From b667237b3a84f601ef5a707ce8eb861c3a5002d3 Mon Sep 17 00:00:00 2001
From: Tobias Brunner <tobias@strongswan.org>
Date: Tue, 28 Sep 2021 19:38:22 +0200
Subject: [PATCH] cert-cache: Prevent crash due to integer overflow/sign change

random() allocates values in the range [0, RAND_MAX], with RAND_MAX usually
equaling INT_MAX = 2^31-1.  Previously, values between 0 and 31 were added
directly to that offset before applying`% CACHE_SIZE` to get an index into
the cache array.  If the random value was very high, this resulted in an
integer overflow and a negative index value and, therefore, an out-of-bounds
access of the array and in turn dereferencing invalid pointers when trying
to acquire the read lock.  This most likely results in a segmentation fault.

Fixes: 764e8b2211ce ("reimplemented certificate cache")
Fixes: CVE-2021-41991
---
 src/libstrongswan/credentials/sets/cert_cache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libstrongswan/credentials/sets/cert_cache.c b/src/libstrongswan/credentials/sets/cert_cache.c
index f1579c60a9bc..ceebb3843725 100644
--- a/src/libstrongswan/credentials/sets/cert_cache.c
+++ b/src/libstrongswan/credentials/sets/cert_cache.c
@@ -151,7 +151,7 @@ static void cache(private_cert_cache_t *this,
 	for (try = 0; try < REPLACE_TRIES; try++)
 	{
 		/* replace a random relation */
-		offset = random();
+		offset = random() % CACHE_SIZE;
 		for (i = 0; i < CACHE_SIZE; i++)
 		{
 			rel = &this->relations[(i + offset) % CACHE_SIZE];
-- 
2.25.1

