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[Samba] [cifs-utils PATCH v3 4/4] cifs.upcall: allow scraping of KRB5CCNAME out of initiating task's /proc/<pid>/environ file




Chad reported that he was seeing a regression in cifs-utils-6.6. Prior
to that, cifs.upcall was able to find credcaches in non-default FILE:
locations, but with the rework of that code, that ability was lost.

Unfortunately, the krb5 library design doesn't really take into account
the fact that we might need to find a credcache in a process that isn't
descended from the session.

When the kernel does an upcall, it passes several bits of info about the
task that initiated the upcall. One of those things is the PID (the
tgid, in particular). We can use that info to reach into the
/proc/<pid>/environ file for the process, and grab whatever value of
$KRB5CCNAME is there.

Then, after switching credentials, set $KRB5CCNAME in the environment
to the same value before opening the credcache, to hint to the krb5
libs where they ought to look.

This new behavior is on by default, but can be disabled by having
request-key pass a '-E' flag to cifs.upcall.

Reported-by: Chad William Seys <cwseys@xxxxxxxxxxxxxxxx>
Signed-off-by: Jeff Layton <jlayton@xxxxxxxxx>
---
 cifs.upcall.8.in |   9 ++++
 cifs.upcall.c    | 150 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 152 insertions(+), 7 deletions(-)

diff --git a/cifs.upcall.8.in b/cifs.upcall.8.in
index 50f79d18331d..e1f3956e176a 100644
--- a/cifs.upcall.8.in
+++ b/cifs.upcall.8.in
@@ -38,6 +38,15 @@ for a particular key type\&. While it can be run directly from the command\-line
 This option is deprecated and is currently ignored\&.
 .RE
 .PP
+\-\-no-env-probe|\-E
+.RS 4
+Normally, cifs.upcall will probe the environment variable space of the process
+that initiated the upcall in order to fetch the value of $KRB5CCNAME. This can
+assist the program with finding credential caches in non-default locations. If
+this option is set, then the program won't do this and will rely on finding
+credcaches in the default locations specified in krb5.conf.
+.RE
+.PP
 \--krb5conf=/path/to/krb5.conf|-k /path/to/krb5.conf
 .RS 4
 This option allows administrators to set an alternate location for the
diff --git a/cifs.upcall.c b/cifs.upcall.c
index 25af0fb6957b..c9ddd35856a9 100644
--- a/cifs.upcall.c
+++ b/cifs.upcall.c
@@ -40,6 +40,7 @@
 #include <dirent.h>
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <fcntl.h>
 #include <unistd.h>
 #include <keyutils.h>
 #include <time.h>
@@ -212,11 +213,125 @@ err_cache:
 	return credtime;
 }
 
+#define	ENV_PATH_FMT			"/proc/%d/environ"
+#define	ENV_PATH_MAXLEN			(6 + 10 + 8 + 1)
+
+#define	ENV_NAME			"KRB5CCNAME"
+#define	ENV_PREFIX			"KRB5CCNAME="
+#define	ENV_PREFIX_LEN			11
+
+#define	ENV_BUF_START			(4096)
+#define	ENV_BUF_MAX			(131072)
+
+/**
+ * get_cachename_from_process_env - scrape value of $KRB5CCNAME out of the
+ * 				    initiating process' environment.
+ * @pid: initiating pid value from the upcall string
+ *
+ * Open the /proc/<pid>/environ file for the given pid, and scrape it for
+ * KRB5CCNAME entries.
+ *
+ * We start with a page-size buffer, and then progressively double it until
+ * we can slurp in the whole thing.
+ *
+ * Note that this is not entirely reliable. If the process is sitting in a
+ * container or something, then this is almost certainly not going to point
+ * where you expect.
+ *
+ * Probably it just won't work, but could a user use this to trick cifs.upcall
+ * into reading a file outside the container, by setting KRB5CCNAME in a
+ * crafty way?
+ */
+static char *
+get_cachename_from_process_env(pid_t pid)
+{
+	int fd, ret;
+	ssize_t buflen;
+	ssize_t bufsize = ENV_BUF_START;
+	char pathname[ENV_PATH_MAXLEN];
+	char *cachename = NULL;
+	char *buf = NULL, *pos;
+
+	if (!pid) {
+		syslog(LOG_DEBUG, "%s: pid == 0\n", __func__);
+		return NULL;
+	}
+
+	pathname[ENV_PATH_MAXLEN - 1] = '\0';
+	ret = snprintf(pathname, ENV_PATH_MAXLEN, ENV_PATH_FMT, pid);
+	if (ret >= ENV_PATH_MAXLEN) {
+		syslog(LOG_DEBUG, "%s: unterminated path!\n", __func__);
+		return NULL;
+	}
+
+	syslog(LOG_DEBUG, "%s: pathname=%s\n", __func__, pathname);
+	fd = open(pathname, O_RDONLY);
+	if (fd < 0) {
+		syslog(LOG_DEBUG, "%s: open failed: %d\n", __func__, errno);
+		return NULL;
+	}
+retry:
+	if (bufsize > ENV_BUF_MAX) {
+		syslog(LOG_DEBUG, "%s: buffer too big: %zd\n",
+							__func__, bufsize);
+		goto out_close;
+	}
+
+	buf = malloc(bufsize);
+	if (!buf) {
+		syslog(LOG_DEBUG, "%s: malloc failure\n", __func__);
+		goto out_close;
+	}
+
+	buflen = read(fd, buf, bufsize);
+	if (buflen < 0) {
+		syslog(LOG_DEBUG, "%s: read failed: %d\n", __func__, errno);
+		goto out_close;
+	}
+
+	if (buflen >= bufsize) {
+		/* We read to the end of the buffer. Double and try again */
+		syslog(LOG_DEBUG, "%s: read to end of buffer (%zu bytes)\n",
+					__func__, bufsize);
+		free(buf);
+		bufsize *= 2;
+		if (lseek(fd, 0, SEEK_SET) < 0)
+			goto out_close;
+		goto retry;
+	}
+
+	pos = buf;
+	while (buflen > 0) {
+		size_t len = strnlen(pos, buflen);
+
+		if (len > ENV_PREFIX_LEN &&
+		    !memcmp(pos, ENV_PREFIX, ENV_PREFIX_LEN)) {
+			cachename = strndup(pos + ENV_PREFIX_LEN,
+							len - ENV_PREFIX_LEN);
+			syslog(LOG_DEBUG, "%s: cachename = %s\n",
+							__func__, cachename);
+			break;
+		}
+		buflen -= (len + 1);
+		pos += (len + 1);
+	}
+out_close:
+	free(buf);
+	close(fd);
+	return cachename;
+}
+
 static krb5_ccache
-get_default_cc(void)
+get_existing_cc(const char *env_cachename)
 {
 	krb5_error_code ret;
 	krb5_ccache cc;
+	char *cachename;
+
+	if (env_cachename) {
+		if (setenv(ENV_NAME, env_cachename, 1))
+			syslog(LOG_DEBUG, "%s: failed to setenv %d\n", __func__, errno);
+	}
 
