run-init: new applet

function                                             old     new   delta
switch_root_main                                     354     637    +283
drop_usermodehelper                                    -     157    +157
cap_name_to_number                                     -      77     +77
packed_usage                                       31707   31743     +36
applet_names                                        2665    2674      +9
applet_main                                         1544    1548      +4
applet_install_loc                                   193     194      +1
setpriv_main                                         933     928      -5
getcaps                                              131     122      -9
parse_cap                                            117      29     -88
------------------------------------------------------------------------------
(add/remove: 3/0 grow/shrink: 5/3 up/down: 567/-102)          Total: 465 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/libbb.h

@@ -1475,6 +1475,15 @@ const char *get_shell_name(void) FAST_FUNC;
 
 unsigned cap_name_to_number(const char *cap) FAST_FUNC;
 void printf_cap(const char *pfx, unsigned cap_no) FAST_FUNC;
+void drop_capability(int cap_ordinal) FAST_FUNC;
+/* Structures inside "struct caps" are Linux-specific and libcap-specific: */
+#define DEFINE_STRUCT_CAPS \
+struct caps { \
+	struct __user_cap_header_struct header; \
+	unsigned u32s; \
+	struct __user_cap_data_struct data[2]; \
+}
+void getcaps(void *caps) FAST_FUNC;
 
 unsigned cap_name_to_number(const char *name) FAST_FUNC;
 void printf_cap(const char *pfx, unsigned cap_no) FAST_FUNC;

klibc-utils/run-init.c

@@ -0,0 +1,27 @@
+/*
+ * run-init implementation for busybox
+ *
+ * Copyright (c) 2017 Denys Vlasenko <vda.linux@gmail.com>
+ *
+ * Licensed under GPLv2, see file LICENSE in this source tree.
+ */
+//config:config RUN_INIT
+//config:	bool "run-init"
+//config:	default y
+//config:	select PLATFORM_LINUX
+//config:	help
+//config:	The run-init utility is used from initramfs to select a new
+//config:	root device. Under initramfs, you have to use this instead of
+//config:	pivot_root.
+//config:
+//config:	Booting with initramfs extracts a gzipped cpio archive into rootfs
+//config:	(which is a variant of ramfs/tmpfs). Because rootfs can't be moved
+//config:	or unmounted, pivot_root will not work from initramfs. Instead,
+//config:	run-init deletes everything out of rootfs (including itself),
+//config:	does a mount --move that overmounts rootfs with the new root, and
+//config:	then execs the specified init program.
+//config:
+//config:	util-linux has a similar tool, switch-root.
+//config:	run-init differs by also having a "-d CAPS_TO_DROP" option.
+
+/* applet and kbuild hooks are in switch_root.c */

libbb/capability.c

@@ -6,6 +6,14 @@
 //kbuild:lib-$(CONFIG_PLATFORM_LINUX) += capability.o
 
 #include <linux/capability.h>
+// #include <sys/capability.h>
+// This header is in libcap, but the functions are in libc.
+// Comment in the header says this above capset/capget:
+/* system calls - look to libc for function to system call mapping */
+extern int capset(cap_user_header_t header, cap_user_data_t data);
+extern int capget(cap_user_header_t header, const cap_user_data_t data);
+// so for bbox, let's just repeat the declarations.
+// This way, libcap needs not be installed in build environment.
 #include "libbb.h"
 
 static const char *const capabilities[] = {
@@ -77,3 +85,42 @@ void FAST_FUNC printf_cap(const char *pfx, unsigned cap_no)
 	}
 	printf("%scap_%u", pfx, cap_no);
 }
+
+DEFINE_STRUCT_CAPS;
+
+void FAST_FUNC getcaps(void *arg)
+{
+	static const uint8_t versions[] = {
+		_LINUX_CAPABILITY_U32S_3, /* = 2 (fits into byte) */
+		_LINUX_CAPABILITY_U32S_2, /* = 2 */
+		_LINUX_CAPABILITY_U32S_1, /* = 1 */
+	};
+	int i;
+	struct caps *caps = arg;
+
+	caps->header.pid = 0;
+	for (i = 0; i < ARRAY_SIZE(versions); i++) {
+		caps->header.version = versions[i];
+		if (capget(&caps->header, NULL) == 0)
+			goto got_it;
+	}
+	bb_simple_perror_msg_and_die("capget");
+ got_it:
+
+	switch (caps->header.version) {
+		case _LINUX_CAPABILITY_VERSION_1:
+			caps->u32s = _LINUX_CAPABILITY_U32S_1;
+			break;
+		case _LINUX_CAPABILITY_VERSION_2:
+			caps->u32s = _LINUX_CAPABILITY_U32S_2;
+			break;
+		case _LINUX_CAPABILITY_VERSION_3:
+			caps->u32s = _LINUX_CAPABILITY_U32S_3;
+			break;
+		default:
+			bb_error_msg_and_die("unsupported capability version");
+	}
+
+	if (capget(&caps->header, caps->data) != 0)
+		bb_simple_perror_msg_and_die("capget");
+}

util-linux/setpriv.c

@@ -124,48 +124,7 @@ enum {
 };
 
