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Johannes Findeisen 2026-09-29 09:39:24 +02:00
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/*
* ============================================================================
* foowosd.c -- "foowosd": an INTENTIONALLY VULNERABLE daemon that becomes
* root the honest way: by being STARTED as root.
* ============================================================================
*
* PURPOSE
* -------
* This is the "no setuid bit" companion to the other two labs:
*
* food / fooc a plain daemon: the overflow gives you a user shell
* foosd / foosc a SETUID-root daemon: root arrives via the +s bit
* foowosd/ foowosc THIS one: no +s bit anywhere. Root arrives because
* somebody STARTED the process as root.
*
* The setuid bit is not the only way a process ends up privileged. Any
* daemon launched by root -- a `sudo ./foowosd`, a systemd unit with
* `User=root`, an init script -- has real uid 0, effective uid 0, and saved
* uid 0. To the kernel and to every access-control check it makes, that
* process IS root, indistinguishable from one that arrived there via +s.
* And an overflow in a root process is a root exploit, filesystem
* attributes notwithstanding.
*
* THAT is the lesson of this file: the setuid bit is a *transfer vehicle*
* for privilege, not the privilege itself. "I don't have SUID binaries" is
* NOT the same as "I am not vulnerable to privilege escalation". If your
* daemon runs as root and it has a reachable memory-safety bug, you have a
* root-exploit -- with or without the letter 's' in anyone's file mode.
*
* WHY THE EXPLOIT HERE IS DIFFERENT FROM THE SUID LAB -- ruid
* ----------------------------------------------------------
* A setuid-root binary gives the process euid 0 but LEAVES ruid at the
* launching user's id (1000). bash and dash notice `euid != ruid` at
* startup and reset euid = ruid -- the shell's own guard against this
* attack -- which is why foosc's shellcode had to call setreuid(0,0) first.
*
* A daemon *started* as root has ruid == euid == 0. There is no mismatch
* for the shell's guard to notice, so a plain `execve("/bin/sh")` keeps
* root -- no setreuid needed. The same 23 bytes that pwnd `food` in the
* parent lab, byte for byte, open a *root* shell against this daemon,
* because the process they run in is already fully root. The shellcode
* chosen for foowosc therefore does not contain a setreuid prefix.
*
* SAFETY RAILS (identical policy to the SUID lab -- a root daemon is no
* less dangerous because it got there without +s)
* -------------------------------------------------
* * Binds 127.0.0.1 by default and REFUSES a non-loopback bind unless you
* pass -L. A root daemon on a real interface is a remote root service.
* * Logs at startup whether it is running as root or as a normal user, so
* you always know which exploit outcome to expect.
* * Same deliberate bugs as food/foosd, so the whole toolchain
* (objdump-based offset discovery, leak parsing, alignment fix, pty
* harness) carries over unchanged.
*
* Build: make foowosd
* make run-root (needs sudo; starts the daemon as real root)
* make run-root-ns (no sudo: user-namespace root, for verification)
* make run (baseline: starts it as your normal user)
*
* HOW TO BECOME ROOT HERE -- and how NOT to
* -----------------------------------------
* START AS ROOT: sudo make run-root -> ruid=0 euid=0
* START AS ROOT (ns): make run-root-ns -> namespaced 0/0 (test-only)
* PLAIN USER: make run -> ruid=1000 euid=1000
*
* The exploit behaves the same in all three cases -- it just yields a root
* shell in the first two. That "the agency, not the attribute, is what
* matters" property is the whole point of this lab.
*
* THE BUILD FLAGS (same deliberate removals as the other two labs)
* ----------------------------------------------------------------
* -fno-stack-protector no canary: the overflow is not detected
* -no-pie fixed addresses: win() is a constant
* -z execstack executable stack: shellcode can run
*
* `make hardened` re-enables all three; the maliciously shareable lesson is
* that those flags do nothing about the "running as root" design decision.
