crypto/evp: harden RC4_MD5 cipher.

Originally a crash in 32-bit build was reported CHACHA20-POLY1305
cipher. The crash is triggered by truncated packet and is result
of excessive hashing to the edge of accessible memory (or bogus
MAC value is produced if x86 MD5 assembly module is involved). Since
hash operation is read-only it is not considered to be exploitable
beyond a DoS condition.

Thanks to Robert Święcki for report.

CVE-2017-3731

Reviewed-by: Rich Salz <rsalz@openssl.org>

diff --git a/crypto/evp/e_rc4_hmac_md5.c b/crypto/evp/e_rc4_hmac_md5.c
index 828ed64415b429ec7e5e33a0365a88e0faed6e93..8ab18c1413a46cf6017e2b5cf1e0d6853b4f0830 100644 (file)
--- a/crypto/evp/e_rc4_hmac_md5.c
+++ b/crypto/evp/e_rc4_hmac_md5.c
@@ -219,6 +219,8 @@ static int rc4_hmac_md5_ctrl(EVP_CIPHER_CTX *ctx, int type, int arg,
             len = p[arg - 2] << 8 | p[arg - 1];
 
             if (!EVP_CIPHER_CTX_encrypting(ctx)) {
+                if (len < MD5_DIGEST_LENGTH)
+                    return -1;
                 len -= MD5_DIGEST_LENGTH;
                 p[arg - 2] = len >> 8;
                 p[arg - 1] = len;