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#include <sys/prctl.h>
#include <linux/capability.h>
#ifndef _LINUX_CAPABILITY_VERSION_3
#define _LINUX_CAPABILITY_VERSION_3 0x20080522
#define _LINUX_CAPABILITY_U32S_3 2
#endif
/* CAP_TO_MASK is missing in CentOS header files */
#ifndef CAP_TO_MASK
#define CAP_TO_MASK(x) (1 << ((x) & 31))
#endif
/* capset() prototype is missing ... */
int capset(cap_user_header_t hdrp, const cap_user_data_t datap);
static inline int
set_capabilities(u32 caps)
{
struct __user_cap_header_struct cap_hdr;
struct __user_cap_data_struct cap_dat[_LINUX_CAPABILITY_U32S_3];
int err;
cap_hdr.version = _LINUX_CAPABILITY_VERSION_3;
cap_hdr.pid = 0;
memset(cap_dat, 0, sizeof(cap_dat));
cap_dat[0].effective = cap_dat[0].permitted = caps;
err = capset(&cap_hdr, cap_dat);
if (!err)
return 0;
/* Kernel may support do not support our version of capability interface.
The last call returned supported version so we just retry it. */
if (errno == EINVAL)
{
err = capset(&cap_hdr, cap_dat);
if (!err)
return 0;
}
return -1;
}
static void
drop_uid(uid_t uid)
{
u32 caps =
CAP_TO_MASK(CAP_NET_BIND_SERVICE) |
CAP_TO_MASK(CAP_NET_BROADCAST) |
CAP_TO_MASK(CAP_NET_ADMIN) |
CAP_TO_MASK(CAP_NET_RAW);
/* change effective user ID to be able to switch to that
user ID completely after dropping CAP_SETUID */
if (seteuid(uid) < 0)
die("seteuid: %m");
/* restrict the capabilities */
if (set_capabilities(caps) < 0)
die("capset: %m");
/* keep the capabilities after dropping root ID */
if (prctl(PR_SET_KEEPCAPS, 1) < 0)
die("prctl: %m");
/* completely switch to the unprivileged user ID */
if (setresuid(uid, uid, uid) < 0)
die("setresuid: %m");
}
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