*
* Usage: ./foowosd [-h HOST] [-p PORT] [-d] [-L]
* ============================================================================
*/
/* Request the gnu decls we need (dprintf, etc.). */
#define _GNU_SOURCE
#include <arpa/inet.h> /* inet_pton(): parse "127.0.0.1" into bytes. */
#include <errno.h> /* errno, strerror(). */
#include <fcntl.h> /* dup2() -- hand the accepted socket to the shell. */
#include <grp.h> /* setgroups(): part of the (never-called) privilege
* drop -- supplementary groups must go first. */
#include <netinet/in.h>/* struct sockaddr_in, htons(). */
#include <signal.h> /* signal(), sigaction(). */
#include <stdarg.h> /* va_list for our log wrapper. */
#include <stdint.h> /* uint16_t. */
#include <stdio.h> /* dprintf, snprintf. */
#include <stdlib.h> /* atoi, _exit. */
#include <string.h> /* memset, strncmp, memchr, strlen. */
#include <sys/socket.h>/* socket, bind, listen, accept. */
#include <sys/stat.h> /* umask. */
#include <sys/types.h> /* ssize_t, pid_t. */
#include <sys/ucontext.h>/* ucontext_t: REG_RIP etc. for the crash reporter. */
#include <sys/wait.h> /* waitpid(). */
#include <unistd.h> /* read, write, dup2, fork, getpid, setsid, chdir. */
/* ------------------------------------------------------------------------- */
/* Configuration constants */
/* ------------------------------------------------------------------------- */
/* Port. 2344 keeps this lab clear of food (2342) and foosd (2343). It is
* above 1024 on purpose: binding it needs NO privilege, so root here is
* pure design smell -- a correct daemon would drop privileges after bind,
* and the lab's whole point is what happens when it does not. */
#define FOOWOSD_PORT 2344
/* Loopback is the ONLY default. -L is required to go further. */
#define FOOWOSD_HOST "127.0.0.1"
/* Size of the overflowed buffer. Same shape as food/foosd so the shared
* objdump-based offset detection works unchanged. */
#define FOOWOSD_BUFSZ 64
/* How much read() accepts. The mismatch with FOOWOSD_BUFSZ IS the bug. */
#define FOOWOSD_READMAX 512
/* Size of the second (format-string demo) buffer. */
#define FOOWOSD_LOGSZ 128
/* ------------------------------------------------------------------------- */
/* Logging (same design as the other labs: the log never reaches the attacker)*/
/* ------------------------------------------------------------------------- */
/* g_logfd -- a private copy of stdout taken BEFORE the socket is dup2()'d
* over fd 1. Every logmsg() line goes here, so a client that overwrites our
* memory or crashes a child never learns internal paths or addresses from
* logs (and never mixes its own bytes with ours). */
static int g_logfd = -1;
/* logmsg() -- timestamped, pid-prefixed line to the log descriptor. One
* write() per line, so forked children cannot interleave mid-line. */
static void logmsg(const char *fmt, ...)
{
char line[1024]; /* Whole-message scratch. */
va_list ap; /* Variadic argument cursor. */
int n; /* Bytes formatted. */
/* va_start MUST precede any use of ap. An uninitialised va_list makes
* vsnprintf walk wild stack memory -- a real bug that was hit in the
* earlier food.c, hence the comment. */
va_start(ap, fmt);
n = vsnprintf(line, sizeof(line) - 32, fmt, ap);
va_end(ap); /* Always pair va_start with va_end. */
if (n < 0)
return;
if (g_logfd >= 0)
dprintf(g_logfd, "[foowosd %d] %s\n", (int)getpid(), line);
}
/* read_exact() / write_all() -- the CORRECT I/O helpers, present so you can
* hold them next to the deliberately broken read() in vulnerable_handler()
* and see the difference: these loop until done and check every result. */
__attribute__((unused))
static ssize_t read_exact(int fd, void *buf, size_t n)
{
size_t got = 0;
while (got < n) {
ssize_t r = read(fd, (char *)buf + got, n - got);
if (r < 0) {
if (errno == EINTR)
continue;
return -1;
}
if (r == 0)
break;
got += (size_t)r;
}
return (ssize_t)got;
}
static ssize_t write_all(int fd, const void *buf, size_t n)
{
size_t sent = 0;
while (sent < n) {
ssize_t w = write(fd, (const char *)buf + sent, n - sent);
if (w <= 0) {
if (w < 0 && errno == EINTR)
continue;
return -1;
}
sent += (size_t)w;
}
return (ssize_t)sent;
}
/* ------------------------------------------------------------------------- */
/* The ret2win target */
/* ------------------------------------------------------------------------- */
/*
* win() -- the "easy" backdoor. The same function as in food.c and foosd.c,
* and the difference between this lab and the SUID lab is contained in it.
*
* In the SUID lab this exact code produced a NON-root shell, because foosd
* had euid 0 but ruid 1000, and bash reset euid = ruid at startup.
