gssd: do a more thorough change of identity after forking

The part of process_krb5_upcall that handles non-machine user creds
first tries to query GSSAPI for credentials. If that fails, it then
falls back to trawling through likely credcache locations to find them
and then points $KRB5CCNAME at it before proceeding. There are a number
of bugs in this code that this patch attempts to address.

The code that queries GSSAPI for credentials does it as root which
almost universally fails to do anything useful unless we happen to be
looking for non-machine root creds. Because of this, gssd almost always
falls back to having to search for credcaches "manually". The code that
handles credential switching is in create_auth_rpc_client, so it's too
late to be of any use here.

Worse yet, for historical reasons the MIT krb5 authors used %{uid} in
the default credcache locations which translates to the real uid. Thus
switching the fsuid or even euid is insufficient. You must switch the
real uid in order to be able to find the proper credcache in most cases.

This patch moves the credential switching to occur much earlier in the
process and has it do a much more thorough job of it. It first drops all
supplimentary groups, then determines a gid to use and switches the gids
and uids to the correct ones. If it can't determine the correct gid to
use, it then tries to look up the one for "nobody" and uses that.

Once the credentials are switched, the forked child now no longer tries
to change them back. It does the downcall with the new credentials and
just exits when it's done.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>

1 files changed, 66 insertions, 16 deletions

diff --git a/utils/gssd/gssd_proc.c b/utils/gssd/gssd_proc.c
index 99537d9..b48d163 100644
--- a/utils/gssd/gssd_proc.c
+++ b/utils/gssd/gssd_proc.c
@@ -834,7 +834,6 @@ create_auth_rpc_client(struct clnt_info *clp,
 	CLIENT			*rpc_clnt = NULL;
 	struct rpc_gss_sec	sec;
 	AUTH			*auth = NULL;
-	uid_t			save_uid = -1;
 	int			retval = -1;
 	OM_uint32		min_stat;
 	char			rpc_errmsg[1024];
@@ -843,16 +842,6 @@ create_auth_rpc_client(struct clnt_info *clp,
 	struct sockaddr		*addr = (struct sockaddr *) &clp->addr;
 	socklen_t		salen;
 
-	/* Create the context as the user (not as root) */
-	save_uid = geteuid();
-	if (setfsuid(uid) != 0) {
-		printerr(0, "WARNING: Failed to setfsuid for "
-			    "user with uid %d\n", uid);
-		goto out_fail;
-	}
-	printerr(2, "creating context using fsuid %d (save_uid %d)\n",
-			uid, save_uid);
-
 	sec.qop = GSS_C_QOP_DEFAULT;
 	sec.svc = RPCSEC_GSS_SVC_NONE;
 	sec.cred = cred;
@@ -951,11 +940,6 @@ create_auth_rpc_client(struct clnt_info *clp,
   out:
 	if (sec.cred != GSS_C_NO_CREDENTIAL)
 		gss_release_cred(&min_stat, &sec.cred);
-	/* Restore euid to original value */
-	if (((int)save_uid != -1) && (setfsuid(save_uid) != (int)uid)) {
-		printerr(0, "WARNING: Failed to restore fsuid"
-			    " to uid %d from %d\n", save_uid, uid);
-	}
 	return retval;
 
   out_fail:
@@ -966,6 +950,64 @@ create_auth_rpc_client(struct clnt_info *clp,
 }
 
 /*
+ * Create the context as the user (not as root).
+ *
+ * Note that we change the *real* uid here, as changing the effective uid is
+ * not sufficient. This is due to an unfortunate historical error in the MIT
+ * krb5 libs, where they used %{uid} in the default_ccache_name. Changing that
+ * now might break some applications so we're sort of stuck with it.
+ *
+ * Unfortunately, doing this leaves the forked child vulnerable to signals and
+ * renicing, but this is the best we can do. In the event that a child is
+ * signalled before downcalling, the kernel will just eventually time out the
+ * upcall attempt.
+ */
+static int
+change_identity(uid_t uid)
+{
+	struct passwd	*pw;
+
+	/* drop list of supplimentary groups first */
+	if (setgroups(0, NULL) != 0) {
+		printerr(0, "WARNING: unable to drop supplimentary groups!");
+		return errno;
+	}
+
+	/* try to get pwent for user */
+	pw = getpwuid(uid);
+	if (!pw) {
+		/* if that doesn't work, try to get one for "nobody" */
+		errno = 0;
+		pw = getpwnam("nobody");
+		if (!pw) {
+			printerr(0, "WARNING: unable to determine gid for uid %u\n", uid);
+			return errno ? errno : ENOENT;
+		}
+	}
+
+	/*
+	 * Switch the GIDs. Note that we leave the saved-set-gid alone in an
+	 * attempt to prevent attacks via ptrace()
+	 */
+	if (setresgid(pw->pw_gid, pw->pw_gid, -1) != 0) {
+		printerr(0, "WARNING: failed to set gid to %u!\n", pw->pw_gid);
+		return errno;
+	}
+
+	/*
+	 * Switch UIDs, but leave saved-set-uid alone to prevent ptrace() by
+	 * other processes running with this uid.
+	 */
+	if (setresuid(uid, uid, -1) != 0) {
+		printerr(0, "WARNING: Failed to setuid for user with uid %u\n",
+				uid);
+		return errno;
+	}
+
+	return 0;
+}
+
+/*
  * this code uses the userland rpcsec gss library to create a krb5
  * context on behalf of the kernel
  */
@@ -1036,6 +1078,14 @@ process_krb5_upcall(struct clnt_info *clp, uid_t uid, int fd, char *tgtname,
 		 service ? service : "<null>");
 	if (uid != 0 || (uid == 0 && root_uses_machine_creds == 0 &&
 				service == NULL)) {
+
+		err = change_identity(uid);
+		if (err) {
+			printerr(0, "WARNING: failed to change identity: %s",
+				 strerror(err));
+			goto out_return_error;
+		}
+
 		/* Tell krb5 gss which credentials cache to use */
 		/* Try first to acquire credentials directly via GSSAPI */
 		err = gssd_acquire_user_cred(uid, &gss_cred);