Commit message: Add a failsafe on the maximum number of Canon MakerNote subtags.

A malicious file could be crafted to cause extremely large values in some
tags without tripping any buffer range checks.  This is bad with the libexif
representation of Canon MakerNotes because some arrays are turned into
individual tags that the application must loop around.

The largest value I've seen for failsafe_size in a (very small) sample of valid
Canon files is <5000.  The limit is set two orders of magnitude larger to avoid
tripping up falsely in case some models use much larger values.

Patch from Google.

CVE-2020-13114
CVE: CVE-2020-13114
CWE: CWE-703
Vulnerability description: An issue was discovered in libexif before 0.6.22. An unrestricted size in handling Canon EXIF MakerNote data could lead to consumption of large amounts of compute time for decoding EXIF data.

Project: libexif
File: exif-mnote-data-canon.c

exif_mnote_data_canon_load (ExifMnoteData *ne,
	const unsigned char *buf, unsigned int buf_size)
{
	ExifMnoteDataCanon *n = (ExifMnoteDataCanon *) ne;
	ExifShort c;
	size_t i, tcount, o, datao;

	if (!n || !buf || !buf_size) {
		exif_log (ne->log, EXIF_LOG_CODE_CORRUPT_DATA,
			  "ExifMnoteCanon", "Short MakerNote");
		return;
	}
	datao = 6 + n->offset;
	if (CHECKOVERFLOW(datao, buf_size, 2)) {
		exif_log (ne->log, EXIF_LOG_CODE_CORRUPT_DATA,
			  "ExifMnoteCanon", "Short MakerNote");
		return;
	}

	/* Read the number of tags */
	c = exif_get_short (buf + datao, n->order);
	datao += 2;

	/* Remove any old entries */
	exif_mnote_data_canon_clear (n);

	/* Reserve enough space for all the possible MakerNote tags */
	n->entries = exif_mem_alloc (ne->mem, sizeof (MnoteCanonEntry) * c);
	if (!n->entries) {
		EXIF_LOG_NO_MEMORY(ne->log, "ExifMnoteCanon", sizeof (MnoteCanonEntry) * c);
		return;
	}

	/* Parse the entries */
	tcount = 0;
	for (i = c, o = datao; i; --i, o += 12) {
		size_t s;

		memset(&n->entries[tcount], 0, sizeof(MnoteCanonEntry));
		if (CHECKOVERFLOW(o,buf_size,12)) {
			exif_log (ne->log, EXIF_LOG_CODE_CORRUPT_DATA,
				"ExifMnoteCanon", "Short MakerNote");
			break;
		}

		n->entries[tcount].tag        = exif_get_short (buf + o, n->order);
		n->entries[tcount].format     = exif_get_short (buf + o + 2, n->order);
		n->entries[tcount].components = exif_get_long (buf + o + 4, n->order);
		n->entries[tcount].order      = n->order;

		exif_log (ne->log, EXIF_LOG_CODE_DEBUG, "ExifMnoteCanon",
			"Loading entry 0x%x ('%s')...", n->entries[tcount].tag,
			 mnote_canon_tag_get_name (n->entries[tcount].tag));

		/* Check if we overflow the multiplication. Use buf_size as the max size for integer overflow detection,
		 * we will check the buffer sizes closer later. */
		if (	exif_format_get_size (n->entries[tcount].format) &&
			buf_size / exif_format_get_size (n->entries[tcount].format) < n->entries[tcount].components
		) {
			exif_log (ne->log, EXIF_LOG_CODE_CORRUPT_DATA,
				  "ExifMnoteCanon", "Tag size overflow detected (%u * %lu)", exif_format_get_size (n->entries[tcount].format), n->entries[tcount].components);
			continue;
		}

		/*
		 * Size? If bigger than 4 bytes, the actual data is not
		 * in the entry but somewhere else (offset).
		 */
		s = exif_format_get_size (n->entries[tcount].format) * 
								  n->entries[tcount].components;
		n->entries[tcount].size = s;
		if (!s) {
			exif_log (ne->log, EXIF_LOG_CODE_CORRUPT_DATA,
				  "ExifMnoteCanon",
				  "Invalid zero-length tag size");
			continue;

		} else {
			size_t dataofs = o + 8;
			if (s > 4) dataofs = exif_get_long (buf + dataofs, n->order) + 6;

			if (CHECKOVERFLOW(dataofs, buf_size, s)) {
				exif_log (ne->log, EXIF_LOG_CODE_DEBUG,
					"ExifMnoteCanon",
					"Tag data past end of buffer (%u > %u)",
					(unsigned)(dataofs + s), buf_size);
				continue;
			}

			n->entries[tcount].data = exif_mem_alloc (ne->mem, s);
			if (!n->entries[tcount].data) {
				EXIF_LOG_NO_MEMORY(ne->log, "ExifMnoteCanon", s);
				continue;
			}
			memcpy (n->entries[tcount].data, buf + dataofs, s);
		}

		/* Tag was successfully parsed */
		++tcount;
	}
	/* Store the count of successfully parsed tags */
	n->count = tcount;
}