Commit message: gif: Fix LZW decoder accepting invalid LZW code.

The code value after a reset wasn't being validated, which means we would
accept invalid codes. This could cause an infinite loop in the decoder.

Fixes CVE-2020-29385

Fixes https://gitlab.gnome.org/GNOME/gdk-pixbuf/-/issues/164
CVE: CVE-2020-29385
CWE: CWE-835
Vulnerability description: GNOME gdk-pixbuf (aka GdkPixbuf) before 2.42.2 allows a denial of service (infinite loop) in lzw.c in the function write_indexes. if c->self_code equals 10, self->code_table[10].extends will assign the value 11 to c. The next execution in the loop will assign self->code_table[11].extends to c, which will give the value of 10. This will make the loop run infinitely. This bug can, for example, be triggered by calling this function with a GIF image with LZW compression that is crafted in a special way.

Project: gdk-pixbuf
File: lzw.c

lzw_decoder_feed (LZWDecoder *self,
                  guint8     *input,
                  gsize       input_length,
                  guint8     *output,
                  gsize       output_length)
{
        gsize i, n_written = 0;

        g_return_val_if_fail (LZW_IS_DECODER (self), 0);

        /* Ignore data after "end of information" codeword */
        if (self->last_code == self->eoi_code)
                return 0;

        /* Processes each octet of input */
        for (i = 0; i < input_length; i++) {
                guint8 d = input[i];
                int n_available;

                /* Process the bits of the octet into codewords */
                for (n_available = 8; n_available > 0; ) {
                        int n_bits, new_bits;

                        /* Extract up the the required number of bits from the octet */
                        n_bits = MIN (self->code_size - self->code_bits, n_available);
                        new_bits = d & ((1 << n_bits) - 1);
                        d = d >> n_bits;
                        n_available -= n_bits;

                        /* Add the new bits to the code until we have a full codeword */
                        self->code = new_bits << self->code_bits | self->code;
                        self->code_bits += n_bits;
                        if (self->code_bits < self->code_size)
                                continue;

                        /* Stop on "end of information" codeword */
                        if (self->code == self->eoi_code) {
                                self->last_code = self->code;
                                return n_written;
                        }

                        /* Reset the code table on "clear" */
                        if (self->code == self->clear_code) {
                                self->code_table_size = self->eoi_code + 1;
                                self->code_size = self->min_code_size;
                        } else {
                                /* Add a new code word if space.
                                 * The first code after a clear is skipped */
                                if (self->last_code != self->clear_code && self->code_table_size < MAX_CODES) {
                                        if (self->code < self->code_table_size)
                                                add_code (self, self->code);
                                        else if (self->code == self->code_table_size)
                                                add_code (self, self->last_code);
                                        else {
                                                /* Invalid code received - just stop here */
                                                self->last_code = self->eoi_code;
                                                return output_length;
                                        }

                                        /* When table is full increase code size */
                                        if (self->code_table_size == (1 << self->code_size) && self->code_size < LZW_CODE_MAX)
                                                self->code_size++;
                                }

                                /* Convert codeword into indexes */
                                n_written += write_indexes (self, output + n_written, output_length - n_written);
                        }

                        self->last_code = self->code;
                        self->code = 0;
                        self->code_bits = 0;

                        /* Out of space */
                        if (n_written >= output_length)
                                return output_length;
                }
        }

        return n_written;
}