 #if ENABLE_FEATURE_SETPRIV_CAPABILITIES
-struct caps {
+DEFINE_STRUCT_CAPS;
-	struct __user_cap_header_struct header;
-	cap_user_data_t data;
-	int u32s;
-};
-
-static void getcaps(struct caps *caps)
-{
-	static const uint8_t versions[] = {
-		_LINUX_CAPABILITY_U32S_3, /* = 2 (fits into byte) */
-		_LINUX_CAPABILITY_U32S_2, /* = 2 */
-		_LINUX_CAPABILITY_U32S_1, /* = 1 */
-	};
-	int i;
-
-	caps->header.pid = 0;
-	for (i = 0; i < ARRAY_SIZE(versions); i++) {
-		caps->header.version = versions[i];
-		if (capget(&caps->header, NULL) == 0)
-			goto got_it;
-	}
-	bb_simple_perror_msg_and_die("capget");
- got_it:
-
-	switch (caps->header.version) {
-		case _LINUX_CAPABILITY_VERSION_1:
-			caps->u32s = _LINUX_CAPABILITY_U32S_1;
-			break;
-		case _LINUX_CAPABILITY_VERSION_2:
-			caps->u32s = _LINUX_CAPABILITY_U32S_2;
-			break;
-		case _LINUX_CAPABILITY_VERSION_3:
-			caps->u32s = _LINUX_CAPABILITY_U32S_3;
-			break;
-		default:
-			bb_error_msg_and_die("unsupported capability version");
-	}
-
-	caps->data = xmalloc(sizeof(caps->data[0]) * caps->u32s);
-	if (capget(&caps->header, caps->data) < 0)
-		bb_simple_perror_msg_and_die("capget");
-}
 
 static unsigned parse_cap(const char *cap)
 {
@@ -195,7 +154,7 @@ static void set_inh_caps(char *capstring)
 
 		cap = parse_cap(capstring);
 		if (CAP_TO_INDEX(cap) >= caps.u32s)
-			bb_error_msg_and_die("invalid capability cap");
+			bb_error_msg_and_die("invalid capability '%s'", capstring);
 
 		if (capstring[0] == '+')
 			caps.data[CAP_TO_INDEX(cap)].inheritable |= CAP_TO_MASK(cap);
@@ -204,11 +163,8 @@ static void set_inh_caps(char *capstring)
 		capstring = strtok(NULL, ",");
 	}
 
-	if ((capset(&caps.header, caps.data)) < 0)
+	if (capset(&caps.header, caps.data) != 0)
 		bb_perror_msg_and_die("capset");
-
-	if (ENABLE_FEATURE_CLEAN_UP)
-		free(caps.data);
 }
 
 static void set_ambient_caps(char *string)
@@ -322,10 +278,9 @@ static int dump(void)
 	bb_putchar('\n');
 # endif
 
-	if (ENABLE_FEATURE_CLEAN_UP) {
+	if (ENABLE_FEATURE_CLEAN_UP)
-		IF_FEATURE_SETPRIV_CAPABILITIES(free(caps.data);)
 		free(gids);
-	}
+
 	return EXIT_SUCCESS;
 }
 #endif /* FEATURE_SETPRIV_DUMP */

util-linux/switch_root.c

@@ -26,20 +26,46 @@
 //config:	list of active mount points. That's why.
 
 //applet:IF_SWITCH_ROOT(APPLET(switch_root, BB_DIR_SBIN, BB_SUID_DROP))
+//                      APPLET_ODDNAME:name      main         location     suid_type     help
+//applet:IF_RUN_INIT(   APPLET_ODDNAME(run-init, switch_root, BB_DIR_SBIN, BB_SUID_DROP, run_init))
 
 //kbuild:lib-$(CONFIG_SWITCH_ROOT) += switch_root.o
+//kbuild:lib-$(CONFIG_RUN_INIT)    += switch_root.o
 
 //usage:#define switch_root_trivial_usage
-//usage:       "[-c /dev/console] NEW_ROOT NEW_INIT [ARGS]"
+//usage:       "[-c CONSOLE_DEV] NEW_ROOT NEW_INIT [ARGS]"
 //usage:#define switch_root_full_usage "\n\n"
 //usage:       "Free initramfs and switch to another root fs:\n"
 //usage:       "chroot to NEW_ROOT, delete all in /, move NEW_ROOT to /,\n"
 //usage:       "execute NEW_INIT. PID must be 1. NEW_ROOT must be a mountpoint.\n"
 //usage:     "\n	-c DEV	Reopen stdio to DEV after switch"
 
