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check capabilities, not effective user ID

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Under Linux, a process may not be run under root, yet it may have a permission
to do what a superuser may do given specific capabilities are granted.

This commit makes iodine not depend on EUID being 0 in order to run
properly. Instead, in presence of libcap-ng, the following capabilities
are being checked:

* `CAP_NET_BIND_SERVICES` for server to bind to a port, lower than
  `/proc/sys/net/ipv4/ip_unprivileged_port_start`
* `CAP_NET_ADMIN` to operate on a TUN device
* `CAP_SETUID` and `CAP_SETGID` in case server is configured to change
  the user it runs on behalf of

This change is handy if iodine is being run under a non-root user, provided
`AmbientCapabilities=` and `CapabilityBoundingSet=` of systemd are employed
in the first place.

Fixes: https://github.com/yarrick/iodine/issues/80
Signed-off-by: Oleksandr Natalenko <oleksandr@redhat.com>
This commit is contained in:
Oleksandr Natalenko 2022-11-02 19:14:09 +01:00 committed by Oleksandr Natalenko
parent 11dd73a646
commit ebf4e7ee9e
5 changed files with 60 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

55
src/common.c
View File

@ -51,6 +51,11 @@
# include <selinux/selinux.h>
#endif
#ifdef HAVE_LIBCAPNG
#include <stdbool.h>
#include <cap-ng.h>
#endif
#include "common.h"
/* The raw header used when not using DNS protocol */
@ -103,12 +108,60 @@ int setgroups(int count, int *groups)
#ifndef WINDOWS32
void
check_privileges(void)
check_privileges(char *username, int port)
{
#if defined HAVE_LIBCAPNG
bool capable = true;
if (capng_get_caps_process() == -1) {
warnx("Unable to get capabilities");
exit(-1);
}
if (!capng_have_capability(CAPNG_EFFECTIVE, CAP_NET_ADMIN)) {
warnx("capabilities: CAP_NET_ADMIN required");
capable = false;
}
if (port) {
unsigned short int ip_unprivileged_port_start = 1024;
FILE *file = fopen("/proc/sys/net/ipv4/ip_unprivileged_port_start", "r");
if (!file) {
warnx("sysctl: unable to get ip_unprivileged_port_start value");
// do not bail out here in case systemd.service has ProcSubset=pid set
} else {
fscanf(file, "%hu", &ip_unprivileged_port_start);
fclose(file);
}
if (port < ip_unprivileged_port_start &&
!capng_have_capability(CAPNG_EFFECTIVE, CAP_NET_BIND_SERVICE)) {
warnx("capabilities: CAP_NET_BIND_SERVICE required");
capable = false;
}
}
if (username) {
if (!capng_have_capability(CAPNG_EFFECTIVE, CAP_SETUID)) {
warnx("capabilities: CAP_SETUID required");
capable = false;
}
if (!capng_have_capability(CAPNG_EFFECTIVE, CAP_SETGID)) {
warnx("capabilities: CAP_SETGID required");
capable = false;
}
}
if (!capable) {
exit(-1);
}
#else
if (geteuid() != 0) {
warnx("Run as root and you'll be happy.");
exit(-1);
}
#endif
}
#endif

4
src/common.h
View File

@ -105,11 +105,11 @@ enum connection {
};
#ifdef WINDOWS32
static inline void check_privileges(void)
static inline void check_privileges(char *, int)
{
}
#else
void check_privileges(void);
void check_privileges(char *, int);
#endif
char *format_addr(struct sockaddr_storage *sockaddr, int sockaddr_len);
int get_addr(char *, int, int, int, struct sockaddr_storage *);

2
src/iodine.c
View File

@ -279,7 +279,7 @@ int main(int argc, char **argv)
}
}
check_privileges();
check_privileges(username, 0);
argc -= optind;
argv += optind;

2
src/iodined.c
View File

@ -2519,7 +2519,7 @@ main(int argc, char **argv)
argc -= optind;
argv += optind;
check_privileges();
check_privileges(username, port);
if (argc != 2)
usage();

2
src/osflags
View File

@ -23,6 +23,7 @@ link)
[ -e /usr/include/selinux/selinux.h ] && FLAGS="$FLAGS -lselinux";
"$PKG_CONFIG" --exists libsystemd-daemon && FLAGS="$FLAGS $($PKG_CONFIG --libs libsystemd-daemon)";
"$PKG_CONFIG" --exists libsystemd && FLAGS="$FLAGS $($PKG_CONFIG --libs libsystemd)";
"$PKG_CONFIG" --exists libcap-ng && FLAGS="$FLAGS $($PKG_CONFIG --libs libcap-ng)";
echo $FLAGS;
;;
esac
@ -43,6 +44,7 @@ cflags)
[ -e /usr/include/selinux/selinux.h ] && FLAGS="$FLAGS -DHAVE_SETCON";
"$PKG_CONFIG" --exists libsystemd-daemon && FLAGS="$FLAGS -DHAVE_SYSTEMD";
"$PKG_CONFIG" --exists libsystemd && FLAGS="$FLAGS -DHAVE_SYSTEMD";
"$PKG_CONFIG" --exists libcap-ng && FLAGS="$FLAGS -DHAVE_LIBCAPNG";
echo $FLAGS;
;;
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