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Linux: Retain CAP_NET_ADMIN when dropping privileges
On Linux, when dropping privileges, interaction with the network configuration, such as tearing down routes or ovpn-dco interfaces will fail when --user/--group are used. This patch sets the CAP_NET_ADMIN capability, which grants the needed privileges during the lifetime of the OpenVPN process when dropping root privileges. Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org> Reviewed-By: David Sommerseth <davids@openvpn.net> Acked-by: Frank Lichtenheld <frank@lichtenheld.com> Message-Id: <20220514103717.235-1-timo@rothenpieler.org> URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24360.html Signed-off-by: Gert Doering <gert@greenie.muc.de>
This commit is contained in:
parent
3cb40b22ae
commit
2e359a0882
19
configure.ac
19
configure.ac
@ -793,6 +793,25 @@ dnl
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esac
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fi
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dnl
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dnl Depend on libcap-ng on Linux
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dnl
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case "$host" in
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*-*-linux*)
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PKG_CHECK_MODULES([LIBCAPNG],
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[libcap-ng],
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[],
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[AC_MSG_ERROR([libcap-ng package not found. Is the development package and pkg-config installed?])]
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)
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AC_CHECK_HEADER([sys/prctl.h],,[AC_MSG_ERROR([sys/prctl.h not found!])])
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CFLAGS="${CFLAGS} ${LIBCAPNG_CFALGS}"
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LIBS="${LIBS} ${LIBCAPNG_LIBS}"
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AC_DEFINE(HAVE_LIBCAPNG, 1, [Enable libcap-ng support])
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;;
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esac
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if test "${with_crypto_library}" = "openssl"; then
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AC_ARG_VAR([OPENSSL_CFLAGS], [C compiler flags for OpenSSL])
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AC_ARG_VAR([OPENSSL_LIBS], [linker flags for OpenSSL])
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@ -11,7 +11,7 @@ Type=notify
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PrivateTmp=true
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WorkingDirectory=/etc/openvpn/client
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ExecStart=@sbindir@/openvpn --suppress-timestamps --nobind --config %i.conf
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CapabilityBoundingSet=CAP_IPC_LOCK CAP_NET_ADMIN CAP_NET_RAW CAP_SETGID CAP_SETUID CAP_SYS_CHROOT CAP_DAC_OVERRIDE
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CapabilityBoundingSet=CAP_IPC_LOCK CAP_NET_ADMIN CAP_NET_RAW CAP_SETGID CAP_SETUID CAP_SETPCAP CAP_SYS_CHROOT CAP_DAC_OVERRIDE
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LimitNPROC=10
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DeviceAllow=/dev/null rw
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DeviceAllow=/dev/net/tun rw
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@ -11,7 +11,7 @@ Type=notify
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PrivateTmp=true
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WorkingDirectory=/etc/openvpn/server
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ExecStart=@sbindir@/openvpn --status %t/openvpn-server/status-%i.log --status-version 2 --suppress-timestamps --config %i.conf
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CapabilityBoundingSet=CAP_IPC_LOCK CAP_NET_ADMIN CAP_NET_BIND_SERVICE CAP_NET_RAW CAP_SETGID CAP_SETUID CAP_SYS_CHROOT CAP_DAC_OVERRIDE CAP_AUDIT_WRITE
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CapabilityBoundingSet=CAP_IPC_LOCK CAP_NET_ADMIN CAP_NET_BIND_SERVICE CAP_NET_RAW CAP_SETGID CAP_SETUID CAP_SETPCAP CAP_SYS_CHROOT CAP_DAC_OVERRIDE CAP_AUDIT_WRITE
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LimitNPROC=10
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DeviceAllow=/dev/null rw
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DeviceAllow=/dev/net/tun rw
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@ -1183,8 +1183,9 @@ do_uid_gid_chroot(struct context *c, bool no_delay)
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{
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if (no_delay)
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{
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platform_group_set(&c0->platform_state_group);
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platform_user_set(&c0->platform_state_user);
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platform_user_group_set(&c0->platform_state_user,
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&c0->platform_state_group,
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c);
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}
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else if (c->first_time)
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{
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@ -29,6 +29,9 @@
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#include "syshead.h"
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#include "openvpn.h"
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#include "options.h"
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#include "buffer.h"
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#include "crypto.h"
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#include "error.h"
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@ -43,6 +46,11 @@
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#include <direct.h>
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#endif
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#ifdef HAVE_LIBCAPNG
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#include <cap-ng.h>
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#include <sys/prctl.h>
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#endif
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/* Redefine the top level directory of the filesystem
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* to restrict access to files for security */
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void
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@ -91,7 +99,7 @@ platform_user_get(const char *username, struct platform_state_user *state)
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return ret;
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}
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void
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static void
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platform_user_set(const struct platform_state_user *state)
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{
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#if defined(HAVE_GETPWNAM) && defined(HAVE_SETUID)
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@ -130,7 +138,7 @@ platform_group_get(const char *groupname, struct platform_state_group *state)
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return ret;
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}
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void
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static void
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platform_group_set(const struct platform_state_group *state)
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{
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#if defined(HAVE_GETGRNAM) && defined(HAVE_SETGID)
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@ -155,6 +163,140 @@ platform_group_set(const struct platform_state_group *state)
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#endif
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}
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/*
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* Determine if we need to retain process capabilities. DCO and SITNL need it.
