setuid examples

examples/ex-setuid.c

@@ -0,0 +1,153 @@
+#define _GNU_SOURCE 
+
+/*
+    To compile and make the exeutable a SUID program:
+
+        gcc -o suid ex-setuid.c && sudo chown root suid && sudo chmod +s suid
+
+    There are four demo modes: u, e, d, and o. For example, 
+
+        ./suid u
+*/
+
+#include    <stdio.h>
+#include    <stdlib.h>
+#include    <string.h>
+#include    <time.h>
+#include    <sys/types.h>
+#include    <unistd.h>
+#include    <errno.h>
+#include    <assert.h>
+
+/* setuid examples */
+
+uid_t   my_uid, my_euid;
+
+void	print_uids();
+
+void myerror(char *s)
+{
+    fprintf(stderr, "myerror:\n");
+
+    uid_t ruid, euid, suid;
+    if (getresuid(&ruid, &euid, &suid)) {
+        fprintf(stderr, "myerror: getresuid() returned error.\n");
+    }
+    else {
+        fprintf(stderr, "myerror: ruid=%d euid=%d suid=%d\n", ruid, euid, suid);
+    }
+
+    if (errno) {
+        perror(s);
+    }
+    else {
+        fprintf(stderr, "Error: %s (errno==0)\n", s);
+    }
+    exit(EXIT_FAILURE);
+}
+
+void	print_uids()
+{
+    // printf("ruid=%d euid=%d\n", getuid(), geteuid());
+    // getresuid() is not in POSIX
+    uid_t ruid, euid, suid;
+    if (getresuid(&ruid, &euid, &suid)) {
+        myerror("getresuid()");
+    }
+    printf("ruid=%d euid=%d saved_uid=%d\n", ruid, euid, suid);
+}
+
+void    my_setuid(int newid)
+{
+    printf("calling setuid(%d) ...\n", newid);
+    int rv = setuid(newid);
+
+    if (rv != 0) {
+        myerror("setuid failed");
+    }
+    printf("New UIDs:\n");
+    print_uids();
+}
+
+void    my_seteuid(int newid)
+{
+    printf("calling seteuid(%d) ...\n", newid);
+    int rv = seteuid(newid);
+
+    if (rv != 0) {
+        myerror("seteuid failed");
+    }
+    printf("New UIDs:\n");
+    print_uids();
+}
+
+void    demo_uid()
+{
+    printf("setuid ...\n");
+    print_uids();
+    my_setuid(my_uid);
+    my_setuid(0);
+}
+
+void demo_euid()
+{
+    printf("seteuid...\n");
+    print_uids();
+    my_seteuid(my_uid);
+    my_seteuid(0);
+    my_seteuid(2000);
+    my_seteuid(my_uid);
+    my_seteuid(2000);
+}
+
+void demo_dash()
+{
+    printf("Call setuid(0) to change all UIDs to 0 ...\n");
+    printf("This defeats dash's countermeasure.\n");
+
+    print_uids();
+    my_setuid(my_euid);
+}
+
+void demo_other()
+{
+    print_uids();
+    my_seteuid(2000);       // success
+    my_seteuid(my_uid);     // success
+    my_seteuid(2000);       // fail
+}
+
+int main(int argc,char* argv[])
+{
+    my_uid = getuid();
+    my_euid = geteuid();
+
+    int mode = 0;
+
+    if (argc != 2 ) {
+        printf("Usage: suid mode\nmode is u, e, d, or 4\n");
+        printf("u: setuid, e: seteuid, d: dash, o: other\n");
+        return 1;
+    }
+
+    mode = argv[1][0];
+
+    switch (mode) {
+        case 'u':
+            demo_uid();
+            break;
+        case 'e':
+            demo_euid();
+            break;
+        case 'd':
+            demo_dash();
+            break;
+        case 'o':
+            demo_other();
+            break;
+        default:
+            printf("Unknown mode %c.\n", mode);
+    }
+    return 0;
+}
+