samba - Feb 2017 - [RFC][cifs-utils PATCH] 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.  This patch adds this ability to cifs.upcall.

I'm not 100% convinced that this is a good idea however, so for now,
this is disabled unless the command line has a '-e' switch. Anyone
wishing to play with this should edit their /etc/request-key.conf files
accordingly.

Signed-off-by: Jeff Layton <jlayton at samba.org>
---
 cifs.upcall.8.in |  10 ++++
 cifs.upcall.c    | 148 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 152 insertions(+), 6 deletions(-)

diff --git a/cifs.upcall.8.in b/cifs.upcall.8.in
index 50f79d18331d..06c6d3a02c2f 100644
--- a/cifs.upcall.8.in
+++ b/cifs.upcall.8.in
@@ -38,6 +38,16 @@ 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
+\-\-env-probe|\-e
+.RS 4
+Allow cifs.upcall to do an environment probe to help find the credential
+cache. This can assist the program with finding credential caches in
+non-default locations. If this is set, then the program will attempt to
+retrieve the KRB5CCNAME environment variable from the process that initiated
+the upcall, and then set that in the upcall process for use when searching for
+a credcache.
+.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 8f146c92b4a5..1b2c6c19e39e 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>
@@ -154,11 +155,126 @@ 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?
+ *
+ * YMMV here.
+ */
+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: %d\n", __func__, errno);
+		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);
+	}
+	free(buf);
+out_close:
+	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) {
@@ -166,6 +282,14 @@ 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;
@@ -173,7 +297,6 @@ get_default_cc(void)
 	return cc;
 }
 
-
 static krb5_ccache
 init_cc_from_keytab(const char *keytab_name, const char *user)
 {
@@ -664,10 +787,11 @@ 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[] = {
+	{"env-probe", 0, NULL, 'e'},
 	{"krb5conf", 1, NULL, 'k'},
 	{"legacy-uid", 0, NULL, 'l'},
 	{"trust-dns", 0, NULL, 't'},
@@ -685,13 +809,14 @@ int main(const int argc, char *const argv[])
 	size_t datalen;
 	unsigned int have;
 	long rc = 1;
-	int c, try_dns = 0, legacy_uid = 0;
+	int c, try_dns = 0, legacy_uid = 0, env_probe = 0;
 	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;
 
 	hostbuf[0] = '\0';
@@ -699,11 +824,15 @@ 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':
+			/* do an environmental probe to get the credcache */
+			env_probe++;
+			break;
 		case 't':
 			try_dns++;
 			break;
@@ -794,6 +923,12 @@ int main(const int argc, char *const argv[])
 		goto out;
 	}
 
+	/*
+	 * Must do this before setuid, as we need ptrace perms to look at
+	 * 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));
@@ -806,7 +941,7 @@ 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);
@@ -959,6 +1094,7 @@ 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

On Sat, 2017-02-11 at 08:41 -0500, Jeff Layton wrote:
> 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.  This patch adds this ability to cifs.upcall.
> 
> I'm not 100% convinced that this is a good idea however, so for now,
> this is disabled unless the command line has a '-e' switch. Anyone
> wishing to play with this should edit their /etc/request-key.conf files
> accordingly.
> 
Meant to put a Reported-by: here for Chad. I'll do that before merging.
> Signed-off-by: Jeff Layton <jlayton at samba.org>
> ---
>  cifs.upcall.8.in |  10 ++++
>  cifs.upcall.c    | 148
++++++++++++++++++++++++++++++++++++++++++++++++++++---
>  2 files changed, 152 insertions(+), 6 deletions(-)
> 
FWIW, this patch works as expected in cursory, light testing. This
should be superior to the earlier method where we scanned in /tmp as
well -- more efficient, and also able to find non-FILE: credcaches, as
well as ones that lie in other places besides /tmp.

My main concern is that we have to do the environ file scraping while
privileged, and that we end up trusting some info from a userland
process that triggered the upcall in KRB5CCNAME. Does this open any
potential attack vectors that I'm not considering?


