Commit message: fixed a incorrect overflow check that could be optimized away.

inspired by:
https://android.googlesource.com/platform/external/libexif/+/8e7345f3bc0bad06ac369d6cbc1124c8ceaf7d4b

https://source.android.com/security/bulletin/2020-11-01

CVE-2020-0452
CVE: CVE-2020-0452
CWE: CWE-787
Vulnerability description: In exif_entry_get_value of exif-entry.c, there is a possible out of bounds write due to an integer overflow. This could lead to remote code execution if a third party app used this library to process remote image data with no additional execution privileges needed. User interaction is not needed for exploitation.Product: AndroidVersions: Android-8.1 Android-9 Android-10 Android-11 Android-8.0Android ID: A-159625731

Project: libexif
File: exif-entry.c

exif_entry_get_value (ExifEntry *e, char *val, unsigned int maxlen)
{
	unsigned int i, j, k;
	ExifShort v_short, v_short2, v_short3, v_short4;
	ExifByte v_byte;
	ExifRational v_rat;
	ExifSRational v_srat;
	char b[64];
	const char *c;
	ExifByteOrder o;
	double d;
	ExifEntry *entry;
	static const struct {
		char label[5];
		char major, minor;
	} versions[] = {
		{"0110", 1,  1},
		{"0120", 1,  2},
		{"0200", 2,  0},
		{"0210", 2,  1},
		{"0220", 2,  2},
		{"0221", 2, 21},
		{"0230", 2,  3},
		{""    , 0,  0}
	};

	(void) bindtextdomain (GETTEXT_PACKAGE, LOCALEDIR);

	if (!e || !e->parent || !e->parent->parent || !maxlen || !val)
		return val;

	/* make sure the returned string is zero terminated */
	/* FIXME: this is inefficient in the case of long buffers and should
	 * instead be taken care of on each write instead. */
	memset (val, 0, maxlen);

	/* We need the byte order */
	o = exif_data_get_byte_order (e->parent->parent);

	/* Sanity check */
	if (e->size != e->components * exif_format_get_size (e->format)) {
		snprintf (val, maxlen, _("Invalid size of entry (%i, "
			"expected %li x %i)."), e->size, e->components,
				exif_format_get_size (e->format));
		return val;
	}

	switch (e->tag) {
	case EXIF_TAG_USER_COMMENT:

		/*
		 * The specification says UNDEFINED, but some
		 * manufacturers don't care and use ASCII. If this is the
		 * case here, only refuse to read it if there is no chance
		 * of finding readable data.
		 */
		if ((e->format != EXIF_FORMAT_ASCII) || 
		    (e->size <= 8) ||
		    ( memcmp (e->data, "ASCII\0\0\0"  , 8) &&
		      memcmp (e->data, "UNICODE\0"    , 8) &&
		      memcmp (e->data, "JIS\0\0\0\0\0", 8) &&
		      memcmp (e->data, "\0\0\0\0\0\0\0\0", 8)))
			CF (e, EXIF_FORMAT_UNDEFINED, val, maxlen);

		/*
		 * Note that, according to the specification (V2.1, p 40),
		 * the user comment field does not have to be 
		 * NULL terminated.
		 */
		if ((e->size >= 8) && !memcmp (e->data, "ASCII\0\0\0", 8)) {
			strncpy (val, (char *) e->data + 8, MIN (e->size - 8, maxlen-1));
			break;
		}
		if ((e->size >= 8) && !memcmp (e->data, "UNICODE\0", 8)) {
			strncpy (val, _("Unsupported UNICODE string"), maxlen-1);
		/* FIXME: use iconv to convert into the locale encoding.
		 * EXIF 2.2 implies (but does not say) that this encoding is
		 * UCS-2.
		 */
			break;
		}
		if ((e->size >= 8) && !memcmp (e->data, "JIS\0\0\0\0\0", 8)) {
			strncpy (val, _("Unsupported JIS string"), maxlen-1);
		/* FIXME: use iconv to convert into the locale encoding */
			break;
		}

		/* Check if there is really some information in the tag. */
		for (i = 0; (i < e->size) &&
			    (!e->data[i] || (e->data[i] == ' ')); i++);
		if (i == e->size) break;

