cifs.upcall: drop capabilities early in program

Much of cifs.upcall can and should be run without elevated privileges.
On entry into the program, drop as many capabilities as we can get away
with, and then always drop any remaining caps after calling setuid().

Signed-off-by: Jeff Layton <jlayton@samba.org>

2 files changed, 65 insertions, 1 deletions

diff --git a/Makefile.am b/Makefile.am
index 2e99e60..4823b63 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -16,7 +16,7 @@ clean-local: clean-local-upcall clean-local-idmap clean-local-cifsacl
 if CONFIG_CIFSUPCALL
 sbin_PROGRAMS += cifs.upcall
 cifs_upcall_SOURCES = cifs.upcall.c data_blob.c asn1.c spnego.c
-cifs_upcall_LDADD = -ltalloc -lkeyutils $(KRB5_LDADD)
+cifs_upcall_LDADD = -ltalloc -lkeyutils $(KRB5_LDADD) $(CAPNG_LDADD)
 man_MANS += cifs.upcall.8
 
 #
diff --git a/cifs.upcall.c b/cifs.upcall.c
index 2b535a1..0bd3dcb 100644
--- a/cifs.upcall.c
+++ b/cifs.upcall.c
@@ -54,6 +54,10 @@
 #include "spnego.h"
 #include "cifs_spnego.h"
 
+#ifdef HAVE_LIBCAP_NG
+#include <cap-ng.h>
+#endif
+
 static krb5_context	context;
 static const char	*prog = "cifs.upcall";
 
@@ -63,6 +67,59 @@ typedef enum _sectype {
 	MS_KRB5
 } sectype_t;
 
+#ifdef HAVE_LIBCAP_NG
+static int
+trim_capabilities(bool need_ptrace)
+{
+	capng_clear(CAPNG_SELECT_BOTH);
+
+	/*
+	 * Need PTRACE and DAC_OVERRIDE for environment scraping, SETGID to
+	 * change gid and grouplist, and SETUID to change uid.
+	 */
+	if (capng_updatev(CAPNG_ADD, CAPNG_PERMITTED|CAPNG_EFFECTIVE,
+			CAP_SETUID, CAP_SETGID, CAP_DAC_OVERRIDE, -1)) {
+		syslog(LOG_ERR, "%s: Unable to update capability set: %m\n", __func__);
+		return 1;
+	}
+
+	if (need_ptrace &&
+	    capng_update(CAPNG_ADD, CAPNG_PERMITTED|CAPNG_EFFECTIVE, CAP_SYS_PTRACE)) {
+		syslog(LOG_ERR, "%s: Unable to update capability set: %m\n", __func__);
+		return 1;
+	}
+
+	if (capng_apply(CAPNG_SELECT_BOTH)) {
+		syslog(LOG_ERR, "%s: Unable to apply capability set: %m\n", __func__);
+		return 1;
+	}
+	return 0;
+}
+
+static int
+drop_all_capabilities(void)
+{
+	capng_clear(CAPNG_SELECT_BOTH);
+	if (capng_apply(CAPNG_SELECT_BOTH)) {
+		syslog(LOG_ERR, "%s: Unable to apply capability set: %m\n", __func__);
+		return 1;
+	}
+	return 0;
+}
+#else /* HAVE_LIBCAP_NG */
+static int
+trim_capabilities(void)
+{
+	return 0;
+}
+
+static int
+drop_all_capabilities(void)
+{
+	return 0;
+}
+#endif /* HAVE_LIBCAP_NG */
+
 /*
  * smb_krb5_principal_get_realm
  *
@@ -733,6 +790,9 @@ int main(const int argc, char *const argv[])
 		}
 	}
 
+	if (trim_capabilities(false))
+		goto out;
+
 	/* is there a key? */
 	if (argc <= optind) {
 		usage();
@@ -837,6 +897,10 @@ int main(const int argc, char *const argv[])
 		goto out;
 	}
 
+	rc = drop_all_capabilities();
+	if (rc)
+		goto out;
+
 	rc = krb5_init_context(&context);
 	if (rc) {
 		syslog(LOG_ERR, "unable to init krb5 context: %ld", rc);