core: refuse doing %h, %s, %U specifier resolving in PID 1

These specifiers require NSS lookups to work, and we really shouldn't do
them from PID 1 hence. With this change they are now only supported for
user systemd instance, or when the configured user for a unit is root.

2 files changed, 50 insertions, 2 deletions

diff --git a/src/shared/util.c b/src/shared/util.c
index b5ffaa1c3c..20aec2a5c9 100644
--- a/src/shared/util.c
+++ b/src/shared/util.c
@@ -5220,10 +5220,10 @@ int make_console_stdio(void) {
 }
 
 int get_home_dir(char **_h) {
-        char *h;
+        struct passwd *p;
         const char *e;
+        char *h;
         uid_t u;
-        struct passwd *p;
 
         assert(_h);
 
@@ -5266,6 +5266,53 @@ int get_home_dir(char **_h) {
         return 0;
 }
 
+int get_shell(char **_s) {
+        struct passwd *p;
+        const char *e;
+        char *s;
+        uid_t u;
+
+        assert(_s);
+
+        /* Take the user specified one */
+        e = getenv("SHELL");
+        if (e) {
+                s = strdup(e);
+                if (!s)
+                        return -ENOMEM;
+
+                *_s = s;
+                return 0;
+        }
+
+        /* Hardcode home directory for root to avoid NSS */
+        u = getuid();
+        if (u == 0) {
+                s = strdup("/bin/sh");
+                if (!s)
+                        return -ENOMEM;
+
+                *_s = s;
+                return 0;
+        }
+
+        /* Check the database... */
+        errno = 0;
+        p = getpwuid(u);
+        if (!p)
+                return errno > 0 ? -errno : -ESRCH;
+
+        if (!path_is_absolute(p->pw_shell))
+                return -EINVAL;
+
+        s = strdup(p->pw_shell);
+        if (!s)
+                return -ENOMEM;
+
+        *_s = s;
+        return 0;
+}
+
 bool filename_is_safe(const char *p) {
 
         if (isempty(p))
diff --git a/src/shared/util.h b/src/shared/util.h
index d5fa81c6a5..6fc77808d4 100644
--- a/src/shared/util.h
+++ b/src/shared/util.h
@@ -552,6 +552,7 @@ bool in_initrd(void);
 void warn_melody(void);
 
 int get_home_dir(char **ret);
+int get_shell(char **_ret);
 
 static inline void freep(void *p) {
         free(*(void**) p);