From 0b421290e05862e1abbb5a82654bd2de9829dd58 Mon Sep 17 00:00:00 2001

From: Patrick Steinhardt <ps@pks.im>

Date: Tue, 10 Apr 2018 12:08:21 +0100

Subject: [PATCH 69/74] setpriv: implement option to set parent death signal

When a process uses the syscall `prctl(PR_SET_PDEATHSIG, ...)`, it will

get notified with a process-defined signal as soon as its parent process

dies. This is for example being used by unshare(1)'s recently added

"--kill-child" option, causing the forked child to be killed as soon as

unshare itself dies.

Unfortunately, some LSMs will cause the parent death signal to be reset

when a process changes credentials, with the most important ones being

SELinux and AppArmor. The following command will thus not work as

expected:

    unshare --fork --kill-child setpriv --reuid user <executable>

As soon as setpriv changes UID, the parent death signal is cleared and

the child will never get signalled when unshare gets killed.

Add a new option "--pdeathsig keep|clear|<signal>". Setting this flag

will cause us to either

- restore the previously active parent death signal as soon as the

  setpriv has applied all credential changes

- clear the parent death signal

- set the parent death signal to "<signal>"

Furthermore, print out the currently set signal when dumping process

state.

[kzak@redhat.com: - small changes in codding style]

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1894192

Signed-off-by: Patrick Steinhardt <ps@pks.im>

Signed-off-by: Karel Zak <kzak@redhat.com>

---

 sys-utils/setpriv.1 |  6 ++++++

 sys-utils/setpriv.c | 49 +++++++++++++++++++++++++++++++++++++++++++++

 2 files changed, 55 insertions(+)

diff --git a/sys-utils/setpriv.1 b/sys-utils/setpriv.1

index b900f6e08..f989bf33c 100644

--- a/sys-utils/setpriv.1

+++ b/sys-utils/setpriv.1

@@ -139,6 +139,12 @@ is cleared by

 .BR execve (2)

 and is therefore not allowed.

 .TP

+.BR "\-\-pdeathsig keep" | clear | <signal>

+Keep, clear or set the parent death signal.  Some LSMs, most notably SELinux and

+AppArmor, clear the signal when the process' credentials change.  Using

+\fB--pdeathsig keep\fR will restore the parent death signal after changing

+credentials to remedy that situation.

+.TP

 .BI \-\-selinux\-label " label"

 Request a particular SELinux transition (using a transition on exec, not

 dyntrans).  This will fail and cause

diff --git a/sys-utils/setpriv.c b/sys-utils/setpriv.c

index 4147978cc..0d3a3b3c9 100644

--- a/sys-utils/setpriv.c

+++ b/sys-utils/setpriv.c

@@ -38,6 +38,7 @@

 #include "strutils.h"

 #include "xalloc.h"

 #include "pathnames.h"

+#include "signames.h"

 #ifndef PR_SET_NO_NEW_PRIVS

 # define PR_SET_NO_NEW_PRIVS 38

@@ -102,6 +103,8 @@ struct privctx {

 	/* securebits */

 	int securebits;

+	/* parent death signal (<0 clear, 0 nothing, >0 signal) */

+	int pdeathsig;

 	/* LSMs */

 	const char *selinux_label;

@@ -135,6 +138,8 @@ static void __attribute__((__noreturn__)) usage(void)

 	fputs(_(" --init-groups               initialize supplementary groups\n"), out);

 	fputs(_(" --groups <group,...>        set supplementary groups\n"), out);

 	fputs(_(" --securebits <bits>         set securebits\n"), out);

+	fputs(_(" --pdeathsig keep|clear|<signame>\n"

+	        "                             set or clear parent death signal\n"), out);

 	fputs(_(" --selinux-label <label>     set SELinux label\n"), out);

 	fputs(_(" --apparmor-profile <pr>     set AppArmor profile\n"), out);

@@ -329,6 +334,24 @@ static void dump_groups(void)

 	free(groups);

 }

+static void dump_pdeathsig(void)

+{

+	int pdeathsig;

+

+	if (prctl(PR_GET_PDEATHSIG, &pdeathsig) != 0) {

+		warn(_("get pdeathsig failed"));

+		return;

+	}

+

+	printf("Parent death signal: ");

+	if (pdeathsig && signum_to_signame(pdeathsig) != NULL)

+		printf("%s\n", signum_to_signame(pdeathsig));

+	else if (pdeathsig)

+		printf("%d\n", pdeathsig);

+	else

+		printf("[none]\n");

+}

+

 static void dump(int dumplevel)

 {

 	int x;

@@ -392,6 +415,7 @@ static void dump(int dumplevel)

 	printf("\n");

 	dump_securebits();

+	dump_pdeathsig();

 	if (access(_PATH_SYS_SELINUX, F_OK) == 0)

 		dump_label(_("SELinux label"));

@@ -438,6 +462,19 @@ static void parse_groups(struct privctx *opts, const char *str)

 	free(groups);

 }

+static void parse_pdeathsig(struct privctx *opts, const char *str)

+{

+	if (!strcmp(str, "keep")) {

+		if (prctl(PR_GET_PDEATHSIG, &opts->pdeathsig) != 0)

+			errx(SETPRIV_EXIT_PRIVERR,

+				 _("failed to get parent death signal"));

+	} else if (!strcmp(str, "clear")) {

+		opts->pdeathsig = -1;

+	} else if ((opts->pdeathsig = signame_to_signum(str)) < 0) {

+		errx(EXIT_FAILURE, _("unknown signal: %s"), str);

+	}

+}

+

 static void do_setresuid(const struct privctx *opts)

 {

 	uid_t ruid, euid, suid;

@@ -711,6 +748,7 @@ int main(int argc, char **argv)

 		LISTCAPS,

 		CAPBSET,

 		SECUREBITS,

+		PDEATHSIG,

 		SELINUX_LABEL,

 		APPARMOR_PROFILE

 	};

@@ -734,6 +772,7 @@ int main(int argc, char **argv)

 		{ "groups",           required_argument, NULL, GROUPS           },

 		{ "bounding-set",     required_argument, NULL, CAPBSET          },

 		{ "securebits",       required_argument, NULL, SECUREBITS       },

+		{ "pdeathsig",        required_argument, NULL, PDEATHSIG,       },

 		{ "selinux-label",    required_argument, NULL, SELINUX_LABEL    },

 		{ "apparmor-profile", required_argument, NULL, APPARMOR_PROFILE },

 		{ "help",             no_argument,       NULL, 'h'              },

@@ -844,6 +883,12 @@ int main(int argc, char **argv)

 				     _("duplicate --groups option"));

 			parse_groups(&opts, optarg);

 			break;

+		case PDEATHSIG:

+			if (opts.pdeathsig)

+				errx(EXIT_FAILURE,

+				     _("duplicate --keep-pdeathsig option"));

+			parse_pdeathsig(&opts, optarg);

+			break;

 		case LISTCAPS:

 			list_caps = 1;

 			break;

@@ -989,6 +1034,10 @@ int main(int argc, char **argv)

 		do_caps(CAP_TYPE_AMBIENT, opts.ambient_caps);

 	}

+	/* Clear or set parent death signal */

+	if (opts.pdeathsig && prctl(PR_SET_PDEATHSIG, opts.pdeathsig < 0 ? 0 : opts.pdeathsig) != 0)

+		err(SETPRIV_EXIT_PRIVERR, _("set parent death signal failed"));

+

 	execvp(argv[optind], argv + optind);

 	errexec(argv[optind]);

 }

-- 

2.31.1