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Fix a bug that can crash deflate on some input when using Z_FIXED.
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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.
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@madler
madler committed Apr 20, 2018
1 parent e99813d commit 5c44459
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Showing 3 changed files with 79 additions and 70 deletions.
74 changes: 54 additions & 20 deletions deflate.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -255,11 +255,6 @@ int ZEXPORT deflateInit2_(strm, level, method, windowBits, memLevel, strategy,
int wrap = 1;
static const char my_version[] = ZLIB_VERSION;

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.
*/

if (version == Z_NULL || version[0] != my_version[0] ||
stream_size != sizeof(z_stream)) {
return Z_VERSION_ERROR;
Expand Down Expand Up @@ -329,9 +324,47 @@ int ZEXPORT deflateInit2_(strm, level, method, windowBits, memLevel, strategy,

s->lit_bufsize = 1 << (memLevel + 6); /* 16K elements by default */

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);
/* 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;

if (s->window == Z_NULL || s->prev == Z_NULL || s->head == Z_NULL ||
s->pending_buf == Z_NULL) {
Expand All @@ -340,8 +373,12 @@ int ZEXPORT deflateInit2_(strm, level, method, windowBits, memLevel, strategy,
deflateEnd (strm);
return Z_MEM_ERROR;
}
s->d_buf = overlay + s->lit_bufsize/sizeof(ush);
s->l_buf = s->pending_buf + (1+sizeof(ush))*s->lit_bufsize;
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.
*/

s->level = level;
s->strategy = strategy;
Expand Down Expand Up @@ -552,7 +589,7 @@ int ZEXPORT deflatePrime (strm, bits, value)

if (deflateStateCheck(strm)) return Z_STREAM_ERROR;
s = strm->state;
if ((Bytef *)(s->d_buf) < s->pending_out + ((Buf_size + 7) >> 3))
if (s->sym_buf < s->pending_out + ((Buf_size + 7) >> 3))
return Z_BUF_ERROR;
do {
put = Buf_size - s->bi_valid;
Expand Down Expand Up @@ -1113,7 +1150,6 @@ int ZEXPORT deflateCopy (dest, source)
#else
deflate_state *ds;
deflate_state *ss;
ushf *overlay;


if (deflateStateCheck(source) || dest == Z_NULL) {
Expand All @@ -1133,8 +1169,7 @@ int ZEXPORT deflateCopy (dest, source)
ds->window = (Bytef *) ZALLOC(dest, ds->w_size, 2*sizeof(Byte));
ds->prev = (Posf *) ZALLOC(dest, ds->w_size, sizeof(Pos));
ds->head = (Posf *) ZALLOC(dest, ds->hash_size, sizeof(Pos));
overlay = (ushf *) ZALLOC(dest, ds->lit_bufsize, sizeof(ush)+2);
ds->pending_buf = (uchf *) overlay;
ds->pending_buf = (uchf *) ZALLOC(dest, ds->lit_bufsize, 4);

if (ds->window == Z_NULL || ds->prev == Z_NULL || ds->head == Z_NULL ||
ds->pending_buf == Z_NULL) {
Expand All @@ -1148,8 +1183,7 @@ int ZEXPORT deflateCopy (dest, source)
zmemcpy(ds->pending_buf, ss->pending_buf, (uInt)ds->pending_buf_size);

ds->pending_out = ds->pending_buf + (ss->pending_out - ss->pending_buf);
ds->d_buf = overlay + ds->lit_bufsize/sizeof(ush);
ds->l_buf = ds->pending_buf + (1+sizeof(ush))*ds->lit_bufsize;
ds->sym_buf = ds->pending_buf + ds->lit_bufsize;

ds->l_desc.dyn_tree = ds->dyn_ltree;
ds->d_desc.dyn_tree = ds->dyn_dtree;
Expand Down Expand Up @@ -1925,7 +1959,7 @@ local block_state deflate_fast(s, flush)
FLUSH_BLOCK(s, 1);
return finish_done;
}
if (s->last_lit)
if (s->sym_next)
FLUSH_BLOCK(s, 0);
return block_done;
}
Expand Down Expand Up @@ -2056,7 +2090,7 @@ local block_state deflate_slow(s, flush)
FLUSH_BLOCK(s, 1);
return finish_done;
}
if (s->last_lit)
if (s->sym_next)
FLUSH_BLOCK(s, 0);
return block_done;
}
Expand Down Expand Up @@ -2131,7 +2165,7 @@ local block_state deflate_rle(s, flush)
FLUSH_BLOCK(s, 1);
return finish_done;
}
if (s->last_lit)
if (s->sym_next)
FLUSH_BLOCK(s, 0);
return block_done;
}
Expand Down Expand Up @@ -2170,7 +2204,7 @@ local block_state deflate_huff(s, flush)
FLUSH_BLOCK(s, 1);
return finish_done;
}
if (s->last_lit)
if (s->sym_next)
FLUSH_BLOCK(s, 0);
return block_done;
}
25 changes: 11 additions & 14 deletions deflate.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -217,7 +217,7 @@ typedef struct internal_state {
/* Depth of each subtree used as tie breaker for trees of equal frequency
*/

uchf *l_buf; /* buffer for literals or lengths */
uchf *sym_buf; /* buffer for distances and literals/lengths */

uInt lit_bufsize;
/* Size of match buffer for literals/lengths. There are 4 reasons for
Expand All @@ -239,13 +239,8 @@ typedef struct internal_state {
* - I can't count above 4
*/

