From 71b0389fbb31833d827f5f0fec18880c2f602753 Mon Sep 17 00:00:00 2001 From: Alexander Bokovoy 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 --- 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