*
* Here the daemon is STARTED as root, so at this instant ruid == euid == 0.
* fork() inherits both ids, execve() changes neither, and bash starts with
* equal uid 0s -- its guard has nothing to reset, so execve("/bin/sh") keeps
* root. "spawn a shell" works against a genuinely-root process; it only
* fails against the half-root (euid-only) state the setuid bit produces.
* That asymmetry -- why one lab needs setreuid and this one does not -- is
* the entire technical heart of the two labs side by side.
*/
__attribute__((noinline, used))
static void win(void)
{
pid_t pid;
logmsg("win() reached -- exec'ing /bin/sh (ruid==euid here, so the shell "
"stays root; contrast with foosd where ruid stayed 1000)");
/* Fork so the daemon's accept-loop child can be reaped and return. */
pid = fork();
if (pid < 0) {
logmsg("win(): fork() failed: %s", strerror(errno));
_exit(1);
}
if (pid > 0) {
waitpid(pid, NULL, 0);
/* Must NOT return: that would pop attacker bytes as the next RIP. */
_exit(0);
}
/* Child. prepare_client_fds() already made fds 0/1/2 the socket. */
execl("/bin/sh", "sh", (char *)NULL);
_exit(127); /* Only reached if exec failed. */
}
/* ------------------------------------------------------------------------- */
/* The vulnerable handler -- Bug #1 and Bug #2 live here */
/* ------------------------------------------------------------------------- */
__attribute__((noinline, used))
static void vulnerable_handler(int fd)
{
char buf[FOOWOSD_BUFSZ]; /* 64 stack bytes. The whole ballgame. */
char line[FOOWOSD_LOGSZ]; /* Second buffer, for the format-string demo. */
ssize_t n; /* Bytes actually read. */
/*
* The BUF= leak -- the same deliberate CWE-200 disclosure as the other
* labs. The stack is ASLR-randomised; without this the shellcode could
* not find itself. Real-world leaks of this kind come from %p format
* bugs, crash dumps, debug endpoints, or serialised uninitialised
* pointers.
*
* FIX: never print addresses to untrusted clients.
*/
dprintf(fd, "BUF=%p\n", (void *)buf);
/*
* ====================================================================
* BUG #1 -- UNBOUNDED COPY INTO A FIXED STACK BUFFER (CWE-120)
* ====================================================================
* Identical to the other labs: 512 bytes are accepted into a 64-byte
* array, so the attacker writes 448 bytes past the end, overwriting the
* saved frame pointer and — 8 bytes later — the saved return address.
* On return, `ret` jumps wherever the attacker said:
*
* [ 64 bytes buf ][ 8 bytes saved rbp ][ 8 bytes RETURN ADDRESS ]
*
* The ONLY difference from food is *what that means*: here the hijacked
* process has real-and-effective uid 0 (it was started as root), so
* "attacker controls RIP" becomes "attacker controls root's RIP" --
* with no setuid bit anywhere on this filesystem.
*
* FIXES (in increasing order of strength):
* 1. n = read(fd, buf, sizeof(buf) - 1); <-- the real fix
* 2. -fstack-protector-strong (canary aborts `ret`)
* 3. do not take network input into fixed stack buffers at all
* And SEPARATELY: never run this daemon as root; and if you must, drop
* privileges the moment you are done binding (see drop_privs()). Memory
* safety and least privilege are two different bugs; fix both.
*/
n = read(fd, buf, FOOWOSD_READMAX); /* <-- CWE-120, THE bug. */
if (n <= 0)
return;
/* Echo back a truncated copy so you can watch the overflow in the log.
* Clamping for display does not undo the overwrite that already happened. */
{
ssize_t show = n < FOOWOSD_BUFSZ ? n : FOOWOSD_BUFSZ;
logmsg("vulnerable_handler: read %zd bytes, echoing %zd", n, show);
(void)write_all(fd, buf, (size_t)show);
}
/*
* ====================================================================
* BUG #2 -- NETWORK DATA USED AS A FORMAT STRING (CWE-134)
* ====================================================================
* Same as the other labs: attacker '%'-specifiers in `buf` could read
* stack words with %x or write memory with %n. Here the process is
* root, so a %n is a write-what-where primitive IN A ROOT PROCESS. It
* runs only on a copy in `line`, and only if the payload contains '%'.
*
* FIX: printf("%s", buf), never printf(buf).
