From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Hans de Goede <hdegoede@redhat.com>
Date: Wed, 13 Nov 2019 13:02:01 +0100
Subject: [PATCH] grub-set-bootflag: Write new env to tmpfile and then rename

Make the grubenv writing code in grub-set-bootflag more robust by
writing the modified grubenv to a tmpfile first and then renaming the
tmpfile over the old grubenv (following symlinks).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
 util/grub-set-bootflag.c | 87 +++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 78 insertions(+), 9 deletions(-)

diff --git a/util/grub-set-bootflag.c b/util/grub-set-bootflag.c
index 3eb04beb5..3b4c25ca2 100644
--- a/util/grub-set-bootflag.c
+++ b/util/grub-set-bootflag.c
@@ -28,7 +28,9 @@
 #include <grub/err.h>
 #include <grub/lib/envblk.h> /* For GRUB_ENVBLK_DEFCFG define */
 #include <errno.h>
+#include <limits.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 
@@ -56,8 +58,10 @@ int main(int argc, char *argv[])
 {
   /* NOTE buf must be at least the longest bootflag length + 4 bytes */
   char env[GRUBENV_SIZE + 1], buf[64], *s;
+  /* +1 for 0 termination, +6 for "XXXXXX" in tmp filename */
+  char env_filename[PATH_MAX + 1], tmp_filename[PATH_MAX + 6 + 1];
   const char *bootflag;
-  int i, len, ret;
+  int i, fd, len, ret;
   FILE *f;
 
   if (argc != 2)
@@ -89,7 +93,32 @@ int main(int argc, char *argv[])
   bootflag = bootflags[i];
   len = strlen (bootflag);
 
-  f = fopen (GRUBENV, "r");
+  /*
+   * Really become root. setuid avoids an user killing us, possibly leaking
+   * the tmpfile. setgid avoids the new grubenv's gid being that of the user.
+   */
+  ret = setuid(0);
+  if (ret)
+    {
+      perror ("Error setuid(0) failed");
+      return 1;
+    }
+
+  ret = setgid(0);
+  if (ret)
+    {
+      perror ("Error setgid(0) failed");
+      return 1;
+    }
+
+  /* Canonicalize GRUBENV filename, resolving symlinks, etc. */
+  if (!realpath(GRUBENV, env_filename))
+    {
+      perror ("Error canonicalizing " GRUBENV " filename");
+      return 1;
+    }
+
+  f = fopen (env_filename, "r");
   if (!f)
     {
       perror ("Error opening " GRUBENV " for reading");
@@ -144,30 +173,70 @@ int main(int argc, char *argv[])
   snprintf(buf, sizeof(buf), "%s=1\n", bootflag);
   memcpy(s, buf, len + 3);
 
-  /* "r+", don't truncate so that the diskspace stays reserved */
-  f = fopen (GRUBENV, "r+");
+
+  /*
+   * Create a tempfile for writing the new env.  Use the canonicalized filename
+   * for the template so that the tmpfile is in the same dir / on same fs.
+   */
+  snprintf(tmp_filename, sizeof(tmp_filename), "%sXXXXXX", env_filename);
+  fd = mkstemp(tmp_filename);
+  if (fd == -1)
+    {
+      perror ("Creating tmpfile failed");
+      return 1;
+    }
+
+  f = fdopen (fd, "w");
   if (!f)
     {
-      perror ("Error opening " GRUBENV " for writing");
+      perror ("Error fdopen of tmpfile failed");
+      unlink(tmp_filename);
       return 1;     
     }
 
   ret = fwrite (env, 1, GRUBENV_SIZE, f);
   if (ret != GRUBENV_SIZE)
     {
-      perror ("Error writing to " GRUBENV);
+      perror ("Error writing tmpfile");
+      unlink(tmp_filename);
       return 1;     
     }
 
   ret = fflush (f);
   if (ret)
     {
-      perror ("Error flushing " GRUBENV);
+      perror ("Error flushing tmpfile");
+      unlink(tmp_filename);
       return 1;     
     }
 
-  fsync (fileno (f));
-  fclose (f);
+  ret = fsync (fileno (f));
+  if (ret)
+    {
+      perror ("Error syncing tmpfile");
+      unlink(tmp_filename);
+      return 1;
+    }
+
+  ret = fclose (f);
+  if (ret)
+    {
+      perror ("Error closing tmpfile");
+      unlink(tmp_filename);
+      return 1;
+    }
+
+  /*
+   * And finally rename the tmpfile with the new env over the old env, the
+   * linux kernel guarantees that this is atomic (from a syscall pov).
+   */
+  ret = rename(tmp_filename, env_filename);
+  if (ret)
+    {
+      perror ("Error renaming tmpfile to " GRUBENV " failed");
+      unlink(tmp_filename);
+      return 1;
+    }
 
   return 0;
 }