rexec: handle legacy kernels

src/lxc/Makefile.am

@@ -31,8 +31,10 @@ noinst_HEADERS = \
 	log.h \
 	lxc.h \
 	lxclock.h \
+	macro.h \
 	monitor.h \
 	namespace.h \
+	rexec.h \
 	start.h \
 	state.h \
 	utils.h \
@@ -122,6 +124,7 @@ liblxc_la_SOURCES = \
 	\
 	caps.c caps.h \
 	lxcseccomp.h \
+	macro.h \
 	mainloop.c mainloop.h \
 	af_unix.c af_unix.h \
 	\
@@ -159,7 +162,7 @@ liblxc_la_SOURCES += ../include/strlcat.c ../include/strlcat.h
 endif
 
 if ENFORCE_MEMFD_REXEC
-liblxc_la_SOURCES += rexec.c
+liblxc_la_SOURCES += rexec.c rexec.h
 endif
 
 AM_CFLAGS = -DLXCROOTFSMOUNT=\"$(LXCROOTFSMOUNT)\" \
@@ -269,7 +272,7 @@ AM_LDFLAGS += -Wl,-rpath -Wl,$(libdir)
 endif
 LDADD=liblxc.la @CAP_LIBS@ @SELINUX_LIBS@ @SECCOMP_LIBS@
 
-lxc_attach_SOURCES = tools/lxc_attach.c tools/arguments.c
+lxc_attach_SOURCES = tools/lxc_attach.c tools/arguments.c rexec.c rexec.h
 lxc_autostart_SOURCES = tools/lxc_autostart.c tools/arguments.c
 lxc_cgroup_SOURCES = tools/lxc_cgroup.c tools/arguments.c
 lxc_config_SOURCES = tools/lxc_config.c tools/arguments.c

src/lxc/commands.h

@@ -29,10 +29,9 @@
 #include <unistd.h>
 
 #include "lxccontainer.h"
+#include "macro.h"
 #include "state.h"
 
-#define LXC_CMD_DATA_MAX (MAXPATHLEN * 2)
-
 /* Length of abstract unix domain socket socket address. */
 #define LXC_AUDS_ADDR_LEN sizeof(((struct sockaddr_un *)0)->sun_path)
 

src/lxc/lsm/lsm.h

@@ -28,10 +28,9 @@ struct lxc_conf;
 
 #include <sys/types.h>
 
+#include "macro.h"
 #include "utils.h"
 
-#define LXC_LSMATTRLEN (5 + (LXC_NUMSTRLEN64) + 7 + 1)
-
 struct lsm_drv {
 	const char *name;
 

