| Message ID | 20180327162419.8962-1-bluca@debian.org |
|---|---|
| State | Superseded, archived |
| Delegated to: | stephen hemminger |
| Headers | show |
| Series | [RFC,iproute2] Drop capabilities if not running ip exec vrf with libcap | expand |
On 3/27/18 10:24 AM, Luca Boccassi wrote: > ip vrf exec requires root or CAP_NET_ADMIN, CAP_SYS_ADMIN and > CAP_DAC_OVERRIDE. It is not possible to run unprivileged commands like > ping as non-root or non-cap-enabled due to this requirement. > To allow users and administrators to safely add the required > capabilities to the binary, drop all capabilities on start if not > invoked with "vrf exec". > Update the manpage with the requirements. > > Signed-off-by: Luca Boccassi <bluca@debian.org> > --- > > I'd like to be able to run ip vrf exec as a normal user, does this approach > sound sensible? Any concerns? Are there any other alternatives? > It would be up to each user/admin/whatever to make sure the program is > built with libcap and to add the capabilities manually, so nothing will be > there by default. This is very similar to a change I recently made for our distribution. I created a separate 'runvrf' command so as to not contaminate 'ip' (the runvrf command has the limitation it can not configure the VRF cgroup so that needs to be done before runvrf). > > configure | 17 +++++++++++++++++ > ip/ip.c | 34 ++++++++++++++++++++++++++++++++++ > man/man8/ip-vrf.8 | 8 ++++++++ > 3 files changed, 59 insertions(+) > > diff --git a/configure b/configure > index f7c2d7a7..5ef5cd4c 100755 > --- a/configure > +++ b/configure > @@ -336,6 +336,20 @@ EOF > rm -f $TMPDIR/strtest.c $TMPDIR/strtest > } > > +check_cap() > +{ > + if ${PKG_CONFIG} libcap --exists > + then > + echo "HAVE_CAP:=y" >>$CONFIG > + echo "yes" > + > + echo 'CFLAGS += -DHAVE_LIBCAP' `${PKG_CONFIG} libcap --cflags` >>$CONFIG > + echo 'LDLIBS +=' `${PKG_CONFIG} libcap --libs` >> $CONFIG > + else > + echo "no" > + fi > +} > + > quiet_config() > { > cat <<EOF > @@ -410,6 +424,9 @@ check_berkeley_db > echo -n "need for strlcpy: " > check_strlcpy > > +echo -n "libcap support: " > +check_cap > + > echo >> $CONFIG > echo "%.o: %.c" >> $CONFIG > echo ' $(QUIET_CC)$(CC) $(CFLAGS) $(EXTRA_CFLAGS) -c -o $@ $<' >> $CONFIG > diff --git a/ip/ip.c b/ip/ip.c > index e0cd96cb..49739571 100644 > --- a/ip/ip.c > +++ b/ip/ip.c > @@ -17,6 +17,10 @@ > #include <netinet/in.h> > #include <string.h> > #include <errno.h> > +#include <sys/types.h> > +#ifdef HAVE_LIBCAP > +#include <sys/capability.h> > +#endif > > #include "SNAPSHOT.h" > #include "utils.h" > @@ -68,6 +72,24 @@ static int do_help(int argc, char **argv) > return 0; > } > > +static void drop_cap(void) > +{ > +#ifdef HAVE_LIBCAP > + /* don't harmstring root/sudo */ > + if (getuid() != 0 && geteuid() != 0) { > + cap_t capabilities; > + capabilities = cap_get_proc(); > + if (!capabilities) > + exit(EXIT_FAILURE); > + if (cap_clear(capabilities) != 0) > + exit(EXIT_FAILURE); > + if (cap_set_proc(capabilities) != 0) > + exit(EXIT_FAILURE); > + cap_free(capabilities); > + } > +#endif > +} You don't need the capabilities after the cgroup has been changed, so you can add a call to drop_cap at the end of vrf_switch. > + > static const struct cmd { > const char *cmd; > int (*func)(int argc, char **argv); > @@ -174,6 +196,18 @@ int main(int argc, char **argv) > char *batch_file = NULL; > int color = 0; > > + /* to run vrf exec without root, capabilities