Add sanity check in ssl3_cbc_digest_record

For SSLv3 the code assumes that |header_length| > |md_block_size|. Whilst this is true for all SSLv3 ciphersuites, this fact is far from obvious by looking at the code. If this were not the case then an integer overflow would occur, leading to a subsequent buffer overflow. Therefore I have added an explicit sanity check to ensure header_length is always valid. Thanks to Kevin Wojtysiak (Int3 Solutions) and Paramjot Oberoi (Int3 Solutions) for reporting this issue. Reviewed-by: Andy Polyakov <appro@openssl.org>
openssl · Apr 30, 2015 · 29b0a15 · 29b0a15
1 parent 9d9e377
commit 29b0a15
Showing 1 changed file with 12 additions and 2 deletions.
diff --git a/ssl/s3_cbc.c b/ssl/s3_cbc.c
@@ -397,12 +397,22 @@ void ssl3_cbc_digest_record(const EVP_MD_CTX *ctx,
 
     if (k > 0) {
         if (is_sslv3) {
+            unsigned overhang;
+
             /*
              * The SSLv3 header is larger than a single block. overhang is
              * the number of bytes beyond a single block that the header
-             * consumes: either 7 bytes (SHA1) or 11 bytes (MD5).
+             * consumes: either 7 bytes (SHA1) or 11 bytes (MD5). There are no
+             * ciphersuites in SSLv3 that are not SHA1 or MD5 based and
+             * therefore we can be confident that the header_length will be
+             * greater than |md_block_size|. However we add a sanity check just
+             * in case
              */
-            unsigned overhang = header_length - md_block_size;
+            if (header_length <= md_block_size) {
+                /* Should never happen */
+                return;
+            }
+            overhang = header_length - md_block_size;
             md_transform(md_state.c, header);
             memcpy(first_block, header + md_block_size, overhang);
             memcpy(first_block + overhang, data, md_block_size - overhang);