*/
if (memchr(buf, '%', (size_t)n) != NULL) {
snprintf(line, sizeof(line), "%.*s", (int)FOOWOSD_LOGSZ - 1, buf);
logmsg("vulnerable_handler: payload contains '%%', echoing it raw");
(void)write_all(fd, line, strlen(line));
}
/* On return the (attacker-controlled) saved return address becomes RIP. */
}
/* ------------------------------------------------------------------------- */
/* Crash reporter (same rationale as the other labs: a crash should tell you */
/* it was malicious; the fault address is the return address the client */
/* supplied). */
/* ------------------------------------------------------------------------- */
static void on_sigsegv(int sig, siginfo_t *si, void *ucv)
{
ucontext_t *uc = (ucontext_t *)ucv;
unsigned long rip = 0, rsp = 0;
if (uc != NULL) {
rip = (unsigned long)uc->uc_mcontext.gregs[REG_RIP];
rsp = (unsigned long)uc->uc_mcontext.gregs[REG_RSP];
}
logmsg("SIGSEGV: faulting address %p", si ? si->si_addr : (void *)0);
logmsg("SIGSEGV: RIP=%#lx RSP=%#lx (RIP is the address the client "
"supplied)", rip, rsp);
logmsg("SIGSEGV: if RIP is a real address the attacker jumped there; "
"if it is an address INSIDE vulnerable_handler itself it IS the "
"`ret` instruction: a ret into a non-canonical address (e.g. "
"0x4141414141414141) faults at the ret, not at the target.");
/* Re-raise with the default disposition so the process still dies, with
* the correct status, rather than re-executing the faulting instruction
* forever (returning from this handler would do exactly that). */
signal(sig, SIG_DFL);
raise(sig);
}
static void install_crash_reporter(void)
{
struct sigaction sa;
memset(&sa, 0, sizeof(sa));
sa.sa_sigaction = on_sigsegv; /* Extended two-argument handler. */
sa.sa_flags = SA_SIGINFO;
sigemptyset(&sa.sa_mask);
if (sigaction(SIGSEGV, &sa, NULL) < 0)
logmsg("sigaction(SIGSEGV) failed: %s", strerror(errno));
if (sigaction(SIGBUS, &sa, NULL) < 0)
logmsg("sigaction(SIGBUS) failed: %s", strerror(errno));
}
/* ------------------------------------------------------------------------- */
/* fd handling */
/* ------------------------------------------------------------------------- */
/* prepare_client_fds() -- put the accepted socket onto fds 0/1/2 so that
* every technique (ret2win, ret2libc, shellcode) produces a shell that
* automatically speaks over the network. */
static void prepare_client_fds(int fd)
{
if (fd != STDIN_FILENO) dup2(fd, STDIN_FILENO);
if (fd != STDOUT_FILENO) dup2(fd, STDOUT_FILENO);
if (fd != STDERR_FILENO) dup2(fd, STDERR_FILENO);
if (fd > STDERR_FILENO) close(fd); /* Don't leak the spare descriptor.*/
}
/* ------------------------------------------------------------------------- */
/* THE INFORMATION LEAK */
/* ------------------------------------------------------------------------- */
/*
* send_leaks() -- tell the attacker:
*
* ids= this process's euid/ruid. THE "AM I ROOT ?" CHECK.
* foowosc prints a loud warning when euid is not 0,
* because without a root daemon there is no root shell
* and the user would otherwise think the exploit broke.
* leak stack=... an address on the stack, for the shellcode
* leak libc=... the real address of read() inside libc, for ret2libc
*
* The `ids=` spelling (rather than "euid="/"ruid=") is deliberate: the test
* harness proves a live shell by grepping the session transcript for the
* strict `id`-output shape "uid=NNN(", and a banner containing "uid=" as
* part of "euid="/"ruid=" would itself satisfy a careless grep. This kind of
* "the probe and the answer must not share a signature" thinking is what you
* do when you write real assertions about untrusted output.
*/
static void send_leaks(int fd)
{
long stack_marker = 0x4141414141414141L; /* Obvious in a debugger. */
ssize_t (*libc_read)(int, void *, size_t);/* Real address of read(). */
libc_read = &read; /* &read resolves through the GOT to libc. */
dprintf(fd, "FOOWOSD 1.0 ids=%d/%d leak stack=%p libc=%p\n",
(int)geteuid(), (int)getuid(),
(void *)&stack_marker, (void *)libc_read);
}
/* THE CORRECT DESIGN, PRESENT BUT NEVER CALLED
* -------------------------------------------
* drop_privs() -- what a well-written daemon would do the moment it no
* longer needs root. Two mistakes to notice, both immune to every compiler
* mitigation:
*
* * ORDER: setgroups() before setgid() before setuid(), and ONLY AFTER
* binding the port and opening any root-only files. Drop first and the
* whole point of root is gone.
