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 65d74ce010f..d1c5e28862b 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>

@@ -54,8 +56,10 @@ 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)

@@ -77,7 +81,32 @@ 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");

@@ -132,30 +161,70 @@ 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;

 }