153 lines
4.0 KiB
Diff
153 lines
4.0 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Hans de Goede <hdegoede@redhat.com>
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Date: Wed, 13 Nov 2019 13:02:01 +0100
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Subject: [PATCH] grub-set-bootflag: Write new env to tmpfile and then rename
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Make the grubenv writing code in grub-set-bootflag more robust by
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writing the modified grubenv to a tmpfile first and then renaming the
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tmpfile over the old grubenv (following symlinks).
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Signed-off-by: Hans de Goede <hdegoede@redhat.com>
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---
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util/grub-set-bootflag.c | 87 +++++++++++++++++++++++++++++++++++++++++++-----
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1 file changed, 78 insertions(+), 9 deletions(-)
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diff --git a/util/grub-set-bootflag.c b/util/grub-set-bootflag.c
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index 3eb04beb5e..3b4c25ca2a 100644
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--- a/util/grub-set-bootflag.c
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+++ b/util/grub-set-bootflag.c
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@@ -28,7 +28,9 @@
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#include <grub/err.h>
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#include <grub/lib/envblk.h> /* For GRUB_ENVBLK_DEFCFG define */
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#include <errno.h>
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+#include <limits.h>
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#include <stdio.h>
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+#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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@@ -56,8 +58,10 @@ int main(int argc, char *argv[])
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{
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/* NOTE buf must be at least the longest bootflag length + 4 bytes */
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char env[GRUBENV_SIZE + 1], buf[64], *s;
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+ /* +1 for 0 termination, +6 for "XXXXXX" in tmp filename */
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+ char env_filename[PATH_MAX + 1], tmp_filename[PATH_MAX + 6 + 1];
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const char *bootflag;
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- int i, len, ret;
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+ int i, fd, len, ret;
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FILE *f;
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if (argc != 2)
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@@ -89,7 +93,32 @@ int main(int argc, char *argv[])
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bootflag = bootflags[i];
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len = strlen (bootflag);
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- f = fopen (GRUBENV, "r");
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+ /*
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+ * Really become root. setuid avoids an user killing us, possibly leaking
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+ * the tmpfile. setgid avoids the new grubenv's gid being that of the user.
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+ */
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+ ret = setuid(0);
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+ if (ret)
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+ {
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+ perror ("Error setuid(0) failed");
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+ return 1;
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+ }
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+
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+ ret = setgid(0);
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+ if (ret)
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+ {
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+ perror ("Error setgid(0) failed");
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+ return 1;
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+ }
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+
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+ /* Canonicalize GRUBENV filename, resolving symlinks, etc. */
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+ if (!realpath(GRUBENV, env_filename))
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+ {
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+ perror ("Error canonicalizing " GRUBENV " filename");
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+ return 1;
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+ }
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+
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+ f = fopen (env_filename, "r");
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if (!f)
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{
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perror ("Error opening " GRUBENV " for reading");
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@@ -144,30 +173,70 @@ int main(int argc, char *argv[])
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snprintf(buf, sizeof(buf), "%s=1\n", bootflag);
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memcpy(s, buf, len + 3);
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- /* "r+", don't truncate so that the diskspace stays reserved */
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- f = fopen (GRUBENV, "r+");
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+
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+ /*
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+ * Create a tempfile for writing the new env. Use the canonicalized filename
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+ * for the template so that the tmpfile is in the same dir / on same fs.
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+ */
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+ snprintf(tmp_filename, sizeof(tmp_filename), "%sXXXXXX", env_filename);
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+ fd = mkstemp(tmp_filename);
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+ if (fd == -1)
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+ {
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+ perror ("Creating tmpfile failed");
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+ return 1;
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+ }
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+
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+ f = fdopen (fd, "w");
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if (!f)
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{
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- perror ("Error opening " GRUBENV " for writing");
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+ perror ("Error fdopen of tmpfile failed");
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+ unlink(tmp_filename);
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return 1;
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}
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ret = fwrite (env, 1, GRUBENV_SIZE, f);
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if (ret != GRUBENV_SIZE)
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{
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- perror ("Error writing to " GRUBENV);
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+ perror ("Error writing tmpfile");
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+ unlink(tmp_filename);
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return 1;
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}
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ret = fflush (f);
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if (ret)
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{
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- perror ("Error flushing " GRUBENV);
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+ perror ("Error flushing tmpfile");
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+ unlink(tmp_filename);
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return 1;
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}
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- fsync (fileno (f));
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- fclose (f);
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+ ret = fsync (fileno (f));
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+ if (ret)
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+ {
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+ perror ("Error syncing tmpfile");
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+ unlink(tmp_filename);
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+ return 1;
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+ }
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+
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+ ret = fclose (f);
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+ if (ret)
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+ {
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+ perror ("Error closing tmpfile");
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+ unlink(tmp_filename);
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+ return 1;
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+ }
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+
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+ /*
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+ * And finally rename the tmpfile with the new env over the old env, the
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+ * linux kernel guarantees that this is atomic (from a syscall pov).
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+ */
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+ ret = rename(tmp_filename, env_filename);
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+ if (ret)
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+ {
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+ perror ("Error renaming tmpfile to " GRUBENV " failed");
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+ unlink(tmp_filename);
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+ return 1;
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+ }
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return 0;
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}
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