		/*
		 * If we reach this point, the tag does not
 		 * comply with the standard but seems to contain data.
		 * Print as much as possible.
		 * Note: make sure we do not overwrite the final \0 at maxlen-1
		 */
		exif_entry_log (e, EXIF_LOG_CODE_DEBUG,
			_("Tag UserComment contains data but is "
			  "against specification."));
 		for (j = 0; (i < e->size) && (j < maxlen-1); i++, j++) {
			exif_entry_log (e, EXIF_LOG_CODE_DEBUG,
				_("Byte at position %i: 0x%02x"), i, e->data[i]);
 			val[j] = isprint (e->data[i]) ? e->data[i] : '.';
		}
		break;

	case EXIF_TAG_EXIF_VERSION:
		CF (e, EXIF_FORMAT_UNDEFINED, val, maxlen);
		CC (e, 4, val, maxlen);
		strncpy (val, _("Unknown Exif Version"), maxlen-1);
		for (i = 0; *versions[i].label; i++) {
			if (!memcmp (e->data, versions[i].label, 4)) {
    				snprintf (val, maxlen,
					_("Exif Version %d.%d"),
					versions[i].major,
					versions[i].minor);
    				break;
			}
		}
		break;
	case EXIF_TAG_FLASH_PIX_VERSION:
		CF (e, EXIF_FORMAT_UNDEFINED, val, maxlen);
		CC (e, 4, val, maxlen);
		if (!memcmp (e->data, "0100", 4))
			strncpy (val, _("FlashPix Version 1.0"), maxlen-1);
		else if (!memcmp (e->data, "0101", 4))
			strncpy (val, _("FlashPix Version 1.01"), maxlen-1);
		else
			strncpy (val, _("Unknown FlashPix Version"), maxlen-1);
		break;
	case EXIF_TAG_COPYRIGHT:
		CF (e, EXIF_FORMAT_ASCII, val, maxlen);

		/*
		 * First part: Photographer.
		 * Some cameras store a string like "   " here. Ignore it.
		 * Remember that a corrupted tag might not be NUL-terminated
		 */
		if (e->size && e->data && match_repeated_char(e->data, ' ', e->size))
			strncpy (val, (char *) e->data, MIN (maxlen-1, e->size));
		else
			strncpy (val, _("[None]"), maxlen-1);
		strncat (val, " ", maxlen-1 - strlen (val));
		strncat (val, _("(Photographer)"), maxlen-1 - strlen (val));

		/* Second part: Editor. */
		strncat (val, " - ", maxlen-1 - strlen (val));
		k = 0;
		if (e->size && e->data) {
			const unsigned char *tagdata = memchr(e->data, 0, e->size);
			if (tagdata++) {
				unsigned int editor_ofs = tagdata - e->data;
				unsigned int remaining = e->size - editor_ofs;
				if (match_repeated_char(tagdata, ' ', remaining)) {
					strncat (val, (const char*)tagdata, MIN (maxlen-1 - strlen (val), remaining));
					++k;
				}
			}
		}
		if (!k)
			strncat (val, _("[None]"), maxlen-1 - strlen (val));
		strncat (val, " ", maxlen-1 - strlen (val));
		strncat (val, _("(Editor)"), maxlen-1 - strlen (val));

		break;
	case EXIF_TAG_FNUMBER:
		CF (e, EXIF_FORMAT_RATIONAL, val, maxlen);
		CC (e, 1, val, maxlen);
		v_rat = exif_get_rational (e->data, o);
		if (!v_rat.denominator) {
			exif_entry_format_value(e, val, maxlen);
			break;
		}
		d = (double) v_rat.numerator / (double) v_rat.denominator;
		snprintf (val, maxlen, "f/%.01f", d);
		break;
	case EXIF_TAG_APERTURE_VALUE:
	case EXIF_TAG_MAX_APERTURE_VALUE:
		CF (e, EXIF_FORMAT_RATIONAL, val, maxlen);
		CC (e, 1, val, maxlen);
		v_rat = exif_get_rational (e->data, o);
		if (!v_rat.denominator || (0x80000000 == v_rat.numerator)) {
			exif_entry_format_value(e, val, maxlen);
			break;
		}
		d = (double) v_rat.numerator / (double) v_rat.denominator;
		snprintf (val, maxlen, _("%.02f EV"), d);
		snprintf (b, sizeof (b), _(" (f/%.01f)"), pow (2, d / 2.));
		strncat (val, b, maxlen-1 - strlen (val));
		break;
	case EXIF_TAG_FOCAL_LENGTH:
		CF (e, EXIF_FORMAT_RATIONAL, val, maxlen);
		CC (e, 1, val, maxlen);
		v_rat = exif_get_rational (e->data, o);
		if (!v_rat.denominator) {
			exif_entry_format_value(e, val, maxlen);
			break;
		}

