journalctl: verify sealed log epochs are continuous

Currently empty epochs are not sealed. This allows an attacker to truncate
a sealed log and continue it without any problems showing when verifying the
log.

This partially addresses CVE-2023-31438. One way to extend this change to
address CVE-2023-31438 completely, would be to verify that there is exactly
one seal per epoch (and not sealing when the epoch has not ended yet).

the change also adds a journal-file flag: HEADER_COMPATIBLE_SEALED_CONTINUOUS
this flag indicates that a journal file is sealed continuously and decides whether
any missing crypto epochs should trigger a warning or an error.

src/shared/journal-file-util.c

@@ -379,20 +379,23 @@ bool journal_file_is_offlining(JournalFile *f) {
         return true;
 }
 
+void journal_file_write_final_tag(JournalFile *f) {
+        assert(f);
+#if HAVE_GCRYPT
+        if (!JOURNAL_HEADER_SEALED(f->header) || !journal_file_writable(f))
+                return;
+
+        int r = journal_file_append_tag(f);
+        if (r < 0)
+                log_debug_errno(r, "Failed to append tag when closing journal: %m");
+#endif
+}
+
 JournalFile* journal_file_offline_close(JournalFile *f) {
         if (!f)
                 return NULL;
 
-#if HAVE_GCRYPT
-        /* Write the final tag */
-        if (JOURNAL_HEADER_SEALED(f->header) && journal_file_writable(f)) {
-                int r;
-
-                r = journal_file_append_tag(f);
-                if (r < 0)
-                        log_error_errno(r, "Failed to append tag when closing journal: %m");
-        }
-#endif
+        journal_file_write_final_tag(f);
 
         if (sd_event_source_get_enabled(f->post_change_timer, NULL) > 0)
                 journal_file_post_change(f);
@@ -435,6 +438,7 @@ int journal_file_rotate(
         assert(f);
         assert(*f);
 
+        journal_file_write_final_tag(*f);
         r = journal_file_archive(*f, &path);
         if (r < 0)
                 return r;

src/shared/journal-file-util.h

@@ -5,6 +5,7 @@
 
 int journal_file_set_offline(JournalFile *f, bool wait);
 bool journal_file_is_offlining(JournalFile *f);
+void journal_file_write_final_tag(JournalFile *f);
 JournalFile* journal_file_offline_close(JournalFile *f);
 DEFINE_TRIVIAL_CLEANUP_FUNC(JournalFile*, journal_file_offline_close);