Add cap reduction support to enable use as SUID

The ideal way to use qemu-bridge-helper is to give it an fscap of using:

 setcap cap_net_admin=ep qemu-bridge-helper

Unfortunately, most distros still do not have a mechanism to package files
with fscaps applied.  This means they'll have to SUID the qemu-bridge-helper
binary.

To improve security, use libcap to reduce our capability set to just
cap_net_admin, then reduce privileges down to the calling user.  This is
hopefully close to equivalent to fscap support from a security perspective.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Richa Marwaha <rmarwah@linux.vnet.ibm.com>
Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

configure

@@ -133,6 +133,7 @@ vnc_thread="no"
 xen=""
 xen_ctrl_version=""
 linux_aio=""
+cap_ng=""
 attr=""
 libattr=""
 xfs=""
@@ -672,6 +673,10 @@ for opt do
   ;;
   --enable-tcg-interpreter) tcg_interpreter="yes"
   ;;
+  --disable-cap-ng)  cap_ng="no"
+  ;;
+  --enable-cap-ng) cap_ng="yes"
+  ;;
   --disable-spice) spice="no"
   ;;
   --enable-spice) spice="yes"
@@ -1053,6 +1058,8 @@ echo "  --disable-vde            disable support for vde network"
 echo "  --enable-vde             enable support for vde network"
 echo "  --disable-linux-aio      disable Linux AIO support"
 echo "  --enable-linux-aio       enable Linux AIO support"
+echo "  --disable-cap-ng         disable libcap-ng support"
+echo "  --enable-cap-ng          enable libcap-ng support"
 echo "  --disable-attr           disables attr and xattr support"
 echo "  --enable-attr            enable attr and xattr support"
 echo "  --disable-blobs          disable installing provided firmware blobs"
@@ -1744,6 +1751,29 @@ EOF
   fi
 fi
 
+##########################################
+# libcap-ng library probe
+if test "$cap_ng" != "no" ; then
+  cap_libs="-lcap-ng"
+  cat > $TMPC << EOF
+#include <cap-ng.h>
+int main(void)
+{
+    capng_capability_to_name(CAPNG_EFFECTIVE);
+    return 0;
+}
+EOF
+  if compile_prog "" "$cap_libs" ; then
+    cap_ng=yes
+    libs_tools="$cap_libs $libs_tools"
+  else
+    if test "$cap_ng" = "yes" ; then
+      feature_not_found "cap_ng"
+    fi
+    cap_ng=no
+  fi
+fi
+
 ##########################################
 # Sound support libraries probe
 
@@ -2876,6 +2906,7 @@ echo "fdatasync         $fdatasync"
 echo "madvise           $madvise"
 echo "posix_madvise     $posix_madvise"
 echo "uuid support      $uuid"
+echo "libcap-ng support $cap_ng"
 echo "vhost-net support $vhost_net"
 echo "Trace backend     $trace_backend"
 echo "Trace output file $trace_file-<pid>"
@@ -2980,6 +3011,9 @@ fi
 if test "$vde" = "yes" ; then
   echo "CONFIG_VDE=y" >> $config_host_mak
 fi
+if test "$cap_ng" = "yes" ; then
+  echo "CONFIG_LIBCAP=y" >> $config_host_mak
+fi
 for card in $audio_card_list; do
     def=CONFIG_`echo $card | tr '[:lower:]' '[:upper:]'`
     echo "$def=y" >> $config_host_mak

qemu-bridge-helper.c

@@ -39,6 +39,10 @@
 
 #include "net/tap-linux.h"
 
+#ifdef CONFIG_LIBCAP
+#include <cap-ng.h>
+#endif
+
 #define DEFAULT_ACL_FILE CONFIG_QEMU_CONFDIR "/bridge.conf"
 
 enum {
@@ -193,6 +197,27 @@ static int send_fd(int c, int fd)
     return sendmsg(c, &msg, 0);
 }
 
+#ifdef CONFIG_LIBCAP
+static int drop_privileges(void)
+{
+    /* clear all capabilities */
+    capng_clear(CAPNG_SELECT_BOTH);
+
+    if (capng_update(CAPNG_ADD, CAPNG_EFFECTIVE | CAPNG_PERMITTED,
+                     CAP_NET_ADMIN) < 0) {
+        return -1;
+    }
+
+    /* change to calling user's real uid and gid, retaining supplemental
+     * groups and CAP_NET_ADMIN */
+    if (capng_change_id(getuid(), getgid(), CAPNG_CLEAR_BOUNDING)) {
+        return -1;
+    }
+
+    return 0;
+}
+#endif
+
 int main(int argc, char **argv)
 {
     struct ifreq ifr;
@@ -207,6 +232,17 @@ int main(int argc, char **argv)
     int access_allowed, access_denied;
     int ret = EXIT_SUCCESS;
 
+#ifdef CONFIG_LIBCAP
+    /* if we're run from an suid binary, immediately drop privileges preserving
+     * cap_net_admin */
+    if (geteuid() == 0 && getuid() != geteuid()) {
+        if (drop_privileges() == -1) {
+            fprintf(stderr, "failed to drop privileges\n");
+            return 1;
+        }
+    }
+#endif
+
     /* parse arguments */
     for (index = 1; index < argc; index++) {
         if (strcmp(argv[index], "--use-vnet") == 0) {