might be set, drop them > + * if not needed as the first thing. > + * execv will drop them for the child command. > + * vrf exec requires: > + * - cap_dac_override to create the cgroup subdir in /sys > + * - cap_sys_admin to load the BPF program > + * - cap_net_admin to set the socket into the cgroup > + */ > + if (argc < 3 || strcmp(argv[1], "vrf") != 0 || > + strcmp(argv[2], "exec") != 0) > + drop_cap(); > + > basename = strrchr(argv[0], '/'); > if (basename == NULL) > basename = argv[0]; > diff --git a/man/man8/ip-vrf.8 b/man/man8/ip-vrf.8 > index 18789339..1a42cebe 100644 > --- a/man/man8/ip-vrf.8 > +++ b/man/man8/ip-vrf.8 > @@ -63,6 +63,14 @@ a VRF other than the default VRF (main table). A command can be run against > the default VRF by passing the "default" as the VRF name. This is useful if > the current shell is associated with another VRF (e.g, Management VRF). > > +This command requires the system to be booted with cgroup v2 (e.g. with systemd, > +add systemd.unified_cgroup_hierarchy=1 to the kernel command line). > + > +This command also requires to be ran as root or with the CAP_SYS_ADMIN, > +CAP_NET_ADMIN and CAP_DAC_OVERRIDE capabilities. If built with libcap and if > +capabilities are added to the ip binary program via setcap, the program will > +drop them as the first thing when invoked, unless the command is vrf exec. > + > .TP > .B ip vrf identify [PID] - Report VRF association for process > .sp >
On Tue, 2018-03-27 at 10:40 -0600, David Ahern wrote: > On 3/27/18 10:24 AM, Luca Boccassi wrote: > > ip vrf exec requires root or CAP_NET_ADMIN, CAP_SYS_ADMIN and > > CAP_DAC_OVERRIDE. It is not possible to run unprivileged commands > > like > > ping as non-root or non-cap-enabled due to this requirement. > > To allow users and administrators to safely add the required > > capabilities to the binary, drop all capabilities on start if not > > invoked with "vrf exec". > > Update the manpage with the requirements. > > > > Signed-off-by: Luca Boccassi <bluca@debian.org> > > --- > > > > I'd like to be able to run ip vrf exec as a normal user, does this > > approach > > sound sensible? Any concerns? Are there any other alternatives? > > It would be up to each user/admin/whatever to make sure the program > > is > > built with libcap and to add the capabilities manually, so nothing > > will be > > there by default. > > This is very similar to a change I recently made for our > distribution. I > created a separate 'runvrf' command so as to not contaminate 'ip' > (the > runvrf command has the limitation it can not configure the VRF cgroup > so > that needs to be done before runvrf). Great, thanks for the feedback! > > > > configure | 17 +++++++++++++++++ > > ip/ip.c | 34 ++++++++++++++++++++++++++++++++++ > > man/man8/ip-vrf.8 | 8 ++++++++ > > 3 files changed, 59 insertions(+) > > > > diff --git a/configure b/configure > > index f7c2d7a7..5ef5cd4c 100755 > > --- a/configure > > +++ b/configure > > @@ -336,6 +336,20 @@ EOF > > rm -f $TMPDIR/strtest.c $TMPDIR/strtest > > } > > > > +check_cap() > > +{ > > + if ${PKG_CONFIG} libcap --exists > > + then > > + echo "HAVE_CAP:=y" >>$CONFIG > > + echo "yes" > > + > > + echo 'CFLAGS += -DHAVE_LIBCAP' `${PKG_CONFIG} > > libcap --cflags` >>$CONFIG > > + echo 'LDLIBS +=' `${PKG_CONFIG} libcap --libs` >> > > $CONFIG > > + else > > + echo "no" > > + fi > > +} > > + > > quiet_config() > > { > > cat <<EOF > > @@ -410,6 +424,9 @@ check_berkeley_db > > echo -n "need for strlcpy: " > > check_strlcpy > > > > +echo -n "libcap support: " > > +check_cap > > + > > echo >> $CONFIG > > echo "%.o: %.c" >> $CONFIG > > echo ' $(QUIET_CC)$(CC) $(CFLAGS) $(EXTRA_CFLAGS) -c -o $@ > > $<' >> $CONFIG > > diff --git a/ip/ip.c b/ip/ip.c > > index e0cd96cb..49739571 100644 > > --- a/ip/ip.c > > +++ b/ip/ip.c > > @@ -17,6 +17,10 @@ > > #include <netinet/in.h> > > #include <string.h> > > #include <errno.h> > > +#include <sys/types.h> > > +#ifdef HAVE_LIBCAP > > +#include <sys/capability.h> > > +#endif > > > > #include "SNAPSHOT.h" > > #include "utils.h" > > @@ -68,6 +72,24 @@ static int do_help(int argc, char **argv) > > return 0; > > } > > > > +static void drop_cap(void) > > +{ > > +#ifdef HAVE_LIBCAP > > + /* don't harmstring root/sudo */ > > + if (getuid() != 0 && geteuid() != 0) { > > + cap_t capabilities; > > + capabilities = cap_get_proc(); > > + if (!capabilities) > > + exit(EXIT_FAILURE); > > + if (cap_clear(capabilities) != 0) > > + exit(EXIT_FAILURE); > > + if (cap_set_proc(capabilities) != 0) > > + exit(EXIT_FAILURE); > > + cap_free(capabilities); > > + } > > +#endif > > +} > > You don't need the capabilities after the cgroup has been changed, so > you can add a call to drop_cap at the end of vrf_switch. Ok, if Stephen finds the approach correct I'll send a v1 with that change (it shouldn't be strictly necessary as execvp which is ran just after vrf_switch drops caps, but it won't hurt either).
On Tue, 27 Mar 2018 17:24:19 +0100 Luca Boccassi <bluca@debian.org> wrote: > ip vrf exec requires root or CAP_NET_ADMIN, CAP_SYS_ADMIN and > CAP_DAC_OVERRIDE. It is not possible to run unprivileged commands like > ping as non-root or non-cap-enabled due to this requirement. > To allow users and administrators to safely add the required > capabilities to the binary, drop all capabilities on start if not > invoked with "vrf exec". > Update the manpage with the requirements. > > Signed-off-by: Luca Boccassi <bluca@debian.org> Gets a little messy, but don't have a better answer. When a command like iproute gets involved in security policy things I become concerned that it may have unexpected consequences.
On Tue, 2018-03-27 at 10:15 -0700, Stephen Hemminger wrote: > On Tue, 27 Mar 2018 17:24:19 +0100 > Luca Boccassi <bluca@debian.org> wrote: > > > ip vrf exec requires root or CAP_NET_ADMIN, CAP_SYS_ADMIN and > > CAP_DAC_OVERRIDE. It is not possible to run unprivileged commands > > like > > ping as non-root or non-cap-enabled due to this requirement. > > To allow users and administrators to safely add the required > > capabilities to the binary, drop all capabilities on start if not > > invoked with "vrf exec". > > Update the manpage with the requirements. > > > > Signed-off-by: Luca Boccassi <bluca@debian.org> > > Gets a little messy, but don't have a better answer. > When a command like iproute gets involved in security policy things > I become concerned that it may have unexpected consequences. Yeah I understand. It requires an explicit action by the sysadmin, to give you plausible deniability :-) I've seen changes to let BPF permissions be managed via an LSM (I think SELinux support is already merged in 4.15), so perhaps one day we'll be able to do the whole shebang (subdir in /sys + load bpf + manipulate cgroup) in a more fine-grained way, but for now I think this will do. I'll send v1 shortly with the change asked by David.
