path: root/debian/patches/84_26-CVE-2020-28014-CVE-2021-27216-Arbitrary-PID-file-cre.patch

From c166890023f56388cb3482cff3def04171a488c4 Mon Sep 17 00:00:00 2001
From: "Heiko Schlittermann (HS12-RIPE)" <hs@schlittermann.de>
Date: Thu, 25 Mar 2021 22:48:09 +0100
Subject: [PATCH 26/29] CVE-2020-28014, CVE-2021-27216: Arbitrary PID file
 creation, clobbering, and deletion

Arbitrary PID file creation, clobbering, and deletion.
Patch provided by Qualys.

(cherry picked from commit 974f32939a922512b27d9f0a8a1cb5dec60e7d37)
(cherry picked from commit 43c6f0b83200b7082353c50187ef75de3704580a)
---
 doc/ChangeLog |   5 +
 src/daemon.c      | 212 ++++++++++++++++++++++++++++++++++++++----
 src/exim.c        |  12 ++-
 test/stderr/0433      |  24 +++++
 4 files changed, 232 insertions(+), 21 deletions(-)

--- a/doc/ChangeLog
+++ b/doc/ChangeLog
@@ -10,13 +10,18 @@ QS/02 PID file creation/deletion: only p
       runtime user.
 
 QS/01 Creation of (database) files in $spool_dir: only uid=0 or the euid of
       the Exim runtime user are allowed to create files.
 
+QS/01 Creation of (database) files in $spool_dir: only uid=0 or the uid of
+      the Exim runtime user are allowed to create files.
 
 HS/01 Handle trailing backslash gracefully. (CVE-2019-15846)
 
+QS/02 PID file creation/deletion: only possible if uid=0 or uid is the Exim
+      runtime user.
+
 
 Since version 4.92
 ------------------
 
 JH/06 Fix buggy handling of autoreply bounce_return_size_limit, and a possible
--- a/src/daemon.c
+++ b/src/daemon.c
@@ -886,10 +886,198 @@ while ((pid = waitpid(-1, &status, WNOHA
     }
   }
 }
 
