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From b3855ff053f5078ec3d3c653cdaedefaa5fc362d Mon Sep 17 00:00:00 2001
From: "djm@openbsd.org" <djm@openbsd.org>
Date: Fri, 18 Sep 2020 05:23:03 +0000
Subject: [PATCH] upstream: tweak the client hostkey preference ordering
algorithm to
prefer the default ordering if the user has a key that matches the
best-preference default algorithm.
feedback and ok markus@
OpenBSD-Commit-ID: a92dd7d7520ddd95c0a16786a7519e6d0167d35f
Upstream-Status: Backport
[https://github.com/openssh/openssh-portable/commit/b3855ff053f5078ec3d3c653cdaedefaa5fc362d]
CVE: CVE-2020-14145
Signed-off-by: Chee Yang Lee <chee.yang.lee@intel.com>
---
sshconnect2.c | 41 ++++++++++++++++++++++++++++++++++++++---
1 file changed, 37 insertions(+), 2 deletions(-)
diff --git a/sshconnect2.c b/sshconnect2.c
index 347e348c60..f64aae66af 100644
--- a/sshconnect2.c
+++ b/sshconnect2.c
@@ -102,12 +102,25 @@ verify_host_key_callback(struct sshkey *hostkey, struct ssh *ssh)
return 0;
}
+/* Returns the first item from a comma-separated algorithm list */
+static char *
+first_alg(const char *algs)
+{
+ char *ret, *cp;
+
+ ret = xstrdup(algs);
+ if ((cp = strchr(ret, ',')) != NULL)
+ *cp = '\0';
+ return ret;
+}
+
static char *
order_hostkeyalgs(char *host, struct sockaddr *hostaddr, u_short port)
{
- char *oavail, *avail, *first, *last, *alg, *hostname, *ret;
+ char *oavail = NULL, *avail = NULL, *first = NULL, *last = NULL;
+ char *alg = NULL, *hostname = NULL, *ret = NULL, *best = NULL;
size_t maxlen;
- struct hostkeys *hostkeys;
+ struct hostkeys *hostkeys = NULL;
int ktype;
u_int i;
@@ -119,6 +132,26 @@ order_hostkeyalgs(char *host, struct sockaddr *hostaddr, u_short port)
for (i = 0; i < options.num_system_hostfiles; i++)
load_hostkeys(hostkeys, hostname, options.system_hostfiles[i]);
+ /*
+ * If a plain public key exists that matches the type of the best
+ * preference HostkeyAlgorithms, then use the whole list as is.
+ * Note that we ignore whether the best preference algorithm is a
+ * certificate type, as sshconnect.c will downgrade certs to
+ * plain keys if necessary.
+ */
+ best = first_alg(options.hostkeyalgorithms);
+ if (lookup_key_in_hostkeys_by_type(hostkeys,
+ sshkey_type_plain(sshkey_type_from_name(best)), NULL)) {
+ debug3("%s: have matching best-preference key type %s, "
+ "using HostkeyAlgorithms verbatim", __func__, best);
+ ret = xstrdup(options.hostkeyalgorithms);
+ goto out;
+ }
+
+ /*
+ * Otherwise, prefer the host key algorithms that match known keys
+ * while keeping the ordering of HostkeyAlgorithms as much as possible.
+ */
oavail = avail = xstrdup(options.hostkeyalgorithms);
maxlen = strlen(avail) + 1;
first = xmalloc(maxlen);
@@ -159,6 +192,8 @@ order_hostkeyalgs(char *host, struct sockaddr *hostaddr, u_short port)
if (*first != '\0')
debug3("%s: prefer hostkeyalgs: %s", __func__, first);
+ out:
+ free(best);
free(first);
free(last);
free(hostname);
|
