rand_method = {
 };
 #endif
 
-static int s2n_rand_init_impl(void)
+static int s2n_rand_init_cb_impl(void)
 {
-OPEN:
-    entropy_fd = open(ENTROPY_SOURCE, ENTROPY_FLAGS);
-    if (entropy_fd == -1) {
-        if (errno == EINTR) {
-            goto OPEN;
-        }
-        POSIX_BAIL(S2N_ERR_OPEN_RANDOM);
-    }
+    /* Currently, s2n-tls may mix in entropy from urandom into every generation of random data. The
+     * file descriptor is opened on initialization for better performance reading from urandom, and
+     * to ensure that urandom is accessible from within a chroot tree.
+     */
+    POSIX_GUARD_RESULT(s2n_rand_device_open(&s2n_dev_urandom));
 
     if (s2n_cpu_supports_rdrand()) {
-        s2n_rand_mix_cb = s2n_rand_rdrand_impl;
+        s2n_rand_mix_cb = s2n_rand_get_entropy_from_rdrand;
     }
 
     return S2N_SUCCESS;
@@ -502,12 +576,14 @@ S2N_RESULT s2n_rand_init(void)
     return S2N_RESULT_OK;
 }
 
-static int s2n_rand_cleanup_impl(void)
+static int s2n_rand_cleanup_cb_impl(void)
 {
-    POSIX_ENSURE(entropy_fd != UNINITIALIZED_ENTROPY_FD, S2N_ERR_NOT_INITIALIZED);
+    POSIX_ENSURE(s2n_dev_urandom.fd != UNINITIALIZED_ENTROPY_FD, S2N_ERR_NOT_INITIALIZED);
 
-    POSIX_GUARD(close(entropy_fd));
-    entropy_fd = UNINITIALIZED_ENTROPY_FD;
+    if (s2n_result_is_ok(s2n_rand_device_validate(&s2n_dev_urandom))) {
+        POSIX_GUARD(close(s2n_dev_urandom.fd));
+    }
+    s2n_dev_urandom.fd = UNINITIALIZED_ENTROPY_FD;
 
     return S2N_SUCCESS;
 }
@@ -530,10 +606,10 @@ S2N_RESULT s2n_rand_cleanup(void)
     }
 #endif
 
-    s2n_rand_init_cb = s2n_rand_init_impl;
-    s2n_rand_cleanup_cb = s2n_rand_cleanup_impl;
-    s2n_rand_seed_cb = s2n_rand_urandom_impl;
-    s2n_rand_mix_cb = s2n_rand_urandom_impl;
+    s2n_rand_init_cb = s2n_rand_init_cb_impl;
+    s2n_rand_cleanup_cb = s2n_rand_cleanup_cb_impl;
+    s2n_rand_seed_cb = s2n_rand_get_entropy_from_urandom;
+    s2n_rand_mix_cb = s2n_rand_get_entropy_from_urandom;
 
     return S2N_RESULT_OK;
 }
@@ -571,11 +647,18 @@ S2N_RESULT s2n_set_private_drbg_for_test(struct s2n_drbg drbg)
     return S2N_RESULT_OK;
 }
 
+S2N_RESULT s2n_rand_set_urandom_for_test()
+{
+    RESULT_ENSURE(s2n_in_unit_test(), S2N_ERR_NOT_IN_UNIT_TEST);
+    s2n_rand_mix_cb = s2n_rand_get_entropy_from_urandom;
+    return S2N_RESULT_OK;
+}
+
 /*
  * volatile is important to prevent the compiler from
  * re-ordering or optimizing the use of RDRAND.
  */
-static int s2n_rand_rdrand_impl(void *data, uint32_t size)
+static int s2n_rand_get_entropy_from_rdrand(void *data, uint32_t size)
 {
 #if defined(__x86_64__) || defined(__i386__)
     struct s2n_blob out = { 0 };

utils/s2n_random.h

@@ -19,6 +19,15 @@
 #include "utils/s2n_blob.h"
 #include "utils/s2n_result.h"
 
+struct s2n_rand_device {
+    const char *source;
+    int fd;
+    dev_t dev;
+    ino_t ino;
+    mode_t mode;
+    dev_t rdev;
+};
+
 S2N_RESULT s2n_rand_init(void);
 S2N_RESULT s2n_rand_cleanup(void);
 S2N_RESULT s2n_get_seed_entropy(struct s2n_blob *blob);