Merge pull request #9811 from poettering/random-seed-tweaks

some random seed handling tweaks

src/random-seed/random-seed.c

@@ -6,6 +6,8 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
+#include "sd-id128.h"
+
 #include "alloc-util.h"
 #include "fd-util.h"
 #include "io-util.h"
@@ -15,15 +17,17 @@
 #include "util.h"
 
 #define POOL_SIZE_MIN 512
+#define POOL_SIZE_MAX (10*1024*1024)
 
 int main(int argc, char *argv[]) {
         _cleanup_close_ int seed_fd = -1, random_fd = -1;
+        bool read_seed_file, write_seed_file;
         _cleanup_free_ void* buf = NULL;
         size_t buf_size = 0;
+        struct stat st;
         ssize_t k;
-        int r, open_rw_error;
         FILE *f;
-        bool refresh_seed_file = true;
+        int r;
 
         if (argc != 2) {
                 log_error("This program requires one argument.");
@@ -46,31 +50,25 @@ int main(int argc, char *argv[]) {
                 fclose(f);
         }
 
-        if (buf_size <= POOL_SIZE_MIN)
+        if (buf_size < POOL_SIZE_MIN)
                 buf_size = POOL_SIZE_MIN;
 
-        buf = malloc(buf_size);
-        if (!buf) {
-                r = log_oom();
-                goto finish;
-        }
-
         r = mkdir_parents_label(RANDOM_SEED, 0755);
         if (r < 0) {
                 log_error_errno(r, "Failed to create directory " RANDOM_SEED_DIR ": %m");
                 goto finish;
         }
 
-        /* When we load the seed we read it and write it to the device
+        /* When we load the seed we read it and write it to the device and then immediately update the saved seed with
-         * and then immediately update the saved seed with new data,
+         * new data, to make sure the next boot gets seeded differently. */
-         * to make sure the next boot gets seeded differently. */
 
         if (streq(argv[1], "load")) {
+                int open_rw_error;
 
                 seed_fd = open(RANDOM_SEED, O_RDWR|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
                 open_rw_error = -errno;
                 if (seed_fd < 0) {
-                        refresh_seed_file = false;
+                        write_seed_file = false;
 
                         seed_fd = open(RANDOM_SEED, O_RDONLY|O_CLOEXEC|O_NOCTTY);
                         if (seed_fd < 0) {
@@ -85,10 +83,13 @@ int main(int argc, char *argv[]) {
 
                                 goto finish;
                         }
-                }
+                } else
+                        write_seed_file = true;
 
                 random_fd = open("/dev/urandom", O_RDWR|O_CLOEXEC|O_NOCTTY, 0600);
                 if (random_fd < 0) {
+                        write_seed_file = false;
+
                         random_fd = open("/dev/urandom", O_WRONLY|O_CLOEXEC|O_NOCTTY, 0600);
                         if (random_fd < 0) {
                                 r = log_error_errno(errno, "Failed to open /dev/urandom: %m");
@@ -96,6 +97,50 @@ int main(int argc, char *argv[]) {
                         }
                 }
 
+                read_seed_file = true;
+
+        } else if (streq(argv[1], "save")) {
+
+                random_fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
+                if (random_fd < 0) {
+                        r = log_error_errno(errno, "Failed to open /dev/urandom: %m");
+                        goto finish;
+                }
+
+                seed_fd = open(RANDOM_SEED, O_WRONLY|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
+                if (seed_fd < 0) {
+                        r = log_error_errno(errno, "Failed to open " RANDOM_SEED ": %m");
+                        goto finish;
+                }
+
+                read_seed_file = false;
+                write_seed_file = true;
+
+        } else {
+                log_error("Unknown verb '%s'.", argv[1]);
+                r = -EINVAL;
+                goto finish;
+        }
+
+        if (fstat(seed_fd, &st) < 0) {
+                r = log_error_errno(errno, "Failed to stat() seed file " RANDOM_SEED ": %m");
+                goto finish;
+        }
+
+        /* If the seed file is larger than what we expect, then honour the existing size and save/restore as much as it says */
+        if ((uint64_t) st.st_size > buf_size)
+                buf_size = MIN(st.st_size, POOL_SIZE_MAX);
+
+        buf = malloc(buf_size);
+        if (!buf) {
+                r = log_oom();
+                goto finish;
+        }
+
+        if (read_seed_file) {
+                sd_id128_t mid;
+                int z;
+
                 k = loop_read(seed_fd, buf, buf_size, false);
                 if (k < 0)
                         r = log_error_errno(k, "Failed to read seed from " RANDOM_SEED ": %m");
@@ -110,27 +155,23 @@ int main(int argc, char *argv[]) {
                                 log_error_errno(r, "Failed to write seed to /dev/urandom: %m");
                 }
 
-        } else if (streq(argv[1], "save")) {
+                /* Let's also write the machine ID into the random seed. Why? As an extra protection against "golden
-
+                 * images" that are put together sloppily, i.e. images which are duplicated on multiple systems but
-                seed_fd = open(RANDOM_SEED, O_WRONLY|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
+                 * where the random seed file is not properly reset. Frequently the machine ID is properly reset on
-                if (seed_fd < 0) {
+                 * those systems however (simply because it's easier to notice, if it isn't due to address clashes and
-                        r = log_error_errno(errno, "Failed to open " RANDOM_SEED ": %m");
+                 * so on, while random seed equivalence is generally not noticed easily), hence let's simply write the
-                        goto finish;
+                 * machined ID into the random pool too. */
+                z = sd_id128_get_machine(&mid);
+                if (z < 0)
+                        log_debug_errno(z, "Failed to get machine ID, ignoring: %m");
+                else {
+                        z = loop_write(random_fd, &mid, sizeof(mid), false);
+                        if (z < 0)
+                                log_debug_errno(z, "Failed to write machine ID to /dev/urandom, ignoring: %m");
                 }
-
-                random_fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
-                if (random_fd < 0) {
-                        r = log_error_errno(errno, "Failed to open /dev/urandom: %m");
-                        goto finish;
-                }
-
-        } else {
-                log_error("Unknown verb '%s'.", argv[1]);
-                r = -EINVAL;
-                goto finish;
         }
 
-        if (refresh_seed_file) {
+        if (write_seed_file) {
 
                 /* This is just a safety measure. Given that we are root and
                  * most likely created the file ourselves the mode and owner