* * PERMANENCE: setuid() to a nonzero value and check it stuck (a root
* process may later regain privileges via the saved id otherwise).
*
* Port 2344 needs no privilege, so the correct design would call this right
* after the listen() succeeds. In this lab it is deliberately absent from
* main(), because the lab NEEDS the accept-loop children to stay root. The
* commented function is your diff: the two missing calls at the point marked
* "*** see drop_privs() ***" below are the entire exploit surface (Bug #3,
* CWE-271: privilege not dropped before handling untrusted input).
*/
__attribute__((unused))
static void drop_privs(void)
{
/* Order matters: setgroups() first (a non-root user may not), then
* setgid(), then setuid(). Never the reverse. */
(void)setgroups(0, NULL); /* Remove all supplementary groups. */
(void)setgid(1000); /* Lose group privileges. */
if (setuid(1000) < 0) /* Any non-zero uid is fine here. */
_exit(1); /* If we cannot drop, FAIL CLOSED. */
/* Verify. getuid()/geteuid() are cheap; a privileged program whose drop
* failed silently is a root hole wearing a costume. */
if (getuid() != 1000 || geteuid() != 1000)
_exit(1);
}
/* ------------------------------------------------------------------------- */
/* Per-connection handling */
/* ------------------------------------------------------------------------- */
static void handle_client(int fd)
{
static const char banner[] =
"FOOWOSD 1.0 - deliberately vulnerable daemon (no setuid bit: root is\n"
"here because this process was started as root).\n"
"Type 'quit' to disconnect. Buffer = 64 bytes, read accepts 512.\n";
prepare_client_fds(fd); /* fds 0,1,2 now all point at the socket. */
install_crash_reporter(); /* Log (g_logfd) lines, not to the socket. */
logmsg("client connected (uid=%d euid=%d)", (int)getuid(), (int)geteuid());
(void)write_all(STDOUT_FILENO, banner, sizeof(banner) - 1);
send_leaks(STDOUT_FILENO);
vulnerable_handler(STDOUT_FILENO);
/* Only reached when the payload did NOT hijack RIP. */
logmsg("vulnerable_handler returned normally -- payload did not hijack RIP");
(void)write_all(STDOUT_FILENO, "OK: no hijack, disconnecting.\n", 29);
}
/* ------------------------------------------------------------------------- */
/* The server loop */
/* ------------------------------------------------------------------------- */
static int make_listener(const char *host, int port)
{
struct sockaddr_in addr;
int fd;
int one = 1;
fd = socket(AF_INET, SOCK_STREAM, 0);
if (fd < 0) {
logmsg("socket() failed: %s", strerror(errno));
return -1;
}
if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)) < 0)
logmsg("setsockopt(SO_REUSEADDR) failed: %s", strerror(errno));
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_port = htons((uint16_t)port);
if (inet_pton(AF_INET, host, &addr.sin_addr) != 1) {
logmsg("bad bind address: %s", host);
close(fd);
return -1;
}
if (port < 1 || port > 65535) {
logmsg("port out of range: %d", port);
close(fd);
return -1;
}
if (bind(fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
logmsg("bind(%s:%d) failed: %s", host, port, strerror(errno));
close(fd);
return -1;
}
if (listen(fd, 16) < 0) {
logmsg("listen() failed: %s", strerror(errno));
close(fd);
return -1;
}
return fd;
}
static void usage(const char *argv0)
{
fprintf(stderr,
"usage: %s [-h HOST] [-p PORT] [-d] [-L]\n"
"\n"
" -h HOST address to bind (default %s -- loopback only!\n"
" -L is required to bind anywhere else)\n"
" -p PORT TCP port to listen on (default %d)\n"
" -d daemonise: fork into the background\n"
" -L ALLOW binding to a non-loopback address (dangerous:\n"
" this daemon exists to be exploited as ROOT)\n"
"\n"
"This lab gives you a ROOT shell only when the daemon was STARTED\n"
"as root (sudo make run-root). There is no setuid bit anywhere.\n"
"Do not run it on any host that matters, never bind it beyond\n"
"loopback, and do not leave it running as root.\n",
argv0, FOOWOSD_HOST, FOOWOSD_PORT);
}
int main(int argc, char **argv)
{
const char *host = FOOWOSD_HOST; /* Bind address. */
int port = FOOWOSD_PORT; /* Bind port. */
int daemonise = 0; /* -d. */
int allow_nonloopback = 0; /* -L. The root-daemon safety guard. */
int lfd; /* Listening socket. */
int i; /* getopt() index. */
while ((i = getopt(argc, argv, ":h:p:dL")) != -1) {
switch (i) {
case 'h': host = optarg; break;
case 'p': port = atoi(optarg); break;
case 'd': daemonise = 1; break;
case 'L': allow_nonloopback = 1; break;
case ':': fprintf(stderr, "missing argument to -%c\n", optopt);
usage(argv[0]);
return 2;
default: usage(argv[0]);
return 2;
}
}
/*
* THE ROOT-DAEMON SAFETY GUARD.
