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Add cap reduction support to enable use as SUID

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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>
This commit is contained in:
Corey Bryant 2012-01-26 09:42:26 -05:00 committed by Anthony Liguori
parent bdef79a299
commit 47e98658f5
2 changed files with 70 additions and 0 deletions
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34
configure vendored
View File

@ -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

36
qemu-bridge-helper.c
View File

@ -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) {