src/lxc/macro.h

+/* liblxcapi
+ *
+ * Copyright © 2018 Christian Brauner <christian.brauner@ubuntu.com>.
+ * Copyright © 2018 Canonical Ltd.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+#ifndef __LXC_MACRO_H
+#define __LXC_MACRO_H
+
+#include <asm/types.h>
+#include <limits.h>
+#include <linux/if_link.h>
+#include <linux/loop.h>
+#include <linux/netlink.h>
+#include <linux/rtnetlink.h>
+#include <linux/types.h>
+#include <stdint.h>
+#include <string.h>
+#include <sys/mount.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <unistd.h>
+
+#ifndef PATH_MAX
+#define PATH_MAX 4096
+#endif
+
+/* Define __S_ISTYPE if missing from the C library. */
+#ifndef __S_ISTYPE
+#define __S_ISTYPE(mode, mask) (((mode)&S_IFMT) == (mask))
+#endif
+
+/* capabilities */
+#ifndef CAP_SYS_ADMIN
+#define CAP_SYS_ADMIN 21
+#endif
+
+#ifndef CAP_SETFCAP
+#define CAP_SETFCAP 31
+#endif
+
+#ifndef CAP_MAC_OVERRIDE
+#define CAP_MAC_OVERRIDE 32
+#endif
+
+#ifndef CAP_MAC_ADMIN
+#define CAP_MAC_ADMIN 33
+#endif
+
+#ifndef CAP_SETUID
+#define CAP_SETUID 7
+#endif
+
+#ifndef CAP_SETGID
+#define CAP_SETGID 6
+#endif
+
+/* prctl */
+#ifndef PR_CAPBSET_READ
+#define PR_CAPBSET_READ 23
+#endif
+
+#ifndef PR_CAPBSET_DROP
+#define PR_CAPBSET_DROP 24
+#endif
+
+/* Control the ambient capability set */
+#ifndef PR_CAP_AMBIENT
+#define PR_CAP_AMBIENT 47
+#endif
+
+#ifndef PR_CAP_AMBIENT_IS_SET
+#define PR_CAP_AMBIENT_IS_SET 1
+#endif
+
+#ifndef PR_CAP_AMBIENT_RAISE
+#define PR_CAP_AMBIENT_RAISE 2
+#endif
+
+#ifndef PR_CAP_AMBIENT_LOWER
+#define PR_CAP_AMBIENT_LOWER 3
+#endif
+
+#ifndef PR_CAP_AMBIENT_CLEAR_ALL
+#define PR_CAP_AMBIENT_CLEAR_ALL 4
+#endif
+
+#ifndef PR_SET_NO_NEW_PRIVS
+#define PR_SET_NO_NEW_PRIVS 38
+#endif
+
+#ifndef PR_GET_NO_NEW_PRIVS
+#define PR_GET_NO_NEW_PRIVS 39
+#endif
+
+/* filesystem magic values */
+#ifndef CGROUP_SUPER_MAGIC
+#define CGROUP_SUPER_MAGIC 0x27e0eb
+#endif
+
+#ifndef CGROUP2_SUPER_MAGIC
+#define CGROUP2_SUPER_MAGIC 0x63677270
+#endif
+
+#ifndef NSFS_MAGIC
+#define NSFS_MAGIC 0x6e736673
+#endif
+
+/* current overlayfs */
+#ifndef OVERLAY_SUPER_MAGIC
+#define OVERLAY_SUPER_MAGIC 0x794c7630
+#endif
+
+/* legacy overlayfs */
+#ifndef OVERLAYFS_SUPER_MAGIC
+#define OVERLAYFS_SUPER_MAGIC 0x794c764f
+#endif
+
+/* Calculate the number of chars needed to represent a given integer as a C
+ * string. Include room for '-' to indicate negative numbers and the \0 byte.
+ * This is based on systemd.
+ */
+#define INTTYPE_TO_STRLEN(type)                   \
+	(2 + (sizeof(type) <= 1                   \
+		  ? 3                             \
+		  : sizeof(type) <= 2             \
+			? 5                       \
+			: sizeof(type) <= 4       \
+			      ? 10                \
+			      : sizeof(type) <= 8 \
+				    ? 20          \
+				    : sizeof(int[-2 * (sizeof(type) > 8)])))
+
+/* Useful macros */
+#define LXC_LINELEN 4096
+#define LXC_IDMAPLEN 4096
+#define LXC_MAX_BUFFER 4096
+#define LXC_NUMSTRLEN64 (INTTYPE_TO_STRLEN(intmax_t))
+
+/* /proc/       =    6
