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Fixes to mkhomedir behavior

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- Provide a switch to restore pre-CVE-2020-10737 behavior
- Always set the home directory permissions according to HOME_MODE

Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
This commit is contained in:
Alexander Bokovoy 2022-12-09 14:23:50 +02:00
parent 85c42911bb
commit f77ee4f2bb
2 changed files with 253 additions and 1 deletions
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246
oddjob-override-mask-fix.patch Normal file
View File

@ -0,0 +1,246 @@
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 1/2] 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.
--
2.38.1
From b800e25258353dbb1a88506123c21ac3298fd2d0 Mon Sep 17 00:00:00 2001
From: Carlos Santos <casantos@redhat.com>
Date: Tue, 18 Oct 2022 08:59:16 -0300
Subject: [PATCH 2/2] Always set the home directory permissions according to
HOME_MODE
Currently the home directory permissions are set by taking the /etc/skel
mode and masking it with HOME_MODE:
override_umask = 0777 & ~get_umask(&configured_umask, "HOME_MODE");
stat(skel, &sb); /* performed by nftw() */
oddjob_selinux_mkdir(newpath, sb->st_mode & ~override_umask, uid, gid);
The problem is that when HOME_MODE is more permissive than /etc/skel,
the masking will not produce the desired result, e.g.
skel_mode = 0755
HOME_MODE = 0775
override_umask = 0777 & ~HOME_MODE /* 0002 */
mode = skel_mode & ~override_umask /* 0755 & 0775 = 0755 */
In order to fix the problem, always use 0777 & ~override_umask for the
top home directory.
Signed-off-by: Carlos Santos <casantos@redhat.com>
Fixes: https://pagure.io/oddjob/issue/17
---
src/mkhomedir.c | 45 +++++++++++++++++++++------------------------
1 file changed, 21 insertions(+), 24 deletions(-)
diff --git a/src/mkhomedir.c b/src/mkhomedir.c
index ac813a9..932918f 100644
--- a/src/mkhomedir.c
+++ b/src/mkhomedir.c
@@ -67,6 +67,7 @@ copy_single_item(const char *source, const struct stat *sb,
{
uid_t uid = pwd->pw_uid;
gid_t gid = pwd->pw_gid;
+ mode_t mode = sb->st_mode & ~override_umask;
int sfd, dfd, i, res;
char target[PATH_MAX + 1], newpath[PATH_MAX + 1];
unsigned char buf[BUFSIZ];
@@ -112,8 +113,7 @@ copy_single_item(const char *source, const struct stat *sb,
oddjob_set_selinux_file_creation_context(newpath,
sb->st_mode |
S_IFREG);
- dfd = open(newpath, O_WRONLY | O_CREAT | O_EXCL,
- sb->st_mode & ~override_umask);
+ dfd = open(newpath, O_WRONLY | O_CREAT | O_EXCL, mode);
if (dfd != -1) {
while ((i = read(sfd, buf, sizeof(buf))) > 0) {
retry_write(dfd, buf, i);
@@ -156,20 +156,22 @@ copy_single_item(const char *source, const struct stat *sb,
}
return 0;
case FTW_D:
- /* It's the home directory itself. Don't give it to the
- * target user just yet to avoid potential race conditions
- * involving symlink attacks when we copy over the skeleton
- * tree. */
- if (status->level == 0 && !owner_mkdir_first) {
- uid = 0;
- gid = 0;
- }
-
/* It's a directory. Make one with the same name and
* permissions, but owned by the target user. */
- res = oddjob_selinux_mkdir(newpath,
- sb->st_mode & ~override_umask,
- uid, gid);
+ if (status->level == 0) {
+ /* It's the home directory itself. Use the configured
+ * (or overriden) mode, not the source mode & umask. */
+ mode = 0777 & ~override_umask;
+
+ /* Don't give it to the target user just yet to avoid
+ * potential race conditions involving symlink attacks
+ * when we copy over the skeleton tree. */
+ if (!owner_mkdir_first) {
+ uid = 0;
+ gid = 0;
+ }
+ }
+ res = oddjob_selinux_mkdir(newpath, mode, uid, gid);
/* on unexpected errors, or if the home directory itself
* suddenly already exists, abort the copy operation. */
@@ -248,12 +250,8 @@ mkhomedir(const char *user, int flags)
return res;
} else {
- if (stat(skel, &st) != 0) {
- st.st_mode = S_IRWXU;
- }
if ((oddjob_selinux_mkdir(pwd->pw_dir,
- st.st_mode &
- ~override_umask,
+ 0777 & ~override_umask,
pwd->pw_uid,
pwd->pw_gid) != 0) &&
(errno != EEXIST)) {
@@ -269,11 +267,11 @@ mkhomedir(const char *user, int flags)
}
static mode_t
-get_umask(int *configured, const char *variable)
+get_umask(int *configured, const char *variable, mode_t default_value)
{
FILE *fp;
char buf[BUFSIZ], *p, *end;
- mode_t mask = umask(0777);
+ mode_t mask = default_value;
long tmp;
size_t vlen = strlen(variable);
@@ -315,11 +313,10 @@ main(int argc, char **argv)
openlog(PACKAGE "-mkhomedir", LOG_PID, LOG_DAEMON);
/* Unlike UMASK, HOME_MODE is the file mode, so needs to be reverted */
- override_umask = 0777 & ~get_umask(&configured_umask, "HOME_MODE");
+ override_umask = 0777 & ~get_umask(&configured_umask, "HOME_MODE", 0);
if (configured_umask == 0) {
- override_umask = get_umask(&configured_umask, "UMASK");
+ override_umask = get_umask(&configured_umask, "UMASK", 022);
}
- umask(override_umask);
skel_dir = "/etc/skel";
while ((i = getopt(argc, argv, "nqfs:u:")) != -1) {
--
2.38.1

8
oddjob.spec
View File

@ -22,9 +22,10 @@
Name: oddjob
Version: 0.34.7
Release: 6%{?dist}
Release: 7%{?dist}
Source0: https://releases.pagure.org/oddjob/oddjob-%{version}.tar.gz
Source1: https://releases.pagure.org/oddjob/oddjob-%{version}.tar.gz.asc
Patch1: oddjob-override-mask-fix.patch
Summary: A D-Bus service which runs odd jobs on behalf of client applications
License: BSD
BuildRequires: make
@ -88,6 +89,7 @@ This package contains a trivial sample oddjob service.
%prep
%setup -q
%patch1 -p1
%build
sample_flag=
@ -247,6 +249,10 @@ fi
exit 0
%changelog
* Fri Dec 09 2022 Alexander Bokovoy <abokovoy@redhat.com> - 0.34.7-7
- Provide a switch to restore pre-CVE-2020-10737 behavior
- Always set the home directory permissions according to HOME_MODE
* Fri Jul 22 2022 Fedora Release Engineering <releng@fedoraproject.org> - 0.34.7-6
- Rebuilt for https://fedoraproject.org/wiki/Fedora_37_Mass_Rebuild