hardening: replace setuid/setgid calls with setresuid/setresgid

when nesting containers and sandboxes, it is possible setuid() fails
silently to reset the saved uid, which is then cleared only by
the next execve. This is solved by replacing setuid() with more
robust setresuid() function calls.

Also add code to drop privileges when entering the run_no_sandbox()
function (along with some minor tidy up).

1 files changed, 7 insertions, 6 deletions

diff --git a/src/firejail/util.c b/src/firejail/util.c
index dd298a31a..52f0f89c5 100644
--- a/src/firejail/util.c
+++ b/src/firejail/util.c
@@ -119,12 +119,13 @@ clean_all:
 // drop privileges
 // - for root group or if nogroups is set, supplementary groups are not configured
 void drop_privs(int nogroups) {
-        EUID_ROOT();
         gid_t gid = getgid();
+        uid_t uid = getuid();
         if (arg_debug)
-                printf("Drop privileges: pid %d, uid %d, gid %d, nogroups %d\n",  getpid(), getuid(), gid, nogroups);
+                printf("Drop privileges: pid %d, uid %d, gid %d, nogroups %d\n",  getpid(), uid, gid, nogroups);
 
         // configure supplementary groups
+        EUID_ROOT();
         if (gid == 0 || nogroups) {
                 if (setgroups(0, NULL) < 0)
                         errExit("setgroups");
@@ -135,10 +136,10 @@ void drop_privs(int nogroups) {
                 clean_supplementary_groups(gid);
 
         // set uid/gid
-        if (setgid(getgid()) < 0)
-                errExit("setgid/getgid");
-        if (setuid(getuid()) < 0)
-                errExit("setuid/getuid");
+        if (setresgid(gid, gid, gid) != 0)
+                errExit("setresgid");
+        if (setresuid(uid, uid, uid) != 0)
+                errExit("setresuid");
 }