+ *                +
+ * <pid-as-str> =   INTTYPE_TO_STRLEN(pid_t)
+ *                +
+ * /fd/         =    4
+ *                +
+ * <fd-as-str>  =   INTTYPE_TO_STRLEN(int)
+ *                +
+ * \0           =    1
+ */
+#define LXC_PROC_PID_FD_LEN \
+	(6 + INTTYPE_TO_STRLEN(pid_t) + 4 + INTTYPE_TO_STRLEN(int) + 1)
+
+/* /proc/        = 6
+ *               +
+ * <pid-as-str>  = INTTYPE_TO_STRLEN(pid_t)
+ *               +
+ * /status       = 7
+ *               +
+ * \0            = 1
+ */
+#define LXC_PROC_STATUS_LEN (6 + INTTYPE_TO_STRLEN(pid_t) + 7 + 1)
+
+/* /proc/        = 6
+ *               +
+ * <pid-as-str>  = INTTYPE_TO_STRLEN(pid_t)
+ *               +
+ * /attr/        = 6
+ *               +
+ * /current      = 8
+ *               +
+ * \0            = 1
+ */
+#define LXC_LSMATTRLEN (6 + INTTYPE_TO_STRLEN(pid_t) + 6 + 8 + 1)
+
+#define LXC_CMD_DATA_MAX (PATH_MAX * 2)
+
+/* loop devices */
+#ifndef LO_FLAGS_AUTOCLEAR
+#define LO_FLAGS_AUTOCLEAR 4
+#endif
+
+#ifndef LOOP_CTL_GET_FREE
+#define LOOP_CTL_GET_FREE 0x4C82
+#endif
+
+/* memfd_create() */
+#ifndef MFD_CLOEXEC
+#define MFD_CLOEXEC 0x0001U
+#endif
+
+#ifndef MFD_ALLOW_SEALING
+#define MFD_ALLOW_SEALING 0x0002U
+#endif
+
+/**
+ * BUILD_BUG_ON - break compile if a condition is true.
+ * @condition: the condition which the compiler should know is false.
+ *
+ * If you have some code which relies on certain constants being equal, or
+ * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
+ * detect if someone changes it.
+ *
+ * The implementation uses gcc's reluctance to create a negative array, but
+ * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments
+ * to inline functions).  So as a fallback we use the optimizer; if it can't
+ * prove the condition is false, it will cause a link error on the undefined
+ * "__build_bug_on_failed".  This error message can be harder to track down
+ * though, hence the two different methods.
+ */
+#ifndef __OPTIMIZE__
+#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2 * !!(condition)]))
+#else
+extern int __build_bug_on_failed;
+#define BUILD_BUG_ON(condition)                              \
+	do {                                                 \
+		((void)sizeof(char[1 - 2 * !!(condition)])); \
+		if (condition)                               \
+			__build_bug_on_failed = 1;           \
+	} while (0)
+#endif
+
+#define lxc_iterate_parts(__iterator, __splitme, __separators)                  \
+	for (char *__p = NULL, *__it = strtok_r(__splitme, __separators, &__p); \
+	     (__iterator = __it);                                               \
+	     __iterator = __it = strtok_r(NULL, __separators, &__p))
+
+#define prctl_arg(x) ((unsigned long)x)
+
+/* networking */
+#ifndef NETLINK_DUMP_STRICT_CHK
+#define NETLINK_DUMP_STRICT_CHK 12
+#endif
+
+#ifndef SOL_NETLINK
+#define SOL_NETLINK 270
+#endif
+
+#ifndef IFLA_LINKMODE
+#define IFLA_LINKMODE 17
+#endif
+
+#ifndef IFLA_LINKINFO
+#define IFLA_LINKINFO 18
+#endif
+