 
+static void
+set_pid_file_path(void)
+{
+if (override_pid_file_path)
+  pid_file_path = override_pid_file_path;
+
+if (!*pid_file_path)
+  pid_file_path = string_sprintf("%s/exim-daemon.pid", spool_directory);
+
+if (pid_file_path[0] != '/')
+  log_write(0, LOG_PANIC_DIE, "pid file path %s must be absolute\n", pid_file_path);
+}
+
+
+enum pid_op { PID_WRITE, PID_CHECK, PID_DELETE };
+
+/* Do various pid file operations as safe as possible. Ideally we'd just
+drop the privileges for creation of the pid file and not care at all about removal of
+the file. FIXME.
+Returns: true on success, false + errno==EACCES otherwise
+*/
+static BOOL
+operate_on_pid_file(const enum pid_op operation, const pid_t pid)
+{
+char pid_line[sizeof(int) * 3 + 2];
+const int pid_len = snprintf(pid_line, sizeof(pid_line), "%d\n", (int)pid);
+BOOL lines_match = FALSE;
+
+char * path = NULL;
+char * base = NULL;
+char * dir = NULL;
+
+const int dir_flags = O_RDONLY | O_NONBLOCK;
+const int base_flags = O_NOFOLLOW | O_NONBLOCK;
+const mode_t base_mode = 0644;
+struct stat sb;
+
+int cwd_fd = -1;
+int dir_fd = -1;
+int base_fd = -1;
+
+BOOL success = FALSE;
+errno = EACCES;
+
+set_pid_file_path();
+if (!f.running_in_test_harness && real_uid != root_uid && real_uid != exim_uid) goto cleanup;
+if (pid_len < 2 || pid_len >= (int)sizeof(pid_line)) goto cleanup;
+
+path = CS string_copy(pid_file_path);
+if ((base = Ustrrchr(path, '/')) == NULL) /* should not happen, but who knows */
+  log_write(0, LOG_MAIN|LOG_PANIC_DIE, "pid file path \"%s\" does not contain a '/'", pid_file_path);
+
+dir = (base != path) ? path : "/";
+*base++ = '\0';
+
+if (!dir || !*dir || *dir != '/') goto cleanup;
+if (!base || !*base || strchr(base, '/') != NULL) goto cleanup;
+
+cwd_fd = open(".", dir_flags);
+if (cwd_fd < 0 || fstat(cwd_fd, &sb) != 0 || !S_ISDIR(sb.st_mode)) goto cleanup;
+dir_fd = open(dir, dir_flags);
+if (dir_fd < 0 || fstat(dir_fd, &sb) != 0 || !S_ISDIR(sb.st_mode)) goto cleanup;
+
+/* emulate openat */
+if (fchdir(dir_fd) != 0) goto cleanup;
+base_fd = open(base, O_RDONLY | base_flags);
+if (fchdir(cwd_fd) != 0)
+  log_write(0, LOG_MAIN|LOG_PANIC_DIE, "can't return to previous working dir: %s", strerror(errno));
+
+if (base_fd >= 0)
+  {
+  char line[sizeof(pid_line)];
+  ssize_t len = -1;
+
+  if (fstat(base_fd, &sb) != 0 || !S_ISREG(sb.st_mode)) goto cleanup;
+  if ((sb.st_mode & 07777) != base_mode || sb.st_nlink != 1) goto cleanup;
+  if (sb.st_size < 2 || sb.st_size >= (off_t)sizeof(line)) goto cleanup;
+
+  len = read(base_fd, line, sizeof(line));
+  if (len != (ssize_t)sb.st_size) goto cleanup;
+  line[len] = '\0';
+
+  if (strspn(line, "0123456789") != (size_t)len-1) goto cleanup;
+  if (line[len-1] != '\n') goto cleanup;
+  lines_match = (len == pid_len && strcmp(line, pid_line) == 0);
+  }
+
+if (operation == PID_WRITE)
+  {
+  if (!lines_match)
+    {
+    if (base_fd >= 0)
+      {
+      int error = -1;
+      /* emulate unlinkat */
+      if (fchdir(dir_fd) != 0) goto cleanup;
+      error = unlink(base);
+      if (fchdir(cwd_fd) != 0)
+        log_write(0, LOG_MAIN|LOG_PANIC_DIE, "can't return to previous working dir: %s", strerror(errno));
+      if (error) goto cleanup;
+      (void)close(base_fd);
+      base_fd = -1;
+     }
+    /* emulate openat */
+    if (fchdir(dir_fd) != 0) goto cleanup;
+    base_fd = open(base, O_WRONLY | O_CREAT | O_EXCL | base_flags, base_mode);
+    if (fchdir(cwd_fd) != 0)
+        log_write(0, LOG_MAIN|LOG_PANIC_DIE, "can't return to previous working dir: %s", strerror(errno));
+    if (base_fd < 0) goto cleanup;
+    if (fchmod(base_fd, base_mode) != 0) goto cleanup;
+    if (write(base_fd, pid_line, pid_len) != pid_len) goto cleanup;
+    DEBUG(D_any) debug_printf("pid written to %s\n", pid_file_path);
+    }
+  }
+else
+  {
+  if (!lines_match) goto cleanup;
+  if (operation == PID_DELETE)
+    {
+    int error = -1;
+    /* emulate unlinkat */
+    if (fchdir(dir_fd) != 0) goto cleanup;
+    error = unlink(base);
+    if (fchdir(cwd_fd) != 0)
+        log_write(0, LOG_MAIN|LOG_PANIC_DIE, "can't return to previous working dir: %s", strerror(errno));
+    if (error) goto cleanup;
+    }
+  }
+
+success = TRUE;
+errno = 0;
+
+cleanup:
+if (cwd_fd >= 0) (void)close(cwd_fd);
+if (dir_fd >= 0) (void)close(dir_fd);
+if (base_fd >= 0) (void)close(base_fd);
+return success;
+}
+
+
+/* Remove the daemon's pidfile.  Note: runs with root privilege,
+as a direct child of the daemon.  Does not return. */
+
+void
+delete_pid_file(void)
+{
+const BOOL success = operate_on_pid_file(PID_DELETE, getppid());
+
+DEBUG(D_any)
+  debug_printf("delete pid file %s %s: %s\n", pid_file_path,
+    success ? "success" : "failure", strerror(errno));
+
+exim_exit(EXIT_SUCCESS, US"");
+}
+
+
+/* Called by the daemon; exec a child to get the pid file deleted
+since we may require privs for the containing directory */
+
+static void
+daemon_die(void)
+{
+int pid;
+
+DEBUG(D_any) debug_printf("SIGTERM/SIGINT seen\n");
+#if defined(SUPPORT_TLS) && (defined(EXIM_HAVE_INOTIFY) || defined(EXIM_HAVE_KEVENT))
+tls_watch_invalidate();
+#endif
+
+if (f.running_in_test_harness || write_pid)
+  {
+  if ((pid = fork()) == 0)
+    {
+    if (override_pid_file_path)
+      (void)child_exec_exim(CEE_EXEC_PANIC, FALSE, NULL, FALSE, 3,
+	"-oP", override_pid_file_path, "-oPX");
+    else
+      (void)child_exec_exim(CEE_EXEC_PANIC, FALSE, NULL, FALSE, 1, "-oPX");
+
+    /* Control never returns here. */
+    }
+  if (pid > 0)
+    child_close(pid, 1);
+  }
+exim_exit(EXIT_SUCCESS, US"");
+}
+
+
 
