Something went wrong. Try again.
Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Something went wrong. Try again.
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380#include <signal.h>#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <errno.h>#include <fcntl.h>#include <string.h>#include <stddef.h>#include <sys/sysmacros.h>#include <sys/types.h>#include <sys/wait.h>#include <sys/socket.h>#include <sys/stat.h>#include <sys/mman.h>#include <sys/syscall.h>#include <sys/user.h>#include <sys/ioctl.h>#include <sys/ptrace.h>#include <sys/mount.h>#include <linux/limits.h>#include <linux/filter.h>#include <linux/seccomp.h>
#define ARRAY_SIZE(x) (sizeof(x) / sizeof(*(x)))
static int seccomp(unsigned int op, unsigned int flags, void *args){ errno = 0; return syscall(__NR_seccomp, op, flags, args);}
static int send_fd(int sock, int fd){ struct msghdr msg = {}; struct cmsghdr *cmsg; int *fd_ptr; char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c'; struct iovec io = { .iov_base = &c, .iov_len = 1, };
msg.msg_iov = &io; msg.msg_iovlen = 1; msg.msg_control = buf; msg.msg_controllen = sizeof(buf); cmsg = CMSG_FIRSTHDR(&msg); cmsg->cmsg_level = SOL_SOCKET; cmsg->cmsg_type = SCM_RIGHTS; cmsg->cmsg_len = CMSG_LEN(sizeof(int)); fd_ptr = (int *)CMSG_DATA(cmsg); *fd_ptr = fd; msg.msg_controllen = cmsg->cmsg_len;
if (sendmsg(sock, &msg, 0) < 0) { perror("sendmsg"); return -1; }
return 0;}
static int recv_fd(int sock){ struct msghdr msg = {}; struct cmsghdr *cmsg; int *fd_ptr; char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c'; struct iovec io = { .iov_base = &c, .iov_len = 1, };
msg.msg_iov = &io; msg.msg_iovlen = 1; msg.msg_control = buf; msg.msg_controllen = sizeof(buf);
if (recvmsg(sock, &msg, 0) < 0) { perror("recvmsg"); return -1; }
cmsg = CMSG_FIRSTHDR(&msg); fd_ptr = (int *)CMSG_DATA(cmsg);
return *fd_ptr;}
static int user_trap_syscall(int nr, unsigned int flags){ struct sock_filter filter[] = { BPF_STMT(BPF_LD+BPF_W+BPF_ABS, offsetof(struct seccomp_data, nr)), BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1), BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_USER_NOTIF), BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), };
struct sock_fprog prog = { .len = (unsigned short)ARRAY_SIZE(filter), .filter = filter, };
return seccomp(SECCOMP_SET_MODE_FILTER, flags, &prog);}
static int handle_req(struct seccomp_notif *req, struct seccomp_notif_resp *resp, int listener){ char path[PATH_MAX], source[PATH_MAX], target[PATH_MAX]; int ret = -1, mem;
resp->id = req->id; resp->error = -EPERM; resp->val = 0;
if (req->data.nr != __NR_mount) { fprintf(stderr, "huh? trapped something besides mount? %d\n", req->data.nr); return -1; }
/* Only allow bind mounts. */ if (!(req->data.args[3] & MS_BIND)) return 0;
/* * Ok, let's read the task's memory to see where they wanted their * mount to go. */ snprintf(path, sizeof(path), "/proc/%d/mem", req->pid); mem = open(path, O_RDONLY); if (mem < 0) { perror("open mem"); return -1; }
/* * Now we avoid a TOCTOU: we referred to a pid by its pid, but since * the pid that made the syscall may have died, we need to confirm that * the pid is still valid after we open its /proc/pid/mem file. We can * ask the listener fd this as follows. * * Note that this check should occur *after* any task-specific * resources are opened, to make sure that the task has not died and * we're not wrongly reading someone else's state in order to make * decisions. */ if (ioctl(listener, SECCOMP_IOCTL_NOTIF_ID_VALID, &req->id) < 0) { fprintf(stderr, "task died before we could map its memory\n"); goto out; }
/* * Phew, we've got the right /proc/pid/mem. Now we can read it. Note * that to avoid another TOCTOU, we should read all of the pointer args * before we decide to allow the syscall. */ if (lseek(mem, req->data.args[0], SEEK_SET) < 0) { perror("seek"); goto out; }
ret = read(mem, source, sizeof(source)); if (ret < 0) { perror("read"); goto out; }
