initial commit

.gitignore

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+/bin

Makefile

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+CFLAGS=-O3 -Wall -lpam -lpam_misc
+BIN=udo
+
+all:
+	mkdir -p bin
+	gcc $(CFLAGS) $(wildcard src/*.c) -o bin/udo
+
+install: all
+	cp bin/udo /usr/local/bin/udo
+	chown root:root /usr/local/bin/udo
+	chmod 6755 /usr/local/bin/udo
+	cp pam.d_udo /etc/pam.d/udo
+
+uninstall:
+	rm /usr/local/bin/udo
+	rm /etc/pam.d/udo
+
+clean:
+	rm -r bin/*

README.md

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+# udo
+
+`udo` is an extremely stripped-down sudo/doas imitation.
+
+## config
+
+the config file is located at `/etc/udo.conf`. it is a space-separated list of UIDs that are allowed to use `udo`. this file should probably contain `0` at some point.
+
+## installation
+
+`sudo make install`
+
+## uninstallation
+
+`sudo make uninstall`, but why would you want to?
+
+## why does it segfault?
+
+it only segfaults if you have done something wrong. do not do wrong things.

pam.d_udo

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+#%PAM-1.0
+auth        include    system-auth
+account     include    system-auth
+session     include    system-auth

src/main.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h> 
+#include <string.h>
+#include "main.h"
+#include <pwd.h>
+#include <security/pam_appl.h>
+#include <security/pam_misc.h>
+
+int main(int argc, char** argv) {
+    if(argc == 1) {
+        printf("insufficient\n");
+        return 0;
+    }
+    uid_t uid = getuid();
+    Config config = load_config();
+
+    struct passwd* pwd = getpwuid(uid);
+    char* user = pwd->pw_name;
+
+    int result = validate(user);
+    if(result) {
+        return 2;
+    }
+
+    char* cmd = argv[1];
+    char* args[argc];
+
+    for(int i = 0; i < argc-1; i++) {
+        args[i] = malloc(strlen(argv[i+1]));
+        strcpy(args[i], argv[i+1]);        
+    }
+    args[argc-1] = NULL;
+
+    if(allowed(uid, &config)) {
+        setuid(geteuid());
+        setgid(getegid());
+        int x = execvp(cmd, args);
+        return x;
+    } else {
+        fprintf(stderr, "nope\n");
+        return 1;
+    }
+}
+
+int validate(char* user) {
+    pam_handle_t *pamh = NULL;
+    struct pam_conv conv = {misc_conv, NULL};
+
+    int status = pam_start("udo", user, &conv, &pamh);
+    if(status != PAM_SUCCESS) {
+        printf("bad: %d\n", status);
+        return 1;
+    }
+
+    status = pam_authenticate(pamh, 0);
+    if(status != PAM_SUCCESS) {
+        printf("wrong: %d\n", status);
+        return 1;
+    }
+    status = pam_acct_mgmt(pamh, 0);
+
+    if (pam_end(pamh,status) != PAM_SUCCESS) { 
+        pamh = NULL;
+        exit(2);
+    }
+
+    if(status != PAM_SUCCESS) {
+        printf("don't: %d\n", status);
+        return 1;
+    }
+    return 0;
+}
+
+int allowed(uid_t uid, Config* config) {
+    for(int i = 0; i < config->len; i++) {
+        if(config->uids[i] == uid) {
+            return 1;
+        }
+    }
+    return 0;
+}
+
+Config load_config() {
+    FILE* fp = fopen("/etc/udo.conf", "r");
+    char* contents = malloc(CHUNK_SIZE + 1);
+    int offset = 0;
+    char buf[CHUNK_SIZE];
+    int read = CHUNK_SIZE;
+    while(read == CHUNK_SIZE) {
+        read = fread(buf, 1, CHUNK_SIZE, fp);
+        memcpy(contents + offset, buf, read);
+        offset += read;
+        if(read == CHUNK_SIZE) {
+            contents = realloc(contents, offset + CHUNK_SIZE + 1);
+        }
+    }
+    fclose(fp);
+    contents[offset] = '\0';
+
+    Config config;
+    config.uids = malloc(1 * sizeof(uid_t));
+    config.len = 0;
+
+    char* ctx;
+    char* word = strtok_r(contents, " ", &ctx);
+    while(word != NULL) {
+        uid_t uid = atoi(word);
+        config.uids = realloc(config.uids, (config.len+1) * sizeof(uid_t));
+        config.uids[config.len++] = uid;
+        word = strtok_r(NULL, "\n", &ctx);
+    }
+
+    return config;
+}

src/main.h

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+#include <stddef.h>
+#include <sys/types.h>
+
+#define CHUNK_SIZE 10
+
+typedef struct {
+    uid_t* uids;
+    int len;
+} Config;
+
+int main(int argc, char** argv);
+int allowed(uid_t uid, Config* config);
+int validate(char* user);
+Config load_config();