		/*
		 * For calculation of the 35mm equivalent,
		 * Minolta cameras need a multiplier that depends on the
		 * camera model.
		 */
		d = 0.;
		entry = exif_content_get_entry (
			e->parent->parent->ifd[EXIF_IFD_0], EXIF_TAG_MAKE);
		if (entry && entry->data && entry->size >= 7 &&
		    !strncmp ((char *)entry->data, "Minolta", 7)) {
			entry = exif_content_get_entry (
					e->parent->parent->ifd[EXIF_IFD_0],
					EXIF_TAG_MODEL);
			if (entry && entry->data && entry->size >= 8) {
				if (!strncmp ((char *)entry->data, "DiMAGE 7", 8))
					d = 3.9;
				else if (!strncmp ((char *)entry->data, "DiMAGE 5", 8))
					d = 4.9;
			}
		}
		if (d)
			snprintf (b, sizeof (b), _(" (35 equivalent: %.0f mm)"),
				  (d * (double) v_rat.numerator /
				       (double) v_rat.denominator));
		else
			b[0] = 0;

		d = (double) v_rat.numerator / (double) v_rat.denominator;
		snprintf (val, maxlen, "%.1f mm", d);
		strncat (val, b, maxlen-1 - strlen (val));
		break;
	case EXIF_TAG_SUBJECT_DISTANCE:
		CF (e, EXIF_FORMAT_RATIONAL, val, maxlen);
		CC (e, 1, val, maxlen);
		v_rat = exif_get_rational (e->data, o);
		if (!v_rat.denominator) {
			exif_entry_format_value(e, val, maxlen);
			break;
		}
		d = (double) v_rat.numerator / (double) v_rat.denominator;
		snprintf (val, maxlen, "%.1f m", d);
		break;
	case EXIF_TAG_EXPOSURE_TIME:
		CF (e, EXIF_FORMAT_RATIONAL, val, maxlen);
		CC (e, 1, val, maxlen);
		v_rat = exif_get_rational (e->data, o);
		if (!v_rat.denominator) {
			exif_entry_format_value(e, val, maxlen);
			break;
		}
		d = (double) v_rat.numerator / (double) v_rat.denominator;
		if (d < 1 && d)
			snprintf (val, maxlen, _("1/%.0f"), 1. / d);
		else
			snprintf (val, maxlen, "%.0f", d);
		strncat (val, _(" sec."), maxlen-1 - strlen (val));
		break;
	case EXIF_TAG_SHUTTER_SPEED_VALUE:
		CF (e, EXIF_FORMAT_SRATIONAL, val, maxlen);
		CC (e, 1, val, maxlen);
		v_srat = exif_get_srational (e->data, o);
		if (!v_srat.denominator) {
			exif_entry_format_value(e, val, maxlen);
			break;
		}
		d = (double) v_srat.numerator / (double) v_srat.denominator;
		snprintf (val, maxlen, _("%.02f EV"), d);
		if (pow (2, d))
			d = 1. / pow (2, d);
		if (d < 1 && d)
		  snprintf (b, sizeof (b), _(" (1/%.0f sec.)"), 1. / d);
		else
		  snprintf (b, sizeof (b), _(" (%.0f sec.)"), d);
		strncat (val, b, maxlen-1 - strlen (val));
		break;
	case EXIF_TAG_BRIGHTNESS_VALUE:
		CF (e, EXIF_FORMAT_SRATIONAL, val, maxlen);
		CC (e, 1, val, maxlen);
		v_srat = exif_get_srational (e->data, o);
		if (!v_srat.denominator) {
			exif_entry_format_value(e, val, maxlen);
			break;
		}
		d = (double) v_srat.numerator / (double) v_srat.denominator;
		snprintf (val, maxlen, _("%.02f EV"), d);
		snprintf (b, sizeof (b), _(" (%.02f cd/m^2)"),
			1. / (M_PI * 0.3048 * 0.3048) * pow (2, d));
		strncat (val, b, maxlen-1 - strlen (val));
		break;
	case EXIF_TAG_FILE_SOURCE:
		CF (e, EXIF_FORMAT_UNDEFINED, val, maxlen);
		CC (e, 1, val, maxlen);
		v_byte = e->data[0];
		if (v_byte == 3)
			strncpy (val, _("DSC"), maxlen-1);
		else
			snprintf (val, maxlen, _("Internal error (unknown "
				  "value %i)"), v_byte);
		break;
	case EXIF_TAG_COMPONENTS_CONFIGURATION:
		CF (e, EXIF_FORMAT_UNDEFINED, val, maxlen);
		CC (e, 4, val, maxlen);
		for (i = 0; i < 4; i++) {
			switch (e->data[i]) {
			case 0: c = _("-"); break;
			case 1: c = _("Y"); break;
			case 2: c = _("Cb"); break;
			case 3: c = _("Cr"); break;
			case 4: c = _("R"); break;
			case 5: c = _("G"); break;
			case 6: c = _("B"); break;
			default: c = _("Reserved"); break;
			}
			strncat (val, c, maxlen-1 - strlen (val));
