Backport of:

From 6a51b9e1d0cf0bf8515f7201b68fb0a3482b3dc1 Mon Sep 17 00:00:00 2001
From: Matt Caswell <matt@openssl.org>
Date: Tue, 2 Feb 2021 17:17:23 +0000
Subject: [PATCH] Don't overflow the output length in EVP_CipherUpdate calls

CVE-2021-23840

Reviewed-by: Paul Dale <pauli@openssl.org>
---
 crypto/err/openssl.txt   |  3 ++-
 crypto/evp/evp_enc.c     | 27 +++++++++++++++++++++++++++
 crypto/evp/evp_err.c     |  4 +++-
 include/openssl/evperr.h |  7 +++----
 4 files changed, 35 insertions(+), 6 deletions(-)

--- a/crypto/evp/evp_enc.c
+++ b/crypto/evp/evp_enc.c
@@ -354,6 +354,19 @@ int EVP_EncryptUpdate(EVP_CIPHER_CTX *ct
             return 1;
         } else {
             j = bl - i;
+
+            /*
+             * Once we've processed the first j bytes from in, the amount of
+             * data left that is a multiple of the block length is:
+             * (inl - j) & ~(bl - 1)
+             * We must ensure that this amount of data, plus the one block that
+             * we process from ctx->buf does not exceed INT_MAX
+             */
+            if (((inl - j) & ~(bl - 1)) > INT_MAX - bl) {
+                EVPerr(EVP_F_EVP_ENCRYPTDECRYPTUPDATE,
+                       EVP_R_OUTPUT_WOULD_OVERFLOW);
+                return 0;
+            }
             memcpy(&(ctx->buf[i]), in, j);
             if (!M_do_cipher(ctx, out, ctx->buf, bl))
                 return 0;
@@ -455,6 +468,19 @@ int EVP_DecryptUpdate(EVP_CIPHER_CTX *ct
     OPENSSL_assert(b <= sizeof ctx->final);
 
     if (ctx->final_used) {
+        /*
+         * final_used is only ever set if buf_len is 0. Therefore the maximum
+         * length output we will ever see from evp_EncryptDecryptUpdate is
+         * the maximum multiple of the block length that is <= inl, or just:
+         * inl & ~(b - 1)
+         * Since final_used has been set then the final output length is:
+         * (inl & ~(b - 1)) + b
+         * This must never exceed INT_MAX
+         */
+        if ((inl & ~(b - 1)) > INT_MAX - b) {
+            EVPerr(EVP_F_EVP_DECRYPTUPDATE, EVP_R_OUTPUT_WOULD_OVERFLOW);
+            return 0;
+        }
         memcpy(out, ctx->final, b);
         out += b;
         fix_len = 1;
--- a/crypto/evp/evp_err.c
+++ b/crypto/evp/evp_err.c
@@ -215,6 +215,7 @@ static ERR_STRING_DATA EVP_str_reasons[]
     {ERR_REASON(EVP_R_OPERATION_NOT_SUPPORTED_FOR_THIS_KEYTYPE),
      "operation not supported for this keytype"},
     {ERR_REASON(EVP_R_OPERATON_NOT_INITIALIZED), "operaton not initialized"},
+    {ERR_REASON(EVP_R_OUTPUT_WOULD_OVERFLOW), "output would overflow"},
     {ERR_REASON(EVP_R_PKCS8_UNKNOWN_BROKEN_TYPE),
      "pkcs8 unknown broken type"},
     {ERR_REASON(EVP_R_PRIVATE_KEY_DECODE_ERROR), "private key decode error"},
--- a/crypto/evp/evp.h
+++ b/crypto/evp/evp.h
@@ -1509,6 +1509,7 @@ void ERR_load_EVP_strings(void);
 # define EVP_R_NO_VERIFY_FUNCTION_CONFIGURED              105
 # define EVP_R_OPERATION_NOT_SUPPORTED_FOR_THIS_KEYTYPE   150
 # define EVP_R_OPERATON_NOT_INITIALIZED                   151
+# define EVP_R_OUTPUT_WOULD_OVERFLOW                      184
 # define EVP_R_PKCS8_UNKNOWN_BROKEN_TYPE                  117
 # define EVP_R_PRIVATE_KEY_DECODE_ERROR                   145
 # define EVP_R_PRIVATE_KEY_ENCODE_ERROR                   146