+//usage:#define run_init_trivial_usage
+//usage:       "[-d CAP,CAP...] [-c CONSOLE_DEV] NEW_ROOT NEW_INIT [ARGS]"
+//usage:#define run_init_full_usage "\n\n"
+//usage:       "Free initramfs and switch to another root fs:\n"
+//usage:       "chroot to NEW_ROOT, delete all in /, move NEW_ROOT to /,\n"
+//usage:       "execute NEW_INIT. PID must be 1. NEW_ROOT must be a mountpoint.\n"
+//usage:     "\n	-c DEV	Reopen stdio to DEV after switch"
+//usage:     "\n	-d CAPS	Drop capabilities"
+
 #include <sys/vfs.h>
 #include <sys/mount.h>
+#if ENABLE_RUN_INIT
+# include <sys/prctl.h>
+# include <linux/capability.h>
+// #include <sys/capability.h>
+// This header is in libcap, but the functions are in libc.
+// Comment in the header says this above capset/capget:
+/* system calls - look to libc for function to system call mapping */
+extern int capset(cap_user_header_t header, cap_user_data_t data);
+extern int capget(cap_user_header_t header, const cap_user_data_t data);
+// so for bbox, let's just repeat the declarations.
+// This way, libcap needs not be installed in build environment.
+#endif
+
 #include "libbb.h"
+
 // Make up for header deficiencies
 #ifndef RAMFS_MAGIC
 # define RAMFS_MAGIC ((unsigned)0x858458f6)
@@ -89,6 +115,84 @@ static void delete_contents(const char *directory, dev_t rootdev)
 	}
 }
 
+#if ENABLE_RUN_INIT
+DEFINE_STRUCT_CAPS;
+
+static void drop_capset(int cap_idx)
+{
+	struct caps caps;
+
+	/* Get the current capability mask */
+	getcaps(&caps);
+
+	/* Drop the bit */
+	caps.data[CAP_TO_INDEX(cap_idx)].inheritable &= ~CAP_TO_MASK(cap_idx);
+
+	/* And drop the capability. */
+	if (capset(&caps.header, caps.data) != 0)
+		bb_perror_msg_and_die("capset");
+}
+
+static void drop_bounding_set(int cap_idx)
+{
+	int ret;
+
+	ret = prctl(PR_CAPBSET_READ, cap_idx, 0, 0, 0);
+	if (ret < 0)
+		bb_perror_msg_and_die("prctl: %s", "PR_CAPBSET_READ");
+
+	if (ret == 1) {
+		ret = prctl(PR_CAPBSET_DROP, cap_idx, 0, 0, 0);
+		if (ret != 0)
+			bb_perror_msg_and_die("prctl: %s", "PR_CAPBSET_DROP");
+	}
+}
+
+static void drop_usermodehelper(const char *filename, int cap_idx)
+{
+	unsigned lo, hi;
+	char buf[sizeof(int)*3 * 2 + 8];
+	int fd;
+	int ret;
+
+	ret = open_read_close(filename, buf, sizeof(buf) - 1);
+	if (ret < 0)
+		return; /* assuming files do not exist */
+
+	buf[ret] = '\0';
+	ret = sscanf(buf, "%u %u", &lo, &hi);
+	if (ret != 2)
+		bb_perror_msg_and_die("can't parse file '%s'", filename);
+
+	if (cap_idx < 32)
+		lo &= ~(1 << cap_idx);
+	else
+		hi &= ~(1 << (cap_idx - 32));
+
+	fd = xopen(filename, O_WRONLY);
+	fdprintf(fd, "%u %u", lo, hi);
+	close(fd);
+}
+
+static void drop_capabilities(char *string)
+{
+	char *cap;
+
+	cap = strtok(string, ",");
+	while (cap) {
+		unsigned cap_idx;
+
+		cap_idx = cap_name_to_number(cap);
+		drop_usermodehelper("/proc/sys/kernel/usermodehelper/bset", cap_idx);
+		drop_usermodehelper("/proc/sys/kernel/usermodehelper/inheritable", cap_idx);
+		drop_bounding_set(cap_idx);
+		drop_capset(cap_idx);
+		bb_error_msg("dropped capability: %s", cap);
+		cap = strtok(NULL, ",");
+	}
+}
+#endif
+
 int switch_root_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int switch_root_main(int argc UNUSED_PARAM, char **argv)
 {
@@ -98,7 +202,25 @@ int switch_root_main(int argc UNUSED_PARAM, char **argv)
 	dev_t rootdev;
 
 	// Parse args (-c console). '+': stop at first non-option
-	getopt32(argv, "^+" "c:" "\0" "-2" /* minimum 2 args */, &console);
+	if (ENABLE_SWITCH_ROOT && (!ENABLE_RUN_INIT || applet_name[0] == 's')) {
+		getopt32(argv, "^+"
+			"c:"
+			"\0" "-2" /* minimum 2 args */,
+			&console
+		);
+	} else {
+#if ENABLE_RUN_INIT
+		char *cap_list = NULL;
+		getopt32(argv, "^+"
+			"c:d:"
+			"\0" "-2" /* minimum 2 args */,
+			&console,
+			&cap_list
+		);
+		if (cap_list)
+			drop_capabilities(cap_list);
+#endif
+	}
 	argv += optind;
 	newroot = *argv++;