			if (i < 3)
				strncat (val, " ", maxlen-1 - strlen (val));
		}
		break;
	case EXIF_TAG_EXPOSURE_BIAS_VALUE:
		CF (e, EXIF_FORMAT_SRATIONAL, val, maxlen);
		CC (e, 1, val, maxlen);
		v_srat = exif_get_srational (e->data, o);
		if (!v_srat.denominator) {
			exif_entry_format_value(e, val, maxlen);
			break;
		}
		d = (double) v_srat.numerator / (double) v_srat.denominator;
		snprintf (val, maxlen, _("%.02f EV"), d);
		break;
	case EXIF_TAG_SCENE_TYPE:
		CF (e, EXIF_FORMAT_UNDEFINED, val, maxlen);
		CC (e, 1, val, maxlen);
		v_byte = e->data[0];
		if (v_byte == 1)
			strncpy (val, _("Directly photographed"), maxlen-1);
		else
			snprintf (val, maxlen, _("Internal error (unknown "
				  "value %i)"), v_byte);
		break;
	case EXIF_TAG_YCBCR_SUB_SAMPLING:
		CF (e, EXIF_FORMAT_SHORT, val, maxlen);
		CC (e, 2, val, maxlen);
		v_short  = exif_get_short (e->data, o);
		v_short2 = exif_get_short (
			e->data + exif_format_get_size (e->format),
			o);
		if ((v_short == 2) && (v_short2 == 1))
			strncpy (val, _("YCbCr4:2:2"), maxlen-1);
		else if ((v_short == 2) && (v_short2 == 2))
			strncpy (val, _("YCbCr4:2:0"), maxlen-1);
		else
			snprintf (val, maxlen, "%u, %u", v_short, v_short2);
		break;
	case EXIF_TAG_SUBJECT_AREA:
		CF (e, EXIF_FORMAT_SHORT, val, maxlen);
		switch (e->components) {
		case 2:
			v_short  = exif_get_short (e->data, o);
			v_short2 = exif_get_short (e->data + 2, o);
			snprintf (val, maxlen, "(x,y) = (%i,%i)",
				  v_short, v_short2);
			break;
		case 3:
			v_short  = exif_get_short (e->data, o);
			v_short2 = exif_get_short (e->data + 2, o);
			v_short3 = exif_get_short (e->data + 4, o);
			snprintf (val, maxlen, _("Within distance %i of "
				"(x,y) = (%i,%i)"), v_short3, v_short,
				v_short2);
			break;
		case 4:
			v_short  = exif_get_short (e->data, o);
			v_short2 = exif_get_short (e->data + 2, o);
			v_short3 = exif_get_short (e->data + 4, o);
			v_short4 = exif_get_short (e->data + 6, o);
			snprintf (val, maxlen, _("Within rectangle "
				"(width %i, height %i) around "
				"(x,y) = (%i,%i)"), v_short3, v_short4,
				v_short, v_short2);
			break;
		default:
			snprintf (val, maxlen, _("Unexpected number "
				"of components (%li, expected 2, 3, or 4)."),
				e->components);	
		}
		break;
	case EXIF_TAG_GPS_VERSION_ID:
		/* This is only valid in the GPS IFD */
		CF (e, EXIF_FORMAT_BYTE, val, maxlen);
		CC (e, 4, val, maxlen);
		v_byte = e->data[0];
		snprintf (val, maxlen, "%u", v_byte);
		for (i = 1; i < e->components; i++) {
			v_byte = e->data[i];
			snprintf (b, sizeof (b), ".%u", v_byte);
			strncat (val, b, maxlen-1 - strlen (val));
		}
		break;
	case EXIF_TAG_INTEROPERABILITY_VERSION:
	/* a.k.a. case EXIF_TAG_GPS_LATITUDE: */
		/* This tag occurs in EXIF_IFD_INTEROPERABILITY */
		if (e->format == EXIF_FORMAT_UNDEFINED) {
			strncpy (val, (char *) e->data, MIN (maxlen-1, e->size));
			break;
		}
		/* EXIF_TAG_GPS_LATITUDE is the same numerically as
		 * EXIF_TAG_INTEROPERABILITY_VERSION but in EXIF_IFD_GPS
		 */
		exif_entry_format_value(e, val, maxlen);
		break;
	case EXIF_TAG_GPS_ALTITUDE_REF:
		/* This is only valid in the GPS IFD */
		CF (e, EXIF_FORMAT_BYTE, val, maxlen);
		CC (e, 1, val, maxlen);
		v_byte = e->data[0];
		if (v_byte == 0)
			strncpy (val, _("Sea level"), maxlen-1);
		else if (v_byte == 1)
			strncpy (val, _("Sea level reference"), maxlen-1);
		else
			snprintf (val, maxlen, _("Internal error (unknown "
				  "value %i)"), v_byte);
		break;
	case EXIF_TAG_GPS_TIME_STAMP:
		/* This is only valid in the GPS IFD */
		CF (e, EXIF_FORMAT_RATIONAL, val, maxlen);
		CC (e, 3, val, maxlen);

