fix CVE-2019-9278

avoid the use of unsafe integer overflow checking constructs (unsigned integer operations cannot overflow, so "u1 + u2 > u1" can be optimized away)

check for the actual sizes, which should also handle the overflows
document other places google patched, but do not seem relevant due to other restrictions

fixes https://github.com/libexif/libexif/issues/26

diff --git a/libexif/exif-data.c b/libexif/exif-data.c
index a6f9c94..6332cd1 100644 (file)
--- a/libexif/exif-data.c
+++ b/libexif/exif-data.c
@@ -192,9 +192,15 @@ exif_data_load_data_entry (ExifData *data, ExifEntry *entry,
                doff = offset + 8;
 
        /* Sanity checks */
-       if ((doff + s < doff) || (doff + s < s) || (doff + s > size)) {
+       if (doff >= size) {
                exif_log (data->priv->log, EXIF_LOG_CODE_DEBUG, "ExifData",
-                                 "Tag data past end of buffer (%u > %u)", doff+s, size);       
+                                 "Tag starts past end of buffer (%u > %u)", doff, size);
+               return 0;
+       }
+
+       if (s > size - doff) {
+               exif_log (data->priv->log, EXIF_LOG_CODE_DEBUG, "ExifData",
+                                 "Tag data goes past end of buffer (%u > %u)", doff+s, size);
                return 0;
        }
 
@@ -315,13 +321,14 @@ exif_data_load_data_thumbnail (ExifData *data, const unsigned char *d,
                               unsigned int ds, ExifLong o, ExifLong s)
 {
        /* Sanity checks */
-       if ((o + s < o) || (o + s < s) || (o + s > ds) || (o > ds)) {
-               exif_log (data->priv->log, EXIF_LOG_CODE_DEBUG, "ExifData",
-                         "Bogus thumbnail offset (%u) or size (%u).",
-                         o, s);
+       if (o >= ds) {
+               exif_log (data->priv->log, EXIF_LOG_CODE_DEBUG, "ExifData", "Bogus thumbnail offset (%u).", o);
+               return;
+       }
+       if (s > ds - o) {
+               exif_log (data->priv->log, EXIF_LOG_CODE_DEBUG, "ExifData", "Bogus thumbnail size (%u), max would be %u.", s, ds-o);
                return;
        }
-
        if (data->data) 
                exif_mem_free (data->priv->mem, data->data);
        if (!(data->data = exif_data_alloc (data, s))) {
@@ -947,7 +954,7 @@ exif_data_load_data (ExifData *data, const unsigned char *d_orig,
        exif_log (data->priv->log, EXIF_LOG_CODE_DEBUG, "ExifData", 
                  "IFD 0 at %i.", (int) offset);
 
-       /* Sanity check the offset, being careful about overflow */
+       /* ds is restricted to 16 bit above, so offset is restricted too, and offset+8 should not overflow. */
        if (offset > ds || offset + 6 + 2 > ds)
                return;
 
@@ -956,6 +963,7 @@ exif_data_load_data (ExifData *data, const unsigned char *d_orig,
 
        /* IFD 1 offset */
        n = exif_get_short (d + 6 + offset, data->priv->order);
+       /* offset < 2<<16, n is 16 bit at most, so this op will not overflow */
        if (offset + 6 + 2 + 12 * n + 4 > ds)
                return;
 
@@ -964,8 +972,8 @@ exif_data_load_data (ExifData *data, const unsigned char *d_orig,
                exif_log (data->priv->log, EXIF_LOG_CODE_DEBUG, "ExifData",
                          "IFD 1 at %i.", (int) offset);
 
-               /* Sanity check. */
-               if (offset > ds || offset + 6 > ds) {
+               /* Sanity check. ds is ensured to be above 6 above, offset is 16bit */
+               if (offset > ds - 6) {
                        exif_log (data->priv->log, EXIF_LOG_CODE_CORRUPT_DATA,
                                  "ExifData", "Bogus offset of IFD1.");
                } else {