From 576129cd72ae054d246221f111aabf42b9c6d76d Mon Sep 17 00:00:00 2001 From: Matt Caswell Date: Mon, 25 Feb 2019 11:28:32 +0000 Subject: [PATCH] Ensure bn_cmp_words can handle the case where n == 0 Thanks to David Benjamin who reported this, performed the analysis and suggested the patch. I have incorporated some of his analysis in the comments below. This issue can cause an out-of-bounds read. It is believed that this was not reachable until the recent "fixed top" changes. Analysis has so far only identified one code path that can encounter this - although it is possible that others may be found. The one code path only impacts 1.0.2 in certain builds. The fuzzer found a path in RSA where iqmp is too large. If the input is all zeros, the RSA CRT logic will multiply a padded zero by iqmp. Two mitigating factors: - Private keys which trip this are invalid (iqmp is not reduced mod p). Only systems which take untrusted private keys care. - In OpenSSL 1.1.x, there is a check which rejects the oversize iqmp, so the bug is only reproducible in 1.0.2 so far. Fortunately, the bug appears to be relatively harmless. The consequences of bn_cmp_word's misbehavior are: - OpenSSL may crash if the buffers are page-aligned and the previous page is non-existent. - OpenSSL will incorrectly treat two BN_ULONG buffers as not equal when they are equal. - Side channel concerns. The first is indeed a concern and is a DoS bug. The second is fine in this context. bn_cmp_word and bn_cmp_part_words are used to compute abs(a0 - a1) in Karatsuba. If a0 = a1, it does not matter whether we use a0 - a1 or a1 - a0. The third would be worth thinking about, but it is overshadowed by the entire Karatsuba implementation not being constant time. Due to the difficulty of tripping this and the low impact no CVE is felt necessary for this issue. Reviewed-by: Paul Dale Reviewed-by: Viktor Dukhovni (Merged from https://github.com/openssl/openssl/pull/8326) --- crypto/bn/bn_lib.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/crypto/bn/bn_lib.c b/crypto/bn/bn_lib.c index 494ee49bda..e624caf9b4 100644 --- a/crypto/bn/bn_lib.c +++ b/crypto/bn/bn_lib.c @@ -695,6 +695,9 @@ int bn_cmp_words(const BN_ULONG *a, const BN_ULONG *b, int n) int i; BN_ULONG aa, bb; + if (n == 0) + return 0; + aa = a[n - 1]; bb = b[n - 1]; if (aa != bb) -- 2.25.1