		v_rat  = exif_get_rational (e->data, o);
		if (!v_rat.denominator) {
			exif_entry_format_value(e, val, maxlen);
			break;
		}
		i = v_rat.numerator / v_rat.denominator;

		v_rat = exif_get_rational (e->data +
					     exif_format_get_size (e->format),
					   o);
		if (!v_rat.denominator) {
			exif_entry_format_value(e, val, maxlen);
			break;
		}
		j = v_rat.numerator / v_rat.denominator;

		v_rat = exif_get_rational (e->data +
					     2*exif_format_get_size (e->format),
					     o);
		if (!v_rat.denominator) {
			exif_entry_format_value(e, val, maxlen);
			break;
		}
		d = (double) v_rat.numerator / (double) v_rat.denominator;
		snprintf (val, maxlen, "%02u:%02u:%05.2f", i, j, d);
		break;

	case EXIF_TAG_METERING_MODE:
	case EXIF_TAG_COMPRESSION:
	case EXIF_TAG_LIGHT_SOURCE:
	case EXIF_TAG_FOCAL_PLANE_RESOLUTION_UNIT:
	case EXIF_TAG_RESOLUTION_UNIT:
	case EXIF_TAG_EXPOSURE_PROGRAM:
	case EXIF_TAG_FLASH:
	case EXIF_TAG_SUBJECT_DISTANCE_RANGE:
	case EXIF_TAG_COLOR_SPACE:
		CF (e,EXIF_FORMAT_SHORT, val, maxlen);
		CC (e, 1, val, maxlen);
		v_short = exif_get_short (e->data, o);

