CVE-2018-25032 (C, published 2022-03-25T09:15Z)
Description: value: zlib before 1.2.12 allows memory corruption when deflating (i.e., when compressing) if the input has many distant matches.
Severity: MEDIUM (CVSS v3 base score: 7.5)
Weakness: CWE-787 \u2014 Out-of-bounds Write
The product writes data past the end, or before the beginning, of the intended buffer.
Repository: https://github.com/madler/zlib
Files changed:
deflate.c
deflate.h
trees.c
Commit message: Fix a bug that can crash deflate on some input when using Z_FIXED.

This bug was reported by Danilo Ramos of Eideticom, Inc. It has
lain in wait 13 years before being found! The bug was introduced
in zlib 1.2.2.2, with the addition of the Z_FIXED option. That
option forces the use of fixed Huffman codes. For rare inputs with
a large number of distant matches, the pending buffer into which
the compressed data is written can overwrite the distance symbol
table which it overlays. That results in corrupted output due to
invalid distances, and can result in out-of-bound accesses,
crashing the application.

The fix here combines the distance buffer and literal/length
buffers into a single symbol buffer. Now three bytes of pending
buffer space are opened up for each literal or length/distance
pair consumed, instead of the previous two bytes. This assures
that the pending buffer cannot overwrite the symbol table, since
the maximum fixed code compressed length/distance is 31 bits, and
since there are four bytes of pending space for every three bytes
of symbol space.


Vulnerable code:
    ushf *overlay;
    /* We overlay pending_buf and d_buf+l_buf. This works since the average
     * output size for (length,distance) codes is <= 24 bits.
     */

    overlay = (ushf *) ZALLOC(strm, s->lit_bufsize, sizeof(ush)+2);
    s->pending_buf = (uchf *) overlay;
    s->pending_buf_size = (ulg)s->lit_bufsize * (sizeof(ush)+2L);
    s->d_buf = overlay + s->lit_bufsize/sizeof(ush);
    s->l_buf = s->pending_buf + (1+sizeof(ush))*s->lit_bufsize;
    if ((Bytef *)(s->d_buf) < s->pending_out + ((Buf_size + 7) >> 3))
    ushf *overlay;
    overlay = (ushf *) ZALLOC(dest, ds->lit_bufsize, sizeof(ush)+2);
    ds->pending_buf = (uchf *) overlay;
    ds->d_buf = overlay + ds->lit_bufsize/sizeof(ush);
    ds->l_buf = ds->pending_buf + (1+sizeof(ush))*ds->lit_bufsize;
    if (s->last_lit)
    if (s->last_lit)
    if (s->last_lit)
    if (s->last_lit)
    uchf *l_buf;          /* buffer for literals or lengths */
    uInt last_lit;      /* running index in l_buf */

    ushf *d_buf;
    /* Buffer for distances. To simplify the code, d_buf and l_buf have
     * the same number of elements. To use different lengths, an extra flag
     * array would be necessary.
     */
    s->d_buf[s->last_lit] = 0; \
    s->l_buf[s->last_lit++] = cc; \
    flush = (s->last_lit == s->lit_bufsize-1); \
    s->d_buf[s->last_lit] = dist; \
    s->l_buf[s->last_lit++] = len; \
    flush = (s->last_lit == s->lit_bufsize-1); \
    s->last_lit = s->matches = 0;
                s->last_lit));
    s->d_buf[s->last_lit] = (ush)dist;
    s->l_buf[s->last_lit++] = (uch)lc;

#ifdef TRUNCATE_BLOCK
    /* Try to guess if it is profitable to stop the current block here */
    if ((s->last_lit & 0x1fff) == 0 && s->level > 2) {
        /* Compute an upper bound for the compressed length */
        ulg out_length = (ulg)s->last_lit*8L;
        ulg in_length = (ulg)((long)s->strstart - s->block_start);
        int dcode;
        for (dcode = 0; dcode < D_CODES; dcode++) {
            out_length += (ulg)s->dyn_dtree[dcode].Freq *
                (5L+extra_dbits[dcode]);
        }
        out_length >>= 3;
        Tracev((stderr,"\nlast_lit %u, in %ld, out ~%ld(%ld%%) ",
               s->last_lit, in_length, out_length,
               100L - out_length*100L/in_length));
        if (s->matches < s->last_lit/2 && out_length < in_length/2) return 1;
    }
#endif
    return (s->last_lit == s->lit_bufsize-1);
    /* We avoid equality with lit_bufsize because of wraparound at 64K
     * on 16 bit machines and because stored blocks are restricted to
     * 64K-1 bytes.
     */
    unsigned lx = 0;    /* running index in l_buf */
    if (s->last_lit != 0) do {
        dist = s->d_buf[lx];
        lc = s->l_buf[lx++];
        /* Check that the overlay between pending_buf and d_buf+l_buf is ok: */
        Assert((uInt)(s->pending) < s->lit_bufsize + 2*lx,
               "pendingBuf overflow");
    } while (lx < s->last_lit);

