From 71b0389fbb31833d827f5f0fec18880c2f602753 Mon Sep 17 00:00:00 2001
From: Alexander Bokovoy <abokovoy@redhat.com>
Date: Thu, 19 May 2022 13:52:22 +0300
Subject: [PATCH] mkhomedir: add support for pre-CVE-2020-10737 behavior
Pre-CVE-2020-10737 behavior was used to allow creating home directories
on NFS mounts when non-Kerberos authentication method is in use. This is
exactly the case where a race condition addressed by the CVE-2020-10737
fix could have happened. However, there are legit use cases where this
setup is needed.
Add '-f' option to mkhomedir helper to activate previous behavior. In
order to enable it, a change to oddjobd-mkhomedir.conf configuration
file is needed by explicitly adding '-f' option to the executable file
definition.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2050079
Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
---
src/mkhomedir.c | 16 +++++++++++++---
src/oddjobd-mkhomedir.conf.5.in | 9 +++++++++
2 files changed, 22 insertions(+), 3 deletions(-)
diff --git a/src/mkhomedir.c b/src/mkhomedir.c
index be85959..ac813a9 100644
--- a/src/mkhomedir.c
+++ b/src/mkhomedir.c
@@ -53,9 +53,11 @@ static const char *skel;
static const char *skel_dir;
static struct passwd *pwd;
static mode_t override_umask;
+static int owner_mkdir_first = 0;
#define FLAG_POPULATE (1 << 0)
#define FLAG_QUIET (1 << 1)
+#define FLAG_OWNER_MKDIR_FIRST (1 << 2)
/* Given the path of an item somewhere in the skeleton directory, create as
* identical as possible a copy in the destination tree. */
@@ -158,7 +160,7 @@ copy_single_item(const char *source, const struct stat *sb,
* target user just yet to avoid potential race conditions
* involving symlink attacks when we copy over the skeleton
* tree. */
- if (status->level == 0) {
+ if (status->level == 0 && !owner_mkdir_first) {
uid = 0;
gid = 0;
}
@@ -222,6 +224,9 @@ mkhomedir(const char *user, int flags)
pwd->pw_dir);
return HANDLER_INVALID_INVOCATION;
}
+ if (flags & FLAG_OWNER_MKDIR_FIRST) {
+ owner_mkdir_first = 1;
+ }
if ((lstat(pwd->pw_dir, &st) == -1) && (errno == ENOENT)) {
/* Figure out which location we're using as a
* template. */
@@ -237,7 +242,7 @@ mkhomedir(const char *user, int flags)
int res = nftw(get_skel_dir(), copy_single_item, 5,
FTW_PHYS);
/* only now give ownership to the target user */
- if (res == 0) {
+ if (res == 0 && !owner_mkdir_first) {
res = chown(pwd->pw_dir, pwd->pw_uid, pwd->pw_gid);
}
@@ -317,8 +322,11 @@ main(int argc, char **argv)
umask(override_umask);
skel_dir = "/etc/skel";
- while ((i = getopt(argc, argv, "nqs:u:")) != -1) {
+ while ((i = getopt(argc, argv, "nqfs:u:")) != -1) {
switch (i) {
+ case 'f':
+ flags |= FLAG_OWNER_MKDIR_FIRST;
+ break;
case 'n':
flags &= ~FLAG_POPULATE;
break;
@@ -339,6 +347,8 @@ main(int argc, char **argv)
break;
default:
fprintf(stderr, "Valid options:\n"
+ "-f\tCreate home directory initially owned by user, "
+ "not root. See man page for security issues.\n"
"-n\tDo not populate home directories, "
"just create them.\n"
"-q\tDo not print messages when creating "
diff --git a/src/oddjobd-mkhomedir.conf.5.in b/src/oddjobd-mkhomedir.conf.5.in
index d7a2429..6e35ad5 100644
--- a/src/oddjobd-mkhomedir.conf.5.in
+++ b/src/oddjobd-mkhomedir.conf.5.in
@@ -10,6 +10,15 @@ directory.
The mkhomedir helper itself accepts these options:
.TP
+-f
+Restore behavior before CVE-2020-10737 was fixed: create the home directory
+with user's ownership directly rather than create it as a root and only after
+populating it change to the user's ownership. The former behavior is insecure
+but may be used to allow creation of NFS-mounted home directories when
+non-Kerberos authentication is in use. It is prone for a race condition that
+could be exploited in the NFS-mounted home directories use case. To avoid
+CVE-2020-10737, do not use \fB-f\fR option in production environments.
+.TP
-q
Refrain from outputting the usual "Creating home directory..." message when it
creates a home directory.
--
2.37.1