https://github.com/systemd/systemd-stable/commit/ed46ff2bd6ca21d83cae4a94c3ed752ad1b64cce

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Mon, 3 Jan 2022 18:11:32 +0100
Subject: [PATCH] random-seed: hash together old seed and new seed before
 writing out file

If we're consuming an on-disk seed, we usually write out a new one after
consuming it. In that case, we might be at early boot and the randomness
could be rather poor, and the kernel doesn't guarantee that it'll use
the new randomness right away for us. In order to prevent the new
entropy from getting any worse, hash together the old seed and the new
seed, and replace the final bytes of the new seed with the hash output.
This way, entropy strictly increases and never regresses.

(cherry picked from commit da2862ef06f22fc8d31dafced6d2d6dc14f2ee0b)
--- a/src/random-seed/random-seed.c
+++ b/src/random-seed/random-seed.c
@@ -26,6 +26,7 @@
 #include "random-util.h"
 #include "string-util.h"
 #include "sync-util.h"
+#include "sha256.h"
 #include "util.h"
 #include "xattr-util.h"
 
@@ -106,9 +107,11 @@ static int run(int argc, char *argv[]) {
         _cleanup_close_ int seed_fd = -1, random_fd = -1;
         bool read_seed_file, write_seed_file, synchronous;
         _cleanup_free_ void* buf = NULL;
+        struct sha256_ctx hash_state;
+        uint8_t hash[32];
         size_t buf_size;
         struct stat st;
-        ssize_t k;
+        ssize_t k, l;
         int r;
 
         log_setup();
@@ -242,6 +245,16 @@ static int run(int argc, char *argv[]) {
                         if (r < 0)
                                 log_error_errno(r, "Failed to write seed to /dev/urandom: %m");
                 }
+                /* If we're going to later write out a seed file, initialize a hash state with
+                 * the contents of the seed file we just read, so that the new one can't regress
+                 * in entropy. */
+                if (write_seed_file) {
+                        sha256_init_ctx(&hash_state);
+                        if (k < 0)
+                                k = 0;
+                        sha256_process_bytes(&k, sizeof(k), &hash_state);
+                        sha256_process_bytes(buf, k, &hash_state);
+                }
         }
 
         if (write_seed_file) {
@@ -277,6 +290,17 @@ static int run(int argc, char *argv[]) {
                                                        "Got EOF while reading from /dev/urandom.");
                 }
 
+                /* If we previously read in a seed file, then hash the new seed into the old one,
+                 * and replace the last 32 bytes of the seed with the hash output, so that the
+                 * new seed file can't regress in entropy. */
+                if (read_seed_file) {
+                        sha256_process_bytes(&k, sizeof(k), &hash_state);
+                        sha256_process_bytes(buf, k, &hash_state);
+                        sha256_finish_ctx(&hash_state, hash);
+                        l = MIN(k, 32);
+                        memcpy((uint8_t *)buf + k - l, hash, l);
+                }
+
                 r = loop_write(seed_fd, buf, (size_t) k, false);
                 if (r < 0)
                         return log_error_errno(r, "Failed to write new random seed file: %m");