+#ifndef IFLA_NET_NS_PID
+#define IFLA_NET_NS_PID 19
+#endif
+
+#ifndef IFLA_INFO_KIND
+#define IFLA_INFO_KIND 1
+#endif
+
+#ifndef IFLA_VLAN_ID
+#define IFLA_VLAN_ID 1
+#endif
+
+#ifndef IFLA_INFO_DATA
+#define IFLA_INFO_DATA 2
+#endif
+
+#ifndef VETH_INFO_PEER
+#define VETH_INFO_PEER 1
+#endif
+
+#ifndef IFLA_MACVLAN_MODE
+#define IFLA_MACVLAN_MODE 1
+#endif
+
+#ifndef IFLA_NEW_NETNSID
+#define IFLA_NEW_NETNSID 45
+#endif
+
+#ifdef IFLA_IF_NETNSID
+#ifndef IFLA_TARGET_NETNSID
+#define IFLA_TARGET_NETNSID = IFLA_IF_NETNSID
+#endif
+#else
+#define IFLA_IF_NETNSID 46
+#define IFLA_TARGET_NETNSID 46
+#endif
+
+#ifndef IFA_TARGET_NETNSID
+#define IFA_TARGET_NETNSID 10
+#endif
+
+#ifndef IFLA_STATS
+#define IFLA_STATS 7
+#endif
+
+#ifndef IFLA_STATS64
+#define IFLA_STATS64 23
+#endif
+
+#ifndef RTM_NEWNSID
+#define RTM_NEWNSID 88
+#endif
+
+#ifndef RTM_GETNSID
+#define RTM_GETNSID 90
+#endif
+
+#ifndef NLMSG_ERROR
+#define NLMSG_ERROR 0x2
+#endif
+
+#ifndef MACVLAN_MODE_PRIVATE
+#define MACVLAN_MODE_PRIVATE 1
+#endif
+
+#ifndef MACVLAN_MODE_VEPA
+#define MACVLAN_MODE_VEPA 2
+#endif
+
+#ifndef MACVLAN_MODE_BRIDGE
+#define MACVLAN_MODE_BRIDGE 4
+#endif
+
+#ifndef MACVLAN_MODE_PASSTHRU
+#define MACVLAN_MODE_PASSTHRU 8
+#endif
+
+/* Attributes of RTM_NEWNSID/RTM_GETNSID messages */
+enum {
+	__LXC_NETNSA_NONE,
+#define __LXC_NETNSA_NSID_NOT_ASSIGNED -1
+	__LXC_NETNSA_NSID,
+	__LXC_NETNSA_PID,
+	__LXC_NETNSA_FD,
+	__LXC_NETNSA_MAX,
+};
+
+/* Length of abstract unix domain socket socket address. */
+#define LXC_AUDS_ADDR_LEN sizeof(((struct sockaddr_un *)0)->sun_path)
+
+/* mount */
+#ifndef MS_PRIVATE
+#define MS_PRIVATE (1<<18)
+#endif
+
+#ifndef MS_SLAVE
+#define MS_SLAVE (1 << 19)
+#endif
+
+#ifndef MS_LAZYTIME
+#define MS_LAZYTIME (1<<25)
+#endif
+
+#ifndef MS_REC
+#define MS_REC 16384
+#endif
+
+/* open */
+#ifndef O_PATH
+#define O_PATH      010000000
+#endif
+
+#ifndef O_NOFOLLOW
+#define O_NOFOLLOW  00400000
+#endif
+
+/* sockets */
+#ifndef SOCK_CLOEXEC
+#define SOCK_CLOEXEC 02000000
+#endif
+
+/* pointer conversion macros */
+#define PTR_TO_INT(p) ((int)((intptr_t)(p)))
+#define INT_TO_PTR(u) ((void *)((intptr_t)(u)))
+
+#define PTR_TO_INTMAX(p) ((intmax_t)((intptr_t)(p)))
+#define INTMAX_TO_PTR(u) ((void *)((intptr_t)(u)))
+
+#define LXC_INVALID_UID ((uid_t)-1)
+#define LXC_INVALID_GID ((gid_t)-1)
+
+#define STRLITERALLEN(x) (sizeof(""x"") - 1)
+#define STRARRAYLEN(x) (sizeof(x) - 1)
+
+/* Maximum number of bytes sendfile() is able to send in one go. */
+#define LXC_SENDFILE_MAX 0x7ffff000
+
+#define move_ptr(ptr)                                 \
+	({                                            \
+		typeof(ptr) __internal_ptr__ = (ptr); \
+		(ptr) = NULL;                         \
+		__internal_ptr__;                     \
+	})
+
+#define move_fd(fd)                         \
+	({                                  \
+		int __internal_fd__ = (fd); \
+		(fd) = -EBADF;              \
+		__internal_fd__;            \
+	})
+
+#endif /* __LXC_MACRO_H */

