lib: add frr_elevate_privs() wrapper

Used as:
  frr_elevate_privs(&my_privs) {
    ... code ...
  }
and handles privilege raise/lower automatically in conjunction with the
C expression block.  This makes it impossible to accidentally exit a
function with privileges raised (and then running a whole bunch of other
code with privs.)

Signed-off-by: David Lamparter <equinox@diac24.net>

diff --git a/lib/privs.c b/lib/privs.c
index 7c99742d3407bf3454b30ac2c50436b226a8d79f..34905ca480deb04b39eaba94a9c7d51e799b7dea 100644 (file)
--- a/lib/privs.c
+++ b/lib/privs.c
@@ -696,6 +696,41 @@ static int getgrouplist(const char *user, gid_t group, gid_t *groups,
 }
 #endif /* HAVE_GETGROUPLIST */
 
+struct zebra_privs_t *_zprivs_raise(struct zebra_privs_t *privs,
+                                   const char *funcname)
+{
+       int save_errno = errno;
+
+       if (!privs)
+               return NULL;
+
+       errno = 0;
+       if (privs->change(ZPRIVS_RAISE)) {
+               zlog_err("%s: Failed to raise privileges (%s)",
+                        funcname, safe_strerror(errno));
+       }
+       errno = save_errno;
+       privs->raised_in_funcname = funcname;
+       return privs;
+}
+
+void _zprivs_lower(struct zebra_privs_t **privs)
+{
+       int save_errno = errno;
+
+       if (!*privs)
+               return;
+
+       errno = 0;
+       if ((*privs)->change(ZPRIVS_LOWER)) {
+               zlog_err("%s: Failed to lower privileges (%s)",
+                        (*privs)->raised_in_funcname, safe_strerror(errno));
+       }
+       errno = save_errno;
+       (*privs)->raised_in_funcname = NULL;
+       *privs = NULL;
+}
+
 void zprivs_preinit(struct zebra_privs_t *zprivs)
 {
        struct passwd *pwentry = NULL;

diff --git a/lib/privs.h b/lib/privs.h
index 7fe59328b214fc57f4239644f863fbf4bace0d16..b061370b753d2ce2bc88b39699ede003175cb645 100644 (file)
--- a/lib/privs.h
+++ b/lib/privs.h
@@ -62,6 +62,7 @@ struct zebra_privs_t {
        int (*change)(zebra_privs_ops_t); /* change privileges, 0 on success */
        zebra_privs_current_t (*current_state)(
                void); /* current privilege state */
+       const char *raised_in_funcname;
 };
 
 struct zprivs_ids_t {
@@ -81,4 +82,42 @@ extern void zprivs_terminate(struct zebra_privs_t *);
 /* query for runtime uid's and gid's, eg vty needs this */
 extern void zprivs_get_ids(struct zprivs_ids_t *);
 
+/*
+ * Wrapper around zprivs, to be used as:
+ *   frr_elevate_privs(&privs) {
+ *     ... code ...
+ *     if (error)
+ *       break;         -- break can be used to get out of the block
+ *     ... code ...
+ *   }
+ *
+ * The argument to frr_elevate_privs() can be NULL to leave privileges as-is
+ * (mostly useful for conditional privilege-raising, i.e.:)
+ *   frr_elevate_privs(cond ? &privs : NULL) {}
+ *
+ * NB: The code block is always executed, regardless of whether privileges
+ * could be raised or not, or whether NULL was given or not.  This is fully
+ * intentional;  the user may have configured some RBAC or similar that we
+ * are not aware of, but that allows our code to proceed without privileges.
+ *
+ * The point of this wrapper is to prevent accidental bugs where privileges
+ * are elevated but then not dropped.  This can happen when, for example, a
+ * "return", "goto" or "break" in the middle of the elevated-privilege code
+ * skips past the privilege dropping call.
+ *
+ * The macro below uses variable cleanup to drop privileges as soon as the
+ * code block is left in any way (and thus the _privs variable goes out of
+ * scope.)  _once is just a trick to run the loop exactly once.
+ */
+extern struct zebra_privs_t *_zprivs_raise(struct zebra_privs_t *privs,
+                                          const char *funcname);
+extern void _zprivs_lower(struct zebra_privs_t **privs);
+
+#define frr_elevate_privs(privs)                                               \
+       for (struct zebra_privs_t *_once = NULL,                               \
+                                 *_privs __attribute__(                       \
+                                         (unused, cleanup(_zprivs_lower))) =  \
+                                         _zprivs_raise(privs, __func__);      \
+            _once == NULL; _once = (void *)1)
+
 #endif /* _ZEBRA_PRIVS_H */