From 42fa3e66697baa121220b4eacf03607280e4ff89 Mon Sep 17 00:00:00 2001
From: Bernd Edlinger <bernd.edlinger@hotmail.de>
Date: Tue, 2 Jun 2020 11:52:24 +0200
Subject: [PATCH] Fix a buffer overflow in drbg_ctr_generate

This can happen if the 32-bit counter overflows
and the last block is not a multiple of 16 bytes.

Fixes #12012

[extended tests]

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Patrick Steuer <patrick.steuer@de.ibm.com>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/12016)
---
 crypto/rand/drbg_ctr.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/crypto/rand/drbg_ctr.c b/crypto/rand/drbg_ctr.c
index 050ae49652..33e1b324c6 100644
--- a/crypto/rand/drbg_ctr.c
+++ b/crypto/rand/drbg_ctr.c
@@ -366,9 +366,11 @@ __owur static int drbg_ctr_generate(RAND_DRBG *drbg,
         ctr32 = GETU32(ctr->V + 12) + blocks;
         if (ctr32 < blocks) {
             /* 32-bit counter overflow into V. */
-            blocks -= ctr32;
-            buflen = blocks * 16;
-            ctr32 = 0;
+            if (ctr32 != 0) {
+                blocks -= ctr32;
+                buflen = blocks * 16;
+                ctr32 = 0;
+            }
             ctr96_inc(ctr->V);
         }
         PUTU32(ctr->V + 12, ctr32);
-- 
2.39.2