src/lxc/memory_utils.h

+/* liblxcapi
+ *
+ * Copyright © 2019 Christian Brauner <christian.brauner@ubuntu.com>.
+ * Copyright © 2019 Canonical Ltd.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+#ifndef __LXC_MEMORY_UTILS_H
+#define __LXC_MEMORY_UTILS_H
+
+#include <dirent.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "macro.h"
+
+static inline void __auto_free__(void *p)
+{
+	free(*(void **)p);
+}
+
+static inline void __auto_fclose__(FILE **f)
+{
+	if (*f)
+		fclose(*f);
+}
+
+static inline void __auto_closedir__(DIR **d)
+{
+	if (*d)
+		closedir(*d);
+}
+
+#define close_prot_errno_disarm(fd) \
+	if (fd >= 0) {              \
+		int _e_ = errno;    \
+		close(fd);          \
+		errno = _e_;        \
+		fd = -EBADF;        \
+	}
+
+static inline void __auto_close__(int *fd)
+{
+	close_prot_errno_disarm(*fd);
+}
+
+#define __do_close_prot_errno __attribute__((__cleanup__(__auto_close__)))
+#define __do_free __attribute__((__cleanup__(__auto_free__)))
+#define __do_fclose __attribute__((__cleanup__(__auto_fclose__)))
+#define __do_closedir __attribute__((__cleanup__(__auto_closedir__)))
+
+#endif /* __LXC_MEMORY_UTILS_H */

src/lxc/rexec.c

@@ -3,18 +3,19 @@
  * Copyright © 2019 Christian Brauner <christian.brauner@ubuntu.com>.
  * Copyright © 2019 Canonical Ltd.
  *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2, as
- * published by the Free Software Foundation.
- *
- * This program is distributed in the hope that it will be useful,
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+
+ * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License along
- * with this program; if not, write to the Free Software Foundation, Inc.,
- * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
  */
 
 #ifndef _GNU_SOURCE
@@ -32,6 +33,8 @@
 #include <unistd.h>
 
 #include "config.h"
+#include "macro.h"
+#include "memory_utils.h"
 #include "utils.h"
 
 #if IS_BIONIC
@@ -101,51 +104,42 @@ on_error:
 	return NULL;
 }
 
-static int parse_exec_params(char ***argv, char ***envp)
+static int parse_argv(char ***argv)
 {
+	__do_free char *cmdline = NULL;
 	int ret;
-	char *cmdline = NULL, *env = NULL;
-	size_t cmdline_size, env_size;
+	size_t cmdline_size;
 
 	cmdline = file_to_buf("/proc/self/cmdline", &cmdline_size);
 	if (!cmdline)
-		goto on_error;
-
-	env = file_to_buf("/proc/self/environ", &env_size);
-	if (!env)
-		goto on_error;
+		return -1;
 
 	ret = push_vargs(cmdline, cmdline_size, argv);
 	if (ret <= 0)
-		goto on_error;
-
-	ret = push_vargs(env, env_size, envp);
-	if (ret <= 0)
-		goto on_error;
+		return -1;
 
+	move_ptr(cmdline);
 	return 0;
-
-on_error:
-	free(env);
-	free(cmdline);
-
-	return -1;
 }
 
 static int is_memfd(void)
 {
-	int fd, saved_errno, seals;
+	__do_close_prot_errno int fd = -EBADF;
+	int seals;
 
 	fd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC);
 	if (fd < 0)
 		return -ENOTRECOVERABLE;
 
 	seals = fcntl(fd, F_GET_SEALS);
-	saved_errno = errno;
-	close(fd);
-	errno = saved_errno;
-	if (seals < 0)
+	if (seals < 0) {
+		struct stat s = {0};
+
+		if (fstat(fd, &s) == 0)
+			return (s.st_nlink == 0);
+
 		return -EINVAL;
+	}
 
 	return seals == LXC_MEMFD_REXEC_SEALS;
 }
@@ -168,43 +162,122 @@ again:
 	return ret;
 }
 