		/* Search the tag */
		for (i = 0; list2[i].tag && (list2[i].tag != e->tag); i++);
		if (!list2[i].tag) {
			snprintf (val, maxlen, _("Internal error (unknown "
				  "value %i)"), v_short);
			break;
		}

		/* Find the value */
		for (j = 0; list2[i].elem[j].values[0] &&
			    (list2[i].elem[j].index < v_short); j++);
		if (list2[i].elem[j].index != v_short) {
			snprintf (val, maxlen, _("Internal error (unknown "
				  "value %i)"), v_short);
			break;
		}

		/* Find a short enough value */
		memset (val, 0, maxlen);
		for (k = 0; list2[i].elem[j].values[k]; k++) {
			size_t l = strlen (_(list2[i].elem[j].values[k]));
			if ((maxlen > l) && (strlen (val) < l))
				strncpy (val, _(list2[i].elem[j].values[k]), maxlen-1);
		}
		if (!val[0]) snprintf (val, maxlen, "%i", v_short);

		break;

	case EXIF_TAG_PLANAR_CONFIGURATION:
	case EXIF_TAG_SENSING_METHOD:
	case EXIF_TAG_ORIENTATION:
	case EXIF_TAG_YCBCR_POSITIONING:
	case EXIF_TAG_PHOTOMETRIC_INTERPRETATION:
	case EXIF_TAG_CUSTOM_RENDERED:
	case EXIF_TAG_EXPOSURE_MODE:
	case EXIF_TAG_WHITE_BALANCE:
	case EXIF_TAG_SCENE_CAPTURE_TYPE:
	case EXIF_TAG_GAIN_CONTROL:
	case EXIF_TAG_SATURATION:
	case EXIF_TAG_CONTRAST:
	case EXIF_TAG_SHARPNESS:
		CF (e, EXIF_FORMAT_SHORT, val, maxlen);
		CC (e, 1, val, maxlen);
		v_short = exif_get_short (e->data, o);

		/* Search the tag */
		for (i = 0; list[i].tag && (list[i].tag != e->tag); i++);
		if (!list[i].tag) {
			snprintf (val, maxlen, _("Internal error (unknown "
				  "value %i)"), v_short);
			break;
		}

		/* Find the value */
		for (j = 0; list[i].strings[j] && (j < v_short); j++);
		if (!list[i].strings[j])
			snprintf (val, maxlen, "%i", v_short);
		else if (!*list[i].strings[j])
			snprintf (val, maxlen, _("Unknown value %i"), v_short);
		else
			strncpy (val, _(list[i].strings[j]), maxlen-1);
		break;

	case EXIF_TAG_XP_TITLE:
	case EXIF_TAG_XP_COMMENT:
	case EXIF_TAG_XP_AUTHOR:
	case EXIF_TAG_XP_KEYWORDS:
	case EXIF_TAG_XP_SUBJECT:
	{
		unsigned char *utf16;

		/* Sanity check the size to prevent overflow */
		if (e->size+sizeof(uint16_t)+1 < e->size) break;

		/* The tag may not be U+0000-terminated , so make a local
		   U+0000-terminated copy before converting it */
		utf16 = exif_mem_alloc (e->priv->mem, e->size+sizeof(uint16_t)+1);
		if (!utf16) break;
		memcpy(utf16, e->data, e->size);

		/* NUL terminate the string. If the size is odd (which isn't possible
		 * for a valid UTF16 string), then this will overwrite the high byte of
		 * the final half word, plus add a full zero NUL word at the end.
		 */
		utf16[e->size] = 0;
		utf16[e->size+1] = 0;
		utf16[e->size+2] = 0;

		/* Warning! The texts are converted from UTF16 to UTF8 */
		/* FIXME: use iconv to convert into the locale encoding */
		exif_convert_utf16_to_utf8(val, utf16, maxlen);
		exif_mem_free(e->priv->mem, utf16);
		break;
	}

	default:
		/* Use a generic value formatting */
		exif_entry_format_value(e, val, maxlen);
	}

	return val;
}