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From e23745b61a46f034bca3cab9936c24c249afdc7f Mon Sep 17 00:00:00 2001
From: Lukas Fleischer <archlinux@cryptocrack.de>
Date: Sun, 21 Dec 2014 22:17:48 +0100
Subject: [PATCH] Patch sshd for the AUR
* Add SSH_KEY_FINGERPRINT and SSH_KEY variables to the environment of
the AuthorizedKeysCommand which allows for efficiently looking up SSH
keys in the AUR database.
* Remove the secure path check for the AuthorizedKeysCommand. We are
running the sshd under a non-privileged user who has as little
permissions as possible. In particular, he does not own the directory
that contains the scripts for the Git backend.
* Prevent from running the sshd as root.
Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
---
auth2-pubkey.c | 48 +++++++++++++++++++++++++++++++++++++++++++-----
ssh.h | 12 ++++++++++++
sshd.c | 5 +++++
sshd_config.5 | 5 +++++
4 files changed, 65 insertions(+), 5 deletions(-)
diff --git a/auth2-pubkey.c b/auth2-pubkey.c
index 0a3c1de..baf4922 100644
--- a/auth2-pubkey.c
+++ b/auth2-pubkey.c
@@ -510,6 +510,8 @@ user_key_command_allowed2(struct passwd *user_pw, Key *key)
int status, devnull, p[2], i;
pid_t pid;
char *username, errmsg[512];
+ struct sshbuf *b = NULL, *bb = NULL;
+ char *keytext, *uu = NULL;
if (options.authorized_keys_command == NULL ||
options.authorized_keys_command[0] != '/')
@@ -538,11 +540,6 @@ user_key_command_allowed2(struct passwd *user_pw, Key *key)
options.authorized_keys_command, strerror(errno));
goto out;
}
- if (auth_secure_path(options.authorized_keys_command, &st, NULL, 0,
- errmsg, sizeof(errmsg)) != 0) {
- error("Unsafe AuthorizedKeysCommand: %s", errmsg);
- goto out;
- }
if (pipe(p) != 0) {
error("%s: pipe: %s", __func__, strerror(errno));
@@ -568,6 +565,47 @@ user_key_command_allowed2(struct passwd *user_pw, Key *key)
for (i = 0; i < NSIG; i++)
signal(i, SIG_DFL);
+ keytext = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
+ if (setenv(SSH_KEY_FINGERPRINT_ENV_NAME, keytext, 1) == -1) {
+ error("%s: setenv: %s", __func__, strerror(errno));
+ _exit(1);
+ }
+
+ if (!(b = sshbuf_new()) || !(bb = sshbuf_new())) {
+ error("%s: sshbuf_new: %s", __func__, strerror(errno));
+ _exit(1);
+ }
+ if (sshkey_to_blob_buf(key, bb) != 0) {
+ error("%s: sshkey_to_blob_buf: %s", __func__,
+ strerror(errno));
+ _exit(1);
+ }
+ if (!(uu = sshbuf_dtob64(bb))) {
+ error("%s: sshbuf_dtob64: %s", __func__,
+ strerror(errno));
+ _exit(1);
+ }
+ if (sshbuf_putf(b, "%s ", sshkey_ssh_name(key))) {
+ error("%s: sshbuf_putf: %s", __func__,
+ strerror(errno));
+ _exit(1);
+ }
+ if (sshbuf_put(b, uu, strlen(uu) + 1)) {
+ error("%s: sshbuf_put: %s", __func__,
+ strerror(errno));
+ _exit(1);
+ }
+ if (setenv(SSH_KEY_ENV_NAME, sshbuf_ptr(b), 1) == -1) {
+ error("%s: setenv: %s", __func__, strerror(errno));
+ _exit(1);
+ }
+ if (uu)
+ free(uu);
+ if (b)
+ sshbuf_free(b);
+ if (bb)
+ sshbuf_free(bb);
+
if ((devnull = open(_PATH_DEVNULL, O_RDWR)) == -1) {
error("%s: open %s: %s", __func__, _PATH_DEVNULL,
strerror(errno));
diff --git a/ssh.h b/ssh.h
index c94633b..411ea86 100644
--- a/ssh.h
+++ b/ssh.h
@@ -97,3 +97,15 @@
/* Listen backlog for sshd, ssh-agent and forwarding sockets */
#define SSH_LISTEN_BACKLOG 128
+
+/*
+ * Name of the environment variable containing the incoming key passed
+ * to AuthorizedKeysCommand.
+ */
+#define SSH_KEY_ENV_NAME "SSH_KEY"
+
+/*
+ * Name of the environment variable containing the incoming key fingerprint
+ * passed to AuthorizedKeysCommand.
+ */
+#define SSH_KEY_FINGERPRINT_ENV_NAME "SSH_KEY_FINGERPRINT"
diff --git a/sshd.c b/sshd.c
index 4e01855..60c676f 100644
--- a/sshd.c
+++ b/sshd.c
@@ -1424,6 +1424,11 @@ main(int ac, char **av)
av = saved_argv;
#endif
+ if (geteuid() == 0) {
+ fprintf(stderr, "this is a patched version of the sshd that must not be run as root.\n");
+ exit(1);
+ }
+
if (geteuid() == 0 && setgroups(0, NULL) == -1)
debug("setgroups(): %.200s", strerror(errno));
diff --git a/sshd_config.5 b/sshd_config.5
index ef36d33..1d7bade 100644
--- a/sshd_config.5
+++ b/sshd_config.5
@@ -223,6 +223,11 @@ It will be invoked with a single argument of the username
being authenticated, and should produce on standard output zero or
more lines of authorized_keys output (see AUTHORIZED_KEYS in
.Xr sshd 8 ) .
+The key being used for authentication (the key's type and the key text itself,
+separated by a space) will be available in the
+.Ev SSH_KEY
+environment variable, and the fingerprint of the key will be available in the
+.Ev SSH_KEY_FINGERPRINT environment variable.
If a key supplied by AuthorizedKeysCommand does not successfully authenticate
and authorize the user then public key authentication continues using the usual
.Cm AuthorizedKeysFile
--
2.2.1
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