+static int fd_to_fd(int from, int to)
+{
+	for (;;) {
+		uint8_t buf[PATH_MAX];
+		uint8_t *p = buf;
+		ssize_t bytes_to_write;
+		ssize_t bytes_read;
+
+		bytes_read = lxc_read_nointr(from, buf, sizeof buf);
+		if (bytes_read < 0)
+			return -1;
+		if (bytes_read == 0)
+			break;
+
+		bytes_to_write = (size_t)bytes_read;
+		do {
+			ssize_t bytes_written;
+
+			bytes_written = lxc_write_nointr(to, p, bytes_to_write);
+			if (bytes_written < 0)
+				return -1;
+
+			bytes_to_write -= bytes_written;
+			p += bytes_written;
+		} while (bytes_to_write > 0);
+	}
+
+	return 0;
+}
+
 static void lxc_rexec_as_memfd(char **argv, char **envp, const char *memfd_name)
 {
-	int saved_errno;
-	ssize_t bytes_sent;
-	int fd = -1, memfd = -1;
+	__do_close_prot_errno int execfd = -EBADF, fd = -EBADF, memfd = -EBADF,
+				  tmpfd = -EBADF;
+	int ret;
+	ssize_t bytes_sent = 0;
+	struct stat st = {0};
 
 	memfd = memfd_create(memfd_name, MFD_ALLOW_SEALING | MFD_CLOEXEC);
-	if (memfd < 0)
-		return;
+	if (memfd < 0) {
+		char template[PATH_MAX];
+
+		ret = snprintf(template, sizeof(template),
+			       P_tmpdir "/.%s_XXXXXX", memfd_name);
+		if (ret < 0 || (size_t)ret >= sizeof(template))
+			return;
+
+		tmpfd = lxc_make_tmpfile(template, true);
+		if (tmpfd < 0)
+			return;
+
+		ret = fchmod(tmpfd, 0700);
+		if (ret)
+			return;
+	}
 
 	fd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC);
 	if (fd < 0)
-		goto on_error;
+		return;
 
 	/* sendfile() handles up to 2GB. */
-	bytes_sent = lxc_sendfile_nointr(memfd, fd, NULL, LXC_SENDFILE_MAX);
-	saved_errno = errno;
-	close(fd);
-	errno = saved_errno;
-	if (bytes_sent < 0)
-		goto on_error;
+	ret = fstat(fd, &st);
+	if (ret)
+		return;
 
-	if (fcntl(memfd, F_ADD_SEALS, LXC_MEMFD_REXEC_SEALS))
-		goto on_error;
+	while (bytes_sent < st.st_size) {
+		ssize_t sent;
+
+		sent = lxc_sendfile_nointr(memfd >= 0 ? memfd : tmpfd, fd, NULL,
+					   st.st_size - bytes_sent);
+		if (sent < 0) {
+			/* Fallback to shoveling data between kernel- and
+			 * userspace.
+			 */
+			lseek(fd, 0, SEEK_SET);
+			if (fd_to_fd(fd, memfd >= 0 ? memfd : tmpfd))
+				break;
+
+			return;
+		}
+		bytes_sent += sent;
+	}
+	close_prot_errno_disarm(fd);
 
-	fexecve(memfd, argv, envp);
+	if (memfd >= 0) {
+		if (fcntl(memfd, F_ADD_SEALS, LXC_MEMFD_REXEC_SEALS))
+			return;
 
-on_error:
-	saved_errno = errno;
-	close(memfd);
-	errno = saved_errno;
+		execfd = memfd;
+	} else {
+		char procfd[LXC_PROC_PID_FD_LEN];
+
+		ret = snprintf(procfd, sizeof(procfd), "/proc/self/fd/%d", tmpfd);
+		if (ret < 0 || (size_t)ret >= sizeof(procfd))
+			return;
+
+		execfd = open(procfd, O_PATH | O_CLOEXEC);
+		close_prot_errno_disarm(tmpfd);
+
+	}
+	if (execfd < 0)
+		return;
+
+	fexecve(execfd, argv, envp);
 }
 
-static int lxc_rexec(const char *memfd_name)
+/*
+ * Get cheap access to the environment. This must be declared by the user as
+ * mandated by POSIX. The definition is located in unistd.h.
+ */
+extern char **environ;
+
+int lxc_rexec(const char *memfd_name)
 {
 	int ret;
-	char **argv = NULL, **envp = NULL;
+	char **argv = NULL;
 
 	ret = is_memfd();
 	if (ret < 0 && ret == -ENOTRECOVERABLE) {
@@ -216,7 +289,7 @@ static int lxc_rexec(const char *memfd_name)
 		return 0;
 	}
 