if (lseek(mem, req->data.args[1], SEEK_SET) < 0) { perror("seek"); goto out; }
ret = read(mem, target, sizeof(target)); if (ret < 0) { perror("read"); goto out; }
/* * Our policy is to only allow bind mounts inside /tmp. This isn't very * interesting, because we could do unprivlieged bind mounts with user * namespaces already, but you get the idea. */ if (!strncmp(source, "/tmp/", 5) && !strncmp(target, "/tmp/", 5)) { if (mount(source, target, NULL, req->data.args[3], NULL) < 0) { ret = -1; perror("actual mount"); goto out; } resp->error = 0; }
/* Even if we didn't allow it because of policy, generating the * response was be a success, because we want to tell the worker EPERM. */ ret = 0;
out: close(mem); return ret;}
int main(void){ int sk_pair[2], ret = 1, status, listener; pid_t worker = 0 , tracer = 0;
if (socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair) < 0) { perror("socketpair"); return 1; }
worker = fork(); if (worker < 0) { perror("fork"); goto close_pair; }
if (worker == 0) { listener = user_trap_syscall(__NR_mount, SECCOMP_FILTER_FLAG_NEW_LISTENER); if (listener < 0) { perror("seccomp"); exit(1); }
/* * Drop privileges. We definitely can't mount as uid 1000. */ if (setuid(1000) < 0) { perror("setuid"); exit(1); }
/* * Send the listener to the parent; also serves as * synchronization. */ if (send_fd(sk_pair[1], listener) < 0) exit(1); close(listener);
if (mkdir("/tmp/foo", 0755) < 0) { perror("mkdir"); exit(1); }
/* * Try a bad mount just for grins. */ if (mount("/dev/sda", "/tmp/foo", NULL, 0, NULL) != -1) { fprintf(stderr, "huh? mounted /dev/sda?\n"); exit(1); }
if (errno != EPERM) { perror("bad error from mount"); exit(1); }
/* * Ok, we expect this one to succeed. */ if (mount("/tmp/foo", "/tmp/foo", NULL, MS_BIND, NULL) < 0) { perror("mount"); exit(1); }
exit(0); }
/* * Get the listener from the child. */ listener = recv_fd(sk_pair[0]); if (listener < 0) goto out_kill;
/* * Fork a task to handle the requests. This isn't strictly necessary, * but it makes the particular writing of this sample easier, since we * can just wait ofr the tracee to exit and kill the tracer. */ tracer = fork(); if (tracer < 0) { perror("fork"); goto out_kill; }
if (tracer == 0) { struct seccomp_notif *req; struct seccomp_notif_resp *resp; struct seccomp_notif_sizes sizes;
if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) < 0) { perror("seccomp(GET_NOTIF_SIZES)"); goto out_close; }
req = malloc(sizes.seccomp_notif); if (!req) goto out_close;
resp = malloc(sizes.seccomp_notif_resp); if (!resp) goto out_req; memset(resp, 0, sizes.seccomp_notif_resp);
while (1) { memset(req, 0, sizes.seccomp_notif); if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, req)) { perror("ioctl recv"); goto out_resp; }
if (handle_req(req, resp, listener) < 0) goto out_resp;
/* * ENOENT here means that the task may have gotten a * signal and restarted the syscall. It's up to the * handler to decide what to do in this case, but for * the sample code, we just ignore it. Probably * something better should happen, like undoing the * mount, or keeping track of the args to make sure we * don't do it again. */ if (ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, resp) < 0 && errno != ENOENT) { perror("ioctl send"); goto out_resp; } }out_resp: free(resp);out_req: free(req);out_close: close(listener); exit(1); }
close(listener);
if (waitpid(worker, &status, 0) != worker) { perror("waitpid"); goto out_kill; }
if (umount2("/tmp/foo", MNT_DETACH) < 0 && errno != EINVAL) { perror("umount2"); goto out_kill; }
if (remove("/tmp/foo") < 0 && errno != ENOENT) { perror("remove"); exit(1); }
if (!WIFEXITED(status) || WEXITSTATUS(status)) { fprintf(stderr, "worker exited nonzero\n"); goto out_kill; }
ret = 0;
out_kill: if (tracer > 0) kill(tracer, SIGKILL); if (worker > 0) kill(worker, SIGKILL);
close_pair: close(sk_pair[0]); close(sk_pair[1]); return ret;}