netdev - [PATCH net 1/2] ipv6: make exception cache less predictible

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Message-Id: <20210829221615.2057201-2-eric.dumazet@gmail.com>
Date:   Sun, 29 Aug 2021 15:16:14 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Willy Tarreau <w@....eu>, Keyu Man <kman001@....edu>,
        David Ahern <dsahern@...nel.org>, Wei Wang <weiwan@...gle.com>,
        Martin KaFai Lau <kafai@...com>
Subject: [PATCH net 1/2] ipv6: make exception cache less predictible

From: Eric Dumazet <edumazet@...gle.com>

Even after commit 4785305c05b2 ("ipv6: use siphash in rt6_exception_hash()"),
an attacker can still use brute force to learn some secrets from a victim
linux host.

One way to defeat these attacks is to make the max depth of the hash
table bucket a random value.

Before this patch, each bucket of the hash table used to store exceptions
could contain 6 items under attack.

After the patch, each bucket would contains a random number of items,
between 6 and 10. The attacker can no longer infer secrets.

This is slightly increasing memory size used by the hash table,
we do not expect this to be a problem.

Following patch is dealing with the same issue in IPv4.

Fixes: 35732d01fe31 ("ipv6: introduce a hash table to store dst cache")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: Keyu Man <kman001@....edu>
Cc: Wei Wang <weiwan@...gle.com>
Cc: Martin KaFai Lau <kafai@...com>
---
 net/ipv6/route.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index c5e8ecb96426bda619fe242351e40dcf6ff68bcf..60334030210192660a7fa141163f36af7489d0ae 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1657,6 +1657,7 @@ static int rt6_insert_exception(struct rt6_info *nrt,
 	struct in6_addr *src_key = NULL;
 	struct rt6_exception *rt6_ex;
 	struct fib6_nh *nh = res->nh;
+	int max_depth;
 	int err = 0;
 
 	spin_lock_bh(&rt6_exception_lock);
@@ -1711,7 +1712,9 @@ static int rt6_insert_exception(struct rt6_info *nrt,
 	bucket->depth++;
 	net->ipv6.rt6_stats->fib_rt_cache++;
 
-	if (bucket->depth > FIB6_MAX_DEPTH)
+	/* Randomize max depth to avoid some side channels attacks. */
+	max_depth = FIB6_MAX_DEPTH + prandom_u32_max(FIB6_MAX_DEPTH);
+	while (bucket->depth > max_depth)
 		rt6_exception_remove_oldest(bucket);
 
 out:
-- 
2.33.0.259.gc128427fd7-goog