 	ret = krb5_cc_default(context, &cc);
 	if (ret) {
@@ -224,6 +339,14 @@ get_default_cc(void)
 		return NULL;
 	}
 
+	ret = krb5_cc_get_full_name(context, cc, &cachename);
+	if (ret) {
+		syslog(LOG_DEBUG, "%s: krb5_cc_get_full_name failed: %d\n", __func__, ret);
+	} else {
+		syslog(LOG_DEBUG, "%s: default ccache is %s\n", __func__, cachename);
+		krb5_free_string(context, cachename);
+	}
+
 	if (!get_tgt_time(cc)) {
 		krb5_cc_close(context, cc);
 		cc = NULL;
@@ -231,7 +354,6 @@ get_default_cc(void)
 	return cc;
 }
 
-
 static krb5_ccache
 init_cc_from_keytab(const char *keytab_name, const char *user)
 {
@@ -722,10 +844,11 @@ lowercase_string(char *c)
 
 static void usage(void)
 {
-	fprintf(stderr, "Usage: %s [ -K /path/to/keytab] [-k /path/to/krb5.conf] [-t] [-v] [-l] key_serial\n", prog);
+	fprintf(stderr, "Usage: %s [ -K /path/to/keytab] [-k /path/to/krb5.conf] [-E] [-t] [-v] [-l] key_serial\n", prog);
 }
 
 static const struct option long_options[] = {
+	{"no-env-probe", 0, NULL, 'E'},
 	{"krb5conf", 1, NULL, 'k'},
 	{"legacy-uid", 0, NULL, 'l'},
 	{"trust-dns", 0, NULL, 't'},
@@ -744,13 +867,14 @@ int main(const int argc, char *const argv[])
 	unsigned int have;
 	long rc = 1;
 	int c;
-	bool try_dns = false, legacy_uid = false;
+	bool try_dns = false, legacy_uid = false , env_probe = true;
 	char *buf;
 	char hostbuf[NI_MAXHOST], *host;
 	struct decoded_args arg;
 	const char *oid;
 	uid_t uid;
 	char *keytab_name = NULL;
+	char *env_cachename = NULL;
 	krb5_ccache ccache = NULL;
 	struct passwd *pw;
 
@@ -759,11 +883,15 @@ int main(const int argc, char *const argv[])
 
 	openlog(prog, 0, LOG_DAEMON);
 
-	while ((c = getopt_long(argc, argv, "ck:K:ltv", long_options, NULL)) != -1) {
+	while ((c = getopt_long(argc, argv, "cEk:K:ltv", long_options, NULL)) != -1) {
 		switch (c) {
 		case 'c':
 			/* legacy option -- skip it */
 			break;
+		case 'E':
+			/* skip probing initiating process env */
+			env_probe = false;
+			break;
 		case 't':
 			try_dns = true;
 			break;
@@ -789,7 +917,7 @@ int main(const int argc, char *const argv[])
 		}
 	}
 
-	if (trim_capabilities(false))
+	if (trim_capabilities(env_probe))
 		goto out;
 
 	/* is there a key? */
@@ -889,6 +1017,13 @@ int main(const int argc, char *const argv[])
 		goto out;
 	}
 
+	/*
+	 * Must do this before setuid, as we need elevated capabilities to
+	 * look at the environ file.
+	 */
+	env_cachename =
+		get_cachename_from_process_env(env_probe ?  arg.pid : 0);
+
 	rc = setuid(uid);
 	if (rc == -1) {
 		syslog(LOG_ERR, "setuid: %s", strerror(errno));
@@ -907,7 +1042,7 @@ int main(const int argc, char *const argv[])
 		goto out;
 	}
 
-	ccache = get_default_cc();
+	ccache = get_existing_cc(env_cachename);
 	/* Couldn't find credcache? Try to use keytab */
 	if (ccache == NULL && arg.username != NULL)
 		ccache = init_cc_from_keytab(keytab_name, arg.username);
@@ -1060,6 +1195,7 @@ out:
 	SAFE_FREE(arg.ip);
 	SAFE_FREE(arg.username);
 	SAFE_FREE(keydata);
+	SAFE_FREE(env_cachename);
 	syslog(LOG_DEBUG, "Exit status %ld", rc);
 	return rc;
 }
-- 
2.9.3


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