diff --git a/configure b/configure index f7c2d7a7..5ef5cd4c 100755 --- a/configure +++ b/configure @@ -336,6 +336,20 @@ EOF rm -f $TMPDIR/strtest.c $TMPDIR/strtest } +check_cap() +{ + if ${PKG_CONFIG} libcap --exists + then + echo "HAVE_CAP:=y" >>$CONFIG + echo "yes" + + echo 'CFLAGS += -DHAVE_LIBCAP' `${PKG_CONFIG} libcap --cflags` >>$CONFIG + echo 'LDLIBS +=' `${PKG_CONFIG} libcap --libs` >> $CONFIG + else + echo "no" + fi +} + quiet_config() { cat <<EOF @@ -410,6 +424,9 @@ check_berkeley_db echo -n "need for strlcpy: " check_strlcpy +echo -n "libcap support: " +check_cap + echo >> $CONFIG echo "%.o: %.c" >> $CONFIG echo ' $(QUIET_CC)$(CC) $(CFLAGS) $(EXTRA_CFLAGS) -c -o $@ $<' >> $CONFIG diff --git a/ip/ip.c b/ip/ip.c index e0cd96cb..49739571 100644 --- a/ip/ip.c +++ b/ip/ip.c @@ -17,6 +17,10 @@ #include <netinet/in.h> #include <string.h> #include <errno.h> +#include <sys/types.h> +#ifdef HAVE_LIBCAP +#include <sys/capability.h> +#endif #include "SNAPSHOT.h" #include "utils.h" @@ -68,6 +72,24 @@ static int do_help(int argc, char **argv) return 0; } +static void drop_cap(void) +{ +#ifdef HAVE_LIBCAP + /* don't harmstring root/sudo */ + if (getuid() != 0 && geteuid() != 0) { + cap_t capabilities; + capabilities = cap_get_proc(); + if (!capabilities) + exit(EXIT_FAILURE); + if (cap_clear(capabilities) != 0) + exit(EXIT_FAILURE); + if (cap_set_proc(capabilities) != 0) + exit(EXIT_FAILURE); + cap_free(capabilities); + } +#endif +} + static const struct cmd { const char *cmd; int (*func)(int argc, char **argv); @@ -174,6 +196,18 @@ int main(int argc, char **argv) char *batch_file = NULL; int color = 0; + /* to run vrf exec without root, capabilities might be set, drop them + * if not needed as the first thing. + * execv will drop them for the child command. + * vrf exec requires: + * - cap_dac_override to create the cgroup subdir in /sys + * - cap_sys_admin to load the BPF program + * - cap_net_admin to set the socket into the cgroup + */ + if (argc < 3 || strcmp(argv[1], "vrf") != 0 || + strcmp(argv[2], "exec") != 0) + drop_cap(); + basename = strrchr(argv[0], '/'); if (basename == NULL) basename = argv[0]; diff --git a/man/man8/ip-vrf.8 b/man/man8/ip-vrf.8 index 18789339..1a42cebe 100644 --- a/man/man8/ip-vrf.8 +++ b/man/man8/ip-vrf.8 @@ -63,6 +63,14 @@ a VRF other than the default VRF (main table). A command can be run against the default VRF by passing the "default" as the VRF name. This is useful if the current shell is associated with another VRF (e.g, Management VRF). +This command requires the system to be booted with cgroup v2 (e.g. with systemd, +add systemd.unified_cgroup_hierarchy=1 to the kernel command line). + +This command also requires to be ran as root or with the CAP_SYS_ADMIN, +CAP_NET_ADMIN and CAP_DAC_OVERRIDE capabilities. If built with libcap and if +capabilities are added to the ip binary program via setcap, the program will +drop them as the first thing when invoked, unless the command is vrf exec. + .TP .B ip vrf identify [PID] - Report VRF association for process .sp
ip vrf exec requires root or CAP_NET_ADMIN, CAP_SYS_ADMIN and CAP_DAC_OVERRIDE. It is not possible to run unprivileged commands like ping as non-root or non-cap-enabled due to this requirement. To allow users and administrators to safely add the required capabilities to the binary, drop all capabilities on start if not invoked with "vrf exec". Update the manpage with the requirements. Signed-off-by: Luca Boccassi <bluca@debian.org> --- I'd like to be able to run ip vrf exec as a normal user, does this approach sound sensible? Any concerns? Are there any other alternatives? It would be up to each user/admin/whatever to make sure the program is built with libcap and to add the capabilities manually, so nothing will be there by default. configure | 17 +++++++++++++++++ ip/ip.c | 34 ++++++++++++++++++++++++++++++++++ man/man8/ip-vrf.8 | 8 ++++++++ 3 files changed, 59 insertions(+)