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* Enforce it for DCO, but only try and soft-fail for SITNL to keep backwards compat.
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*
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* Returns the tri-state expected by platform_user_group_set.
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* -1: try to keep caps, but continue if impossible
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* 0: don't keep caps
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* 1: keep caps, fail hard if impossible
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*/
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static int
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need_keep_caps(struct context *c)
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{
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if (!c)
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{
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return -1;
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}
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if (dco_enabled(&c->options))
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{
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#ifdef TARGET_LINUX
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/* DCO on Linux does not work at all without CAP_NET_ADMIN */
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return 1;
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#else
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/* Windows/BSD/... has no equivalent capability mechanism */
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return -1;
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#endif
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}
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#ifdef ENABLE_SITNL
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return -1;
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#else
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return 0;
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#endif
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}
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/* Set user and group, retaining neccesary capabilities required by the platform.
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*
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* The keep_caps argument has 3 possible states:
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* >0: Retain capabilities, and fail hard on failure to do so.
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* ==0: Don't attempt to retain any capabilities, just sitch user/group.
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* <0: Try to retain capabilities, but continue on failure.
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*/
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void platform_user_group_set(const struct platform_state_user *user_state,
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const struct platform_state_group *group_state,
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struct context *c)
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{
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int keep_caps = need_keep_caps(c);
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unsigned int err_flags = (keep_caps > 0) ? M_FATAL : M_NONFATAL;
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#ifdef HAVE_LIBCAPNG
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int new_gid = -1, new_uid = -1;
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int res;
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if (keep_caps == 0)
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{
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goto fallback;
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}
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/*
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* new_uid/new_gid defaults to -1, which will not make
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* libcap-ng change the UID/GID unless configured
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*/
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if (group_state->groupname && group_state->gr)
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{
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new_gid = group_state->gr->gr_gid;
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}
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if (user_state->username && user_state->pw)
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{
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new_uid = user_state->pw->pw_uid;
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}
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/* Prepare capabilities before dropping UID/GID */
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capng_clear(CAPNG_SELECT_BOTH);
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res = capng_update(CAPNG_ADD, CAPNG_EFFECTIVE | CAPNG_PERMITTED, CAP_NET_ADMIN);
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if (res < 0)
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{
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msg(err_flags, "capng_update(CAP_NET_ADMIN) failed: %d", res);
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goto fallback;
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}
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/* Change to new UID/GID.
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* capng_change_id() internally calls capng_apply() to apply prepared capabilities.
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*/
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res = capng_change_id(new_uid, new_gid, CAPNG_DROP_SUPP_GRP | CAPNG_CLEAR_BOUNDING);
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if (res == -4 || res == -6)
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{
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/* -4 and -6 mean failure of setuid/gid respectively.
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There is no point for us to continue if those failed. */
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msg(M_ERR, "capng_change_id('%s','%s') failed: %d",
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user_state->username, group_state->groupname, res);
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}
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else if (res == -3)
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{
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msg(M_NONFATAL | M_ERRNO, "capng_change_id() failed applying capabilities");
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msg(err_flags, "NOTE: previous error likely due to missing capability CAP_SETPCAP.");
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goto fallback;
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}
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else if (res < 0)
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{
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msg(err_flags | M_ERRNO, "capng_change_id('%s','%s') failed retaining capabilities: %d",
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user_state->username, group_state->groupname, res);
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goto fallback;
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}
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if (new_uid >= 0)
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{
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msg(M_INFO, "UID set to %s", user_state->username);
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}
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if (new_gid >= 0)
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{
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msg(M_INFO, "GID set to %s", group_state->groupname);
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}
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msg(M_INFO, "Capabilities retained: CAP_NET_ADMIN");
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return;
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fallback:
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/* capng_change_id() can leave this flag clobbered on failure
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* This is working around a bug in libcap-ng, which can leave the flag set
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* on failure: https://github.com/stevegrubb/libcap-ng/issues/33 */
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if (prctl(PR_GET_KEEPCAPS) && prctl(PR_SET_KEEPCAPS, 0) < 0)
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{
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msg(M_ERR, "Clearing KEEPCAPS flag failed");
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}
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#endif /* HAVE_LIBCAPNG */
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if (keep_caps)
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{
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msg(err_flags, "Unable to retain capabilities");
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}
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platform_group_set(group_state);
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platform_user_set(user_state);
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}
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/* Change process priority */
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void
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platform_nice(int niceval)
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#include "basic.h"
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#include "buffer.h"
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/* forward declared to avoid large amounts of extra includes */
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struct context;
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/* Get/Set UID of process */
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struct platform_state_user {
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@ -79,11 +82,12 @@ struct platform_state_group {
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bool platform_user_get(const char *username, struct platform_state_user *state);
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void platform_user_set(const struct platform_state_user *state);
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bool platform_group_get(const char *groupname, struct platform_state_group *state);
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void platform_group_set(const struct platform_state_group *state);
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void platform_user_group_set(const struct platform_state_user *user_state,
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const struct platform_state_group *group_state,
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struct context *c);
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/*
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* Extract UID or GID
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