From bf6405284aa3870a39b402309003633a1c230ed9 Mon Sep 17 00:00:00 2001

From: Aleksa Sarai <asarai@suse.de>

Date: Wed, 9 Jan 2019 13:40:01 +1100

Subject: [PATCH 1/1] nsenter: clone /proc/self/exe to avoid exposing host

 binary to container

There are quite a few circumstances where /proc/self/exe pointing to a

pretty important container binary is a _bad_ thing, so to avoid this we

have to make a copy (preferably doing self-clean-up and not being

writeable).

As a hotfix we require memfd_create(2), but we can always extend this to

use a scratch MNT_DETACH overlayfs or tmpfs. The main downside to this

approach is no page-cache sharing for the runc binary (which overlayfs

would give us) but this is far less complicated.

This is only done during nsenter so that it happens transparently to the

Go code, and any libcontainer users benefit from it. This also makes

ExtraFiles and --preserve-fds handling trivial (because we don't need to

worry about it).

Fixes: CVE-2019-5736

Co-developed-by: Christian Brauner <christian.brauner@ubuntu.com>

Signed-off-by: Aleksa Sarai <asarai@suse.de>

Signed-off-by: Mrunal Patel <mrunalp@gmail.com>

---

 libcontainer/nsenter/cloned_binary.c | 221 +++++++++++++++++++++++++++

 libcontainer/nsenter/nsexec.c        |  11 ++

 2 files changed, 232 insertions(+)

 create mode 100644 libcontainer/nsenter/cloned_binary.c

diff --git a/libcontainer/nsenter/cloned_binary.c b/libcontainer/nsenter/cloned_binary.c

new file mode 100644

index 00000000..d9f6093a

--- /dev/null

+++ b/libcontainer/nsenter/cloned_binary.c

@@ -0,0 +1,221 @@

+#define _GNU_SOURCE

+#include <unistd.h>

+#include <stdio.h>

+#include <stdlib.h>

+#include <stdbool.h>

+#include <string.h>

+#include <limits.h>

+#include <fcntl.h>

+#include <errno.h>

+

+#include <sys/types.h>

+#include <sys/stat.h>

+#include <sys/vfs.h>

+#include <sys/mman.h>

+#include <sys/sendfile.h>

+#include <sys/syscall.h>

+

+#include <linux/magic.h>

+#include <linux/memfd.h>

+

+/* Use our own wrapper for memfd_create. */

+#if !defined(SYS_memfd_create) && defined(__NR_memfd_create)

+#  define SYS_memfd_create __NR_memfd_create

+#endif

+#ifndef SYS_memfd_create

+#  error "memfd_create(2) syscall not supported by this glibc version"

+#endif

+int memfd_create(const char *name, unsigned int flags)

+{

+	return syscall(SYS_memfd_create, name, flags);

+}

+

+/* This comes directly from <linux/fcntl.h>. */

+#ifndef F_LINUX_SPECIFIC_BASE

+#  define F_LINUX_SPECIFIC_BASE 1024

+#endif

+#ifndef F_ADD_SEALS

+#  define F_ADD_SEALS (F_LINUX_SPECIFIC_BASE + 9)

+#  define F_GET_SEALS (F_LINUX_SPECIFIC_BASE + 10)

+#endif

+#ifndef F_SEAL_SEAL

+#  define F_SEAL_SEAL   0x0001	/* prevent further seals from being set */

+#  define F_SEAL_SHRINK 0x0002	/* prevent file from shrinking */

+#  define F_SEAL_GROW   0x0004	/* prevent file from growing */

+#  define F_SEAL_WRITE  0x0008	/* prevent writes */

+#endif

+

+

+#define OUR_MEMFD_COMMENT "runc_cloned:/proc/self/exe"

+#define OUR_MEMFD_SEALS \

+	(F_SEAL_SEAL | F_SEAL_SHRINK | F_SEAL_GROW | F_SEAL_WRITE)

+

+static void *must_realloc(void *ptr, size_t size)

+{

+	void *old = ptr;

+	do {

+		ptr = realloc(old, size);

+	} while(!ptr);

+	return ptr;

+}

+

+/*

+ * Verify whether we are currently in a self-cloned program (namely, is

+ * /proc/self/exe a memfd). F_GET_SEALS will only succeed for memfds (or rather

+ * for shmem files), and we want to be sure it's actually sealed.

+ */

+static int is_self_cloned(void)

+{

+	int fd, seals;

+

+	fd = open("/proc/self/exe", O_RDONLY|O_CLOEXEC);

+	if (fd < 0)

+		return -ENOTRECOVERABLE;

+

+	seals = fcntl(fd, F_