*
* A daemon that is running as root (it was started as root -- there is
* no +s bit here, so this state is easy to forget) and listens on a
* non-loopback interface is a remote root service. Refuse by default,
* document the exception, fail loudly.
*/
if (!allow_nonloopback &&
(strcmp(host, "127.0.0.1") != 0 && strcmp(host, "localhost") != 0 &&
strcmp(host, "::1") != 0)) {
fprintf(stderr,
"foowosd: refusing to bind %s: this daemon may be running as\n"
" root. Loopback is the only permitted default. If you\n"
" really know what you are doing, pass -L.\n", host);
return 1;
}
signal(SIGPIPE, SIG_IGN);
signal(SIGCHLD, SIG_IGN); /* Auto-reap forked children. */
/* Reserve a private log descriptor BEFORE sockets are dup2'd over fd 1. */
g_logfd = dup(STDOUT_FILENO);
if (g_logfd < 0) {
g_logfd = STDOUT_FILENO;
fprintf(stderr, "foowosd: warning: could not reserve a log descriptor\n");
}
/*
* SELF-DIAGNOSIS OF THE "AM I ROOT ?" STATE -- printed once, to the log.
*
* ruid==euid==0 -> started as root: the exploit gives root
* ruid==euid!=0 -> started as a normal user: baseline only
*
* foowosc reads euid over the socket and can warn too; this log line is
* for you at the console.
*/
logmsg("startup: ruid=%d euid=%d %s",
(int)getuid(), (int)geteuid(),
(geteuid() == 0) ? "-> ROOT process"
: "-> NOT root (start as root: make run-root)");
if (geteuid() == 0)
logmsg("startup: WARNING: this daemon is running as root -- no setuid "
"bit involved, just a root-started process. Port %d does not "
"need root; see drop_privs().", port);
lfd = make_listener(host, port);
if (lfd < 0)
return 1;
logmsg("listening on %s:%d (pid %d) -- THIS SERVICE IS INTENTIONALLY "
"VULNERABLE", host, port, (int)getpid());
if (daemonise) {
/* Standard double fork so we cannot acquire a controlling terminal. */
pid_t p1 = fork();
if (p1 < 0) { perror("fork"); return 1; }
if (p1 > 0) _exit(0);
if (setsid() < 0) perror("setsid");
pid_t p2 = fork();
if (p2 < 0) { perror("fork"); return 1; }
if (p2 > 0) _exit(0);
if (chdir("/") < 0) perror("chdir");
umask(022);
}
/* ---- The accept loop. Each child serves one connection. The children
* stay root because the parent was started as root. ---- */
for (;;) {
struct sockaddr_in peer;
socklen_t plen = sizeof(peer);
int cfd;
pid_t pid;
cfd = accept(lfd, (struct sockaddr *)&peer, &plen);
if (cfd < 0) {
if (errno == EINTR || errno == ECONNABORTED)
continue;
logmsg("accept() failed: %s", strerror(errno));
continue;
}
/*
* Fork per connection. The child KEEPS the root privileges -- that
* is Bug #3 in this lab, "no privilege drop before handling
* untrusted input" (CWE-271). See drop_privs() above for the exact
* calls a well-written daemon would make at this point, and why the
* order of those three calls is security-critical.
*/
pid = fork();
if (pid < 0) {
logmsg("fork() failed: %s", strerror(errno));
close(cfd);
continue;
}
if (pid == 0) {
close(lfd);
handle_client(cfd);
_exit(0);
}
close(cfd);
}
}