From f80a4029bb2f43fe5f0270857d48bc9d41d3ea4e Mon Sep 17 00:00:00 2001
From: tb <>
Date: Fri, 13 Jan 2023 14:46:08 +0000
Subject: Prevent 1-byte out-of-bounds read in i2c_ASN1_BIT_STRING

If an ASN.1 BIT STRING a of length > 0 contains only zero bytes in a->data,
this old code would end up reading from a->data[-1]. This may or may not
crash. Luckily, anton observed two openssl-ruby regress test failures in
the last few days, which could eventually be traced back to this (after a
lot of painful digging due to coredumps not working properly).

ok jsing
---
 src/lib/libcrypto/asn1/a_bitstr.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

(limited to 'src/lib')

diff --git a/src/lib/libcrypto/asn1/a_bitstr.c b/src/lib/libcrypto/asn1/a_bitstr.c
index a4a379a9a0..767055144d 100644
--- a/src/lib/libcrypto/asn1/a_bitstr.c
+++ b/src/lib/libcrypto/asn1/a_bitstr.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: a_bitstr.c,v 1.37 2022/11/08 16:48:28 tb Exp $ */
+/* $OpenBSD: a_bitstr.c,v 1.38 2023/01/13 14:46:08 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -255,11 +255,13 @@ i2c_ASN1_BIT_STRING(ASN1_BIT_STRING *a, unsigned char **pp)
 		if (a->flags & ASN1_STRING_FLAG_BITS_LEFT) {
 			bits = (int)a->flags & 0x07;
 		} else {
+			j = 0;
 			for (; len > 0; len--) {
 				if (a->data[len - 1])
 					break;
 			}
-			j = a->data[len - 1];
+			if (len > 0)
+				j = a->data[len - 1];
 			if (j & 0x01)
 				bits = 0;
 			else if (j & 0x02)
-- 
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