-	ret = parse_exec_params(&argv, &envp);
+	ret = parse_argv(&argv);
 	if (ret < 0) {
 		fprintf(stderr,
 			"%s - Failed to parse command line parameters\n",
@@ -224,7 +297,7 @@ static int lxc_rexec(const char *memfd_name)
 		return -1;
 	}
 
-	lxc_rexec_as_memfd(argv, envp, memfd_name);
+	lxc_rexec_as_memfd(argv, environ, memfd_name);
 	fprintf(stderr, "%s - Failed to rexec as memfd\n", strerror(errno));
 	return -1;
 }
@@ -238,7 +311,7 @@ static int lxc_rexec(const char *memfd_name)
  */
 __attribute__((constructor)) static void liblxc_rexec(void)
 {
-	if (lxc_rexec("liblxc")) {
+	if (getenv("LXC_MEMFD_REXEC") && lxc_rexec("liblxc")) {
 		fprintf(stderr, "Failed to re-execute liblxc via memory file descriptor\n");
 		_exit(EXIT_FAILURE);
 	}

src/lxc/rexec.h

+/* liblxcapi
+ *
+ * Copyright © 2019 Christian Brauner <christian.brauner@ubuntu.com>.
+ * Copyright © 2019 Canonical Ltd.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+#ifndef __LXC_REXEC_H
+#define __LXC_REXEC_H
+
+extern int lxc_rexec(const char *memfd_name);
+
+#endif /* __LXC_REXEC_H */

src/lxc/tools/lxc_attach.c

@@ -21,8 +21,9 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-#include "config.h"
-
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE 1
+#endif
 #include <errno.h>
 #include <fcntl.h>
 #include <stdio.h>
@@ -35,15 +36,17 @@
 
 #include <lxc/lxccontainer.h>
 
-#include "attach.h"
 #include "arguments.h"
+#include "attach.h"
 #include "caps.h"
 #include "conf.h"
+#include "config.h"
 #include "confile.h"
 #include "console.h"
-#include "log.h"
 #include "list.h"
+#include "log.h"
 #include "mainloop.h"
+#include "rexec.h"
 #include "utils.h"
 
 #if HAVE_PTY_H
@@ -52,6 +55,23 @@
 #include <../include/openpty.h>
 #endif
 
+/**
+ * This function will copy any binary that calls liblxc into a memory file and
+ * will use the memfd to rexecute the binary. This is done to prevent attacks
+ * through the /proc/self/exe symlink to corrupt the host binary when host and
+ * container are in the same user namespace or have set up an identity id
+ * mapping: CVE-2019-5736.
+ */
+#ifdef ENFORCE_MEMFD_REXEC
+__attribute__((constructor)) static void lxc_attach_rexec(void)
+{
+	if (!getenv("LXC_MEMFD_REXEC") && lxc_rexec("lxc-attach")) {
+		fprintf(stderr, "Failed to re-execute lxc-attach via memory file descriptor\n");
+		_exit(EXIT_FAILURE);
+	}
+}
+#endif
+
 static const struct option my_longopts[] = {
 	{"elevated-privileges", optional_argument, 0, 'e'},
 	{"arch", required_argument, 0, 'a'},

src/lxc/utils.c

@@ -47,6 +47,7 @@
 
 #include "log.h"
 #include "lxclock.h"
+#include "memory_utils.h"
 #include "namespace.h"
 #include "parse.h"
 #include "utils.h"
@@ -2396,7 +2397,8 @@ bool lxc_nic_exists(char *nic)
 
 int lxc_make_tmpfile(char *template, bool rm)
 {
-	int fd, ret;
+	__do_close_prot_errno int fd = -EBADF;
+	int ret;
 	mode_t msk;
 
 	msk = umask(0022);
@@ -2405,16 +2407,17 @@ int lxc_make_tmpfile(char *template, bool rm)
 	if (fd < 0)
 		return -1;
 