 /*************************************************
 *              Exim Daemon Mainline              *
 *************************************************/
 
@@ -1538,32 +1726,18 @@ automatically. Consequently, Exim 4 writ
 
 The variable daemon_write_pid is used to control this. */
 
 if (f.running_in_test_harness || write_pid)
   {
-  FILE *f;
-
-  if (override_pid_file_path)
-    pid_file_path = override_pid_file_path;
-
-  if (pid_file_path[0] == 0)
-    pid_file_path = string_sprintf("%s/exim-daemon.pid", spool_directory);
-
-  if ((f = modefopen(pid_file_path, "wb", 0644)))
-    {
-    (void)fprintf(f, "%d\n", (int)getpid());
-    (void)fclose(f);
-    DEBUG(D_any) debug_printf("pid written to %s\n", pid_file_path);
-    }
-  else
-    DEBUG(D_any)
-      debug_printf("%s\n", string_open_failed(errno, "pid file %s",
-        pid_file_path));
+  const enum pid_op operation = (f.running_in_test_harness
+     || real_uid == root_uid
+     || (real_uid == exim_uid && !override_pid_file_path)) ? PID_WRITE : PID_CHECK;
+  if (!operate_on_pid_file(operation, getpid()))
+    DEBUG(D_any) debug_printf("%s pid file %s: %s\n", (operation == PID_WRITE) ? "write" : "check", pid_file_path, strerror(errno));
   }
 
 /* Set up the handler for SIGHUP, which causes a restart of the daemon. */
-
 sighup_seen = FALSE;
 signal(SIGHUP, sighup_handler);
 
 /* Give up root privilege at this point (assuming that exim_uid and exim_gid
 are not root). The third argument controls the running of initgroups().
--- a/src/exim.c
+++ b/src/exim.c
@@ -3042,12 +3042,20 @@ for (i = 1; i < argc; i++)
 
     else if (Ustrcmp(argrest, "o") == 0) {}
 
     /* -oP <name>: set pid file path for daemon */
 
-    else if (Ustrcmp(argrest, "P") == 0)
-      override_pid_file_path = argv[++i];
+    else if (*argrest == 'P')
+      {
+	if (!f.running_in_test_harness && real_uid != root_uid && real_uid != exim_uid)
+	  exim_fail("exim: only uid=%d or uid=%d can use -oP and -oPX "
+                    "(uid=%d euid=%d | %d)\n",
+                    root_uid, exim_uid, getuid(), geteuid(), real_uid);
+	if (Ustrcmp(argrest, "P") == 0) override_pid_file_path = argv[++i];
+	else if (Ustrcmp(argrest, "PX") == 0) delete_pid_file();
+	else badarg = TRUE;
+      }
 
     /* -or <n>: set timeout for non-SMTP acceptance
        -os <n>: set timeout for SMTP acceptance */
 
     else if (*argrest == 'r' || *argrest == 's')