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djm pushed a commit to branch V_9_6
in repository openssh.

commit 6d51feab157cedf1e7ef5b3f8781ca8ff9c4ab1b
Author: djm@openbsd.org <djm@openbsd.org>
Date: Mon Dec 18 14:48:08 2023 +0000

upstream: ssh-agent: record failed session-bind attempts

Record failed attempts to session-bind a connection and refuse signing
operations on that connection henceforth.

Prevents a future situation where we add a new hostkey type that is not
recognised by an older ssh-agent, that consequently causes session-bind
to fail (this situation is only likely to arise when people mix ssh(1)
and ssh-agent(1) of different versions on the same host). Previously,
after such a failure the agent socket would be considered unbound and
not subject to restriction.

Spotted by Jann Horn

OpenBSD-Commit-ID: b0fdd023e920aa4831413f640de4c5307b53552e
---
ssh-agent.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/ssh-agent.c b/ssh-agent.c
index 12ec66cf..b6a3f481 100644
--- a/ssh-agent.c
+++ b/ssh-agent.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-agent.c,v 1.302 2023/12/18 14:46:56 djm Exp $ */
+/* $OpenBSD: ssh-agent.c,v 1.303 2023/12/18 14:48:08 djm Exp $ */
/*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -130,6 +130,7 @@ typedef struct socket_entry {
struct sshbuf *request;
size_t nsession_ids;
struct hostkey_sid *session_ids;
+ int session_bind_attempted;
} SocketEntry;

u_int sockets_alloc = 0;
@@ -477,6 +478,10 @@ identity_permitted(Identity *id, SocketEntry *e, char *user,
e->nsession_ids, id->ndest_constraints);
if (id->ndest_constraints == 0)
return 0; /* unconstrained */
+ if (e->session_bind_attempted && e->nsession_ids == 0) {
+ error_f("previous session bind failed on socket");
+ return -1;
+ }
if (e->nsession_ids == 0)
return 0; /* local use */
/*
@@ -556,6 +561,12 @@ identity_permitted(Identity *id, SocketEntry *e, char *user,
return 0;
}

+static int
+socket_is_remote(SocketEntry *e)
+{
+ return e->session_bind_attempted || (e->nsession_ids != 0);
+}
+
/* return matching private key for given public key */
static Identity *
lookup_identity(struct sshkey *key)
@@ -1367,7 +1378,7 @@ process_add_identity(SocketEntry *e)
if (strcasecmp(sk_provider, "internal") == 0) {
debug_f("internal provider");
} else {
- if (e->nsession_ids != 0 && !remote_add_provider) {
+ if (socket_is_remote(e) && !remote_add_provider) {
verbose("failed add of SK provider \"%.100s\": "
"remote addition of providers is disabled",
sk_provider);
@@ -1565,7 +1576,7 @@ process_add_smartcard_key(SocketEntry *e)
goto send;
}
dump_dest_constraints(__func__, dest_constraints, ndest_constraints);
- if (e->nsession_ids != 0 && !remote_add_provider) {
+ if (socket_is_remote(e) && !remote_add_provider) {
verbose("failed PKCS#11 add of \"%.100s\": remote addition of "
"providers is disabled", provider);
goto send;
@@ -1680,6 +1691,7 @@ process_ext_session_bind(SocketEntry *e)
u_char fwd = 0;

debug2_f("entering");
+ e->session_bind_attempted = 1;
if ((r = sshkey_froms(e->request, &key)) != 0 ||
(r = sshbuf_froms(e->request, &sid)) != 0 ||
(r = sshbuf_froms(e->request, &sig)) != 0 ||

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