Fixed code:
    /* We overlay pending_buf and sym_buf. This works since the average size
     * for length/distance pairs over any compressed block is assured to be 31
     * bits or less.
     *
     * Analysis: The longest fixed codes are a length code of 8 bits plus 5
     * extra bits, for lengths 131 to 257. The longest fixed distance codes are
     * 5 bits plus 13 extra bits, for distances 16385 to 32768. The longest
     * possible fixed-codes length/distance pair is then 31 bits total.
     *
     * sym_buf starts one-fourth of the way into pending_buf. So there are
     * three bytes in sym_buf for every four bytes in pending_buf. Each symbol
     * in sym_buf is three bytes -- two for the distance and one for the
     * literal/length. As each symbol is consumed, the pointer to the next
     * sym_buf value to read moves forward three bytes. From that symbol, up to
     * 31 bits are written to pending_buf. The closest the written pending_buf
     * bits gets to the next sym_buf symbol to read is just before the last
     * code is written. At that time, 31*(n-2) bits have been written, just
     * after 24*(n-2) bits have been consumed from sym_buf. sym_buf starts at
     * 8*n bits into pending_buf. (Note that the symbol buffer fills when n-1
     * symbols are written.) The closest the writing gets to what is unread is
     * then n+14 bits. Here n is lit_bufsize, which is 16384 by default, and
     * can range from 128 to 32768.
     *
     * Therefore, at a minimum, there are 142 bits of space between what is
     * written and what is read in the overlain buffers, so the symbols cannot
     * be overwritten by the compressed data. That space is actually 139 bits,
     * due to the three-bit fixed-code block header.
     *
     * That covers the case where either Z_FIXED is specified, forcing fixed
     * codes, or when the use of fixed codes is chosen, because that choice
     * results in a smaller compressed block than dynamic codes. That latter
     * condition then assures that the above analysis also covers all dynamic
     * blocks. A dynamic-code block will only be chosen to be emitted if it has
     * fewer bits than a fixed-code block would for the same set of symbols.
     * Therefore its average symbol length is assured to be less than 31. So
     * the compressed data for a dynamic block also cannot overwrite the
     * symbols from which it is being constructed.
     */

    s->pending_buf = (uchf *) ZALLOC(strm, s->lit_bufsize, 4);
    s->pending_buf_size = (ulg)s->lit_bufsize * 4;
    s->sym_buf = s->pending_buf + s->lit_bufsize;
    s->sym_end = (s->lit_bufsize - 1) * 3;
    /* We avoid equality with lit_bufsize*3 because of wraparound at 64K
     * on 16 bit machines and because stored blocks are restricted to
     * 64K-1 bytes.
     */
    if (s->sym_buf < s->pending_out + ((Buf_size + 7) >> 3))
    ds->pending_buf = (uchf *) ZALLOC(dest, ds->lit_bufsize, 4);
    ds->sym_buf = ds->pending_buf + ds->lit_bufsize;
    if (s->sym_next)
    if (s->sym_next)
    if (s->sym_next)
    if (s->sym_next)
    uchf *sym_buf;        /* buffer for distances and literals/lengths */
    uInt sym_next;      /* running index in sym_buf */
    uInt sym_end;       /* symbol table full when sym_next reaches this */
    s->sym_buf[s->sym_next++] = 0; \
    s->sym_buf[s->sym_next++] = 0; \
    s->sym_buf[s->sym_next++] = cc; \
    flush = (s->sym_next == s->sym_end); \
    s->sym_buf[s->sym_next++] = dist; \
    s->sym_buf[s->sym_next++] = dist >> 8; \
    s->sym_buf[s->sym_next++] = len; \
    flush = (s->sym_next == s->sym_end); \
    s->sym_next = s->matches = 0;
                s->sym_next / 3));
    s->sym_buf[s->sym_next++] = dist;
    s->sym_buf[s->sym_next++] = dist >> 8;
    s->sym_buf[s->sym_next++] = lc;
    return (s->sym_next == s->sym_end);
    unsigned sx = 0;    /* running index in sym_buf */
    if (s->sym_next != 0) do {
        dist = s->sym_buf[sx++] & 0xff;
        dist += (unsigned)(s->sym_buf[sx++] & 0xff) << 8;
        lc = s->sym_buf[sx++];
        /* Check that the overlay between pending_buf and sym_buf is ok: */
        Assert(s->pending < s->lit_bufsize + sx, "pendingBuf overflow");
    } while (sx < s->sym_next);