+	if (lxc_set_cloexec(fd))
+		return -1;
+
 	if (!rm)
-		return fd;
+		return move_fd(fd);
 
 	ret = unlink(template);
-	if (ret < 0) {
-		close(fd);
+	if (ret < 0)
 		return -1;
-	}
 
-	return fd;
+	return move_fd(fd);
 }
 
 int parse_byte_size_string(const char *s, int64_t *converted)

src/lxc/utils.h

@@ -45,76 +45,7 @@
 #endif
 
 #include "initutils.h"
-
-/* Define __S_ISTYPE if missing from the C library. */
-#ifndef __S_ISTYPE
-#define __S_ISTYPE(mode, mask) (((mode)&S_IFMT) == (mask))
-#endif
-
-#if HAVE_LIBCAP
-#ifndef CAP_SETFCAP
-#define CAP_SETFCAP 31
-#endif
-
-#ifndef CAP_MAC_OVERRIDE
-#define CAP_MAC_OVERRIDE 32
-#endif
-
-#ifndef CAP_MAC_ADMIN
-#define CAP_MAC_ADMIN 33
-#endif
-#endif
-
-#ifndef PR_CAPBSET_DROP
-#define PR_CAPBSET_DROP 24
-#endif
-
-#ifndef LO_FLAGS_AUTOCLEAR
-#define LO_FLAGS_AUTOCLEAR 4
-#endif
-
-#ifndef CAP_SETUID
-#define CAP_SETUID 7
-#endif
-
-#ifndef CAP_SETGID
-#define CAP_SETGID 6
-#endif
-
-/* needed for cgroup automount checks, regardless of whether we
- * have included linux/capability.h or not */
-#ifndef CAP_SYS_ADMIN
-#define CAP_SYS_ADMIN 21
-#endif
-
-#ifndef CGROUP_SUPER_MAGIC
-#define CGROUP_SUPER_MAGIC 0x27e0eb
-#endif
-
-#ifndef CGROUP2_SUPER_MAGIC
-#define CGROUP2_SUPER_MAGIC 0x63677270
-#endif
-
-#ifndef NSFS_MAGIC
-#define NSFS_MAGIC 0x6e736673
-#endif
-
-/* Useful macros */
-/* Maximum number for 64 bit integer is a string with 21 digits: 2^64 - 1 = 21 */
-#define LXC_NUMSTRLEN64 21
-#define LXC_LINELEN 4096
-#define LXC_IDMAPLEN 4096
-/* /proc/       =    6
- *                +
- * <pid-as-str> =   LXC_NUMSTRLEN64
- *                +
- * /fd/         =    4
- *                +
- * <fd-as-str>  =   LXC_NUMSTRLEN64
- *                +
- * \0           =    1
- */
-#define LXC_PROC_PID_FD_LEN (6 + LXC_NUMSTRLEN64 + 4 + LXC_NUMSTRLEN64 + 1)
+#include "macro.h"
 
 /* returns 1 on success, 0 if there were any failures */
 extern int lxc_rmdir_onedev(char *path, const char *exclude);
@@ -376,32 +307,6 @@ extern struct lxc_popen_FILE *lxc_popen(const char *command);
  */
 extern int lxc_pclose(struct lxc_popen_FILE *fp);
 
-/**
- * BUILD_BUG_ON - break compile if a condition is true.
- * @condition: the condition which the compiler should know is false.
- *
- * If you have some code which relies on certain constants being equal, or
- * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
- * detect if someone changes it.
- *
- * The implementation uses gcc's reluctance to create a negative array, but
- * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments
- * to inline functions).  So as a fallback we use the optimizer; if it can't
- * prove the condition is false, it will cause a link error on the undefined
- * "__build_bug_on_failed".  This error message can be harder to track down
- * though, hence the two different methods.
- */
-#ifndef __OPTIMIZE__
-#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
-#else
-extern int __build_bug_on_failed;
-#define BUILD_BUG_ON(condition)					\
-	do {							\
-		((void)sizeof(char[1 - 2*!!(condition)]));	\
-		if (condition) __build_bug_on_failed = 1;	\
-	} while(0)
-#endif
-
 /*
  * wait on a child we forked
  */