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.
*/
uInt sym_next; /* running index in sym_buf */
uInt sym_end; /* symbol table full when sym_next reaches this */

ulg opt_len; /* bit length of current block with optimal trees */
ulg static_len; /* bit length of current block with static trees */
Expand Down Expand Up @@ -325,20 +320,22 @@ void ZLIB_INTERNAL _tr_stored_block OF((deflate_state *s, charf *buf,

# define _tr_tally_lit(s, c, flush) \
{ uch cc = (c); \
s->d_buf[s->last_lit] = 0; \
s->l_buf[s->last_lit++] = cc; \
s->sym_buf[s->sym_next++] = 0; \
s->sym_buf[s->sym_next++] = 0; \
s->sym_buf[s->sym_next++] = cc; \
s->dyn_ltree[cc].Freq++; \
flush = (s->last_lit == s->lit_bufsize-1); \
flush = (s->sym_next == s->sym_end); \
}
# define _tr_tally_dist(s, distance, length, flush) \
{ uch len = (uch)(length); \
ush dist = (ush)(distance); \
s->d_buf[s->last_lit] = dist; \
s->l_buf[s->last_lit++] = len; \
s->sym_buf[s->sym_next++] = dist; \
s->sym_buf[s->sym_next++] = dist >> 8; \
s->sym_buf[s->sym_next++] = len; \
dist--; \
s->dyn_ltree[_length_code[len]+LITERALS+1].Freq++; \
s->dyn_dtree[d_code(dist)].Freq++; \
flush = (s->last_lit == s->lit_bufsize-1); \
flush = (s->sym_next == s->sym_end); \
}
#else
# define _tr_tally_lit(s, c, flush) flush = _tr_tally(s, 0, c)
Expand Down
50 changes: 14 additions & 36 deletions trees.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -416,7 +416,7 @@ local void init_block(s)

s->dyn_ltree[END_BLOCK].Freq = 1;
s->opt_len = s->static_len = 0L;
s->last_lit = s->matches = 0;
s->sym_next = s->matches = 0;
}

#define SMALLEST 1
Expand Down Expand Up @@ -948,7 +948,7 @@ void ZLIB_INTERNAL _tr_flush_block(s, buf, stored_len, last)

Tracev((stderr, "\nopt %lu(%lu) stat %lu(%lu) stored %lu lit %u ",
opt_lenb, s->opt_len, static_lenb, s->static_len, stored_len,
s->last_lit));
s->sym_next / 3));

if (static_lenb <= opt_lenb) opt_lenb = static_lenb;

Expand Down Expand Up @@ -1017,8 +1017,9 @@ int ZLIB_INTERNAL _tr_tally (s, dist, lc)
unsigned dist; /* distance of matched string */
unsigned lc; /* match length-MIN_MATCH or unmatched char (if dist==0) */
{
s->d_buf[s->last_lit] = (ush)dist;
s->l_buf[s->last_lit++] = (uch)lc;
s->sym_buf[s->sym_next++] = dist;
s->sym_buf[s->sym_next++] = dist >> 8;
s->sym_buf[s->sym_next++] = lc;
if (dist == 0) {
/* lc is the unmatched char */
s->dyn_ltree[lc].Freq++;
Expand All @@ -1033,30 +1034,7 @@ int ZLIB_INTERNAL _tr_tally (s, dist, lc)
s->dyn_ltree[_length_code[lc]+LITERALS+1].Freq++;
s->dyn_dtree[d_code(dist)].Freq++;
}

#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.
*/
return (s->sym_next == s->sym_end);
}

/* ===========================================================================
Expand All @@ -1069,13 +1047,14 @@ local void compress_block(s, ltree, dtree)
{
unsigned dist; /* distance of matched string */
int lc; /* match length or unmatched char (if dist == 0) */
unsigned lx = 0; /* running index in l_buf */
unsigned sx = 0; /* running index in sym_buf */
unsigned code; /* the code to send */
int extra; /* number of extra bits to send */

if (s->last_lit != 0) do {
dist = s->d_buf[lx];
lc = s->l_buf[lx++];
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++];
if (dist == 0) {
send_code(s, lc, ltree); /* send a literal byte */
Tracecv(isgraph(lc), (stderr," '%c' ", lc));
Expand All @@ -1100,11 +1079,10 @@ local void compress_block(s, ltree, dtree)
}
} /* literal or match pair ? */

/* 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");
/* Check that the overlay between pending_buf and sym_buf is ok: */
Assert(s->pending < s->lit_bufsize + sx, "pendingBuf overflow");

} while (lx < s->last_lit);
} while (sx < s->sym_next);

send_code(s, END_BLOCK, ltree);
}
Expand Down

2 comments on commit 5c44459

@vielmetti
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See #605 and CVE-2018-25032

@simonis
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@simonis simonis commented on 5c44459 Apr 4, 2022

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@madler do you agree with Eric Biggers on the oss-security list that this issue (i.e. CVE-2018-25032) can not occur with memLevel == 8?

With memLevel=1, zlib only uses very short blocks, so it's possible for the block header
to bring the average match cost above the 24 bits needed to hit the bug. I'm not seeing
a way for the bug to be reachable with much higher memLevels, notably the default
memLevel of 8; as far as I can tell, the average match cost can't be much over
23 bits in that case.

He has posted a reproducer which works for:

	level=7 (also 8 and 9)
	windowBits=15
	memLevel=1
	strategy=Z_DEFAULT_STRATEGY

i.e.,

    deflateInit2(&strm, 7, Z_DEFLATED, 15, 1, Z_DEFAULT_STRATEGY);

but thinks that it won't be possible to create a similar attack for memLevel == 8.

What do you think?

Thanks a lot,
Volker

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