diff --git a/cifs.upcall.8.in b/cifs.upcall.8.in
> index 50f79d18331d..06c6d3a02c2f 100644
> --- a/cifs.upcall.8.in
> +++ b/cifs.upcall.8.in
> @@ -38,6 +38,16 @@ 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
> +\-\-env-probe|\-e
> +.RS 4
> +Allow cifs.upcall to do an environment probe to help find the credential
> +cache. This can assist the program with finding credential caches in
> +non-default locations. If this is set, then the program will attempt to
> +retrieve the KRB5CCNAME environment variable from the process that
initiated
> +the upcall, and then set that in the upcall process for use when searching
for
> +a credcache.
> +.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 8f146c92b4a5..1b2c6c19e39e 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>
> @@ -154,11 +155,126 @@ 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?
> + *
> + * YMMV here.
> + */
> +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: %d\n", __func__,
errno);
> +		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);
> +	}
> +	free(buf);
> +out_close:
> +	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) {
> @@ -166,6 +282,14 @@ 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;
> @@ -173,7 +297,6 @@ get_default_cc(void)
>  	return cc;
>  }
>  
> -
>  static krb5_ccache
>  init_cc_from_keytab(const char *keytab_name, const char *user)
>  {
> @@ -664,10 +787,11 @@ 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[] = {
> +	{"env-probe", 0, NULL, 'e'},
>  	{"krb5conf", 1, NULL, 'k'},
>  	{"legacy-uid", 0, NULL, 'l'},
>  	{"trust-dns", 0, NULL, 't'},
> @@ -685,13 +809,14 @@ int main(const int argc, char *const argv[])
>  	size_t datalen;
>  	unsigned int have;
>  	long rc = 1;
> -	int c, try_dns = 0, legacy_uid = 0;
> +	int c, try_dns = 0, legacy_uid = 0, env_probe = 0;
>  	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;
>  
>  	hostbuf[0] = '\0';
> @@ -699,11 +824,15 @@ 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':
> +			/* do an environmental probe to get the credcache */
> +			env_probe++;
> +			break;
>  		case 't':
>  			try_dns++;
>  			break;
> @@ -794,6 +923,12 @@ int main(const int argc, char *const argv[])
>  		goto out;
>  	}
>  
> +	/*
> +	 * Must do this before setuid, as we need ptrace perms to look at
> +	 * 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));
> @@ -806,7 +941,7 @@ 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);
> @@ -959,6 +1094,7 @@ 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;
>  }
-- 
Jeff Layton <jlayton at samba.org>

On Mon, 2017-02-13 at 05:02 -0500, Simo Sorce wrote:
> On Sat, 2017-02-11 at 10:16 -0500, Jeff Layton wrote:
> > On Sat, 2017-02-11 at 08:41 -0500, Jeff Layton wrote:
> > > 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.  This patch adds this ability to
cifs.upcall.
> > > 
> > > I'm not 100% convinced that this is a good idea however, so
for
> > > now,
> > > this is disabled unless the command line has a '-e'
switch. Anyone
> > > wishing to play with this should edit their /etc/request-key.conf
> > > files
> > > accordingly.
> > > 
> > 
> > Meant to put a Reported-by: here for Chad. I'll do that before
> > merging.
> > 
> > > > > > Signed-off-by: Jeff Layton <jlayton at
samba.org>
> > > ---
> > >  cifs.upcall.8.in |  10 ++++
> > >  cifs.upcall.c    | 148
> > > ++++++++++++++++++++++++++++++++++++++++++++++++++++---
> > >  2 files changed, 152 insertions(+), 6 deletions(-)
> > > 
> > 
> > FWIW, this patch works as expected in cursory, light testing. This
> > should be superior to the earlier method where we scanned in /tmp as
> > well -- more efficient, and also able to find non-FILE: credcaches,
> > as
> > well as ones that lie in other places besides /tmp.
> > 
> > My main concern is that we have to do the environ file scraping while
> > privileged, and that we end up trusting some info from a userland
> > process that triggered the upcall in KRB5CCNAME. Does this open any
> > potential attack vectors that I'm not considering?
> 
> Well you are operating as the user however I have not seen a setgid
> operation, does this mean cifs.upcall is running with group id 0 ?
> 
> This may give access to files the original user should not be able to
> access or cause files to be created with improper permission.
> 
Yeah, ick. The kernel doesn't send down the gid, so we can't just add
that in directly...

We could do a getpwuid(uid), and then setgid(pw_gid). That might be
problematic for applications that have done a setgid(), but those ought
to be very rare.
> Another thing people may need to pay attention to is SELinux context, I
> am not sure what selinux context cifs.upcall runs into the difference
> may matter in some cases, but in those I think proper SELinux policy
> should be devised, I see no need to change how cifs.upcall operates.
> 
Yeah, that's a different matter. It would be nice to confine
cifs.upcall, but we'd need an SELinux policy for that. Honestly, I'd
rather see us move to gssproxy before spending time on that.

-- 
Jeff Layton <jlayton at samba.org>