From df6b5e29ffea2d5a3e08de92fb765fdb21c7a21e Mon Sep 17 00:00:00 2001 From: Matt Caswell Date: Mon, 19 Sep 2016 11:57:07 +0100 Subject: [PATCH] Excessive allocation of memory in dtls1_preprocess_fragment() This issue is very similar to CVE-2016-6307 described in the previous commit. The underlying defect is different but the security analysis and impacts are the same except that it impacts DTLS. A DTLS message includes 3 bytes for its length in the header for the message. This would allow for messages up to 16Mb in length. Messages of this length are excessive and OpenSSL includes a check to ensure that a peer is sending reasonably sized messages in order to avoid too much memory being consumed to service a connection. A flaw in the logic of version 1.1.0 means that memory for the message is allocated too early, prior to the excessive message length check. Due to way memory is allocated in OpenSSL this could mean an attacker could force up to 21Mb to be allocated to service a connection. This could lead to a Denial of Service through memory exhaustion. However, the excessive message length check still takes place, and this would cause the connection to immediately fail. Assuming that the application calls SSL_free() on the failed conneciton in a timely manner then the 21Mb of allocated memory will then be immediately freed again. Therefore the excessive memory allocation will be transitory in nature. This then means that there is only a security impact if: 1) The application does not call SSL_free() in a timely manner in the event that the connection fails or 2) The application is working in a constrained environment where there is very little free memory or 3) The attacker initiates multiple connection attempts such that there are multiple connections in a state where memory has been allocated for the connection; SSL_free() has not yet been called; and there is insufficient memory to service the multiple requests. Except in the instance of (1) above any Denial Of Service is likely to be transitory because as soon as the connection fails the memory is subsequently freed again in the SSL_free() call. However there is an increased risk during this period of application crashes due to the lack of memory - which would then mean a more serious Denial of Service. This issue does not affect TLS users. Issue was reported by Shi Lei (Gear Team, Qihoo 360 Inc.). CVE-2016-6308 Reviewed-by: Richard Levitte (cherry picked from commit 48c054fec3506417b2598837b8062aae7114c200) --- ssl/statem/statem_dtls.c | 35 ++++++++++++++++++----------------- 1 file changed, 18 insertions(+), 17 deletions(-) diff --git a/ssl/statem/statem_dtls.c b/ssl/statem/statem_dtls.c index de2de09796..043f41b724 100644 --- a/ssl/statem/statem_dtls.c +++ b/ssl/statem/statem_dtls.c @@ -388,6 +388,20 @@ int dtls_get_message(SSL *s, int *mt, unsigned long *len) return 1; } +/* + * dtls1_max_handshake_message_len returns the maximum number of bytes + * permitted in a DTLS handshake message for |s|. The minimum is 16KB, but + * may be greater if the maximum certificate list size requires it. + */ +static unsigned long dtls1_max_handshake_message_len(const SSL *s) +{ + unsigned long max_len = + DTLS1_HM_HEADER_LENGTH + SSL3_RT_MAX_ENCRYPTED_LENGTH; + if (max_len < (unsigned long)s->max_cert_list) + return s->max_cert_list; + return max_len; +} + static int dtls1_preprocess_fragment(SSL *s, struct hm_header_st *msg_hdr) { size_t frag_off, frag_len, msg_len; @@ -397,15 +411,16 @@ static int dtls1_preprocess_fragment(SSL *s, struct hm_header_st *msg_hdr) frag_len = msg_hdr->frag_len; /* sanity checking */ - if ((frag_off + frag_len) > msg_len) { + if ((frag_off + frag_len) > msg_len + || msg_len > dtls1_max_handshake_message_len(s)) { SSLerr(SSL_F_DTLS1_PREPROCESS_FRAGMENT, SSL_R_EXCESSIVE_MESSAGE_SIZE); return SSL_AD_ILLEGAL_PARAMETER; } if (s->d1->r_msg_hdr.frag_off == 0) { /* first fragment */ /* - * msg_len is limited to 2^24, but is effectively checked against max - * above + * msg_len is limited to 2^24, but is effectively checked against + * dtls_max_handshake_message_len(s) above */ if (!BUF_MEM_grow_clean(s->init_buf, msg_len + DTLS1_HM_HEADER_LENGTH)) { SSLerr(SSL_F_DTLS1_PREPROCESS_FRAGMENT, ERR_R_BUF_LIB); @@ -493,20 +508,6 @@ static int dtls1_retrieve_buffered_fragment(SSL *s, int *ok) return 0; } -/* - * dtls1_max_handshake_message_len returns the maximum number of bytes - * permitted in a DTLS handshake message for |s|. The minimum is 16KB, but - * may be greater if the maximum certificate list size requires it. - */ -static unsigned long dtls1_max_handshake_message_len(const SSL *s) -{ - unsigned long max_len = - DTLS1_HM_HEADER_LENGTH + SSL3_RT_MAX_ENCRYPTED_LENGTH; - if (max_len < (unsigned long)s->max_cert_list) - return s->max_cert_list; - return max_len; -} - static int dtls1_reassemble_fragment(SSL *s, const struct hm_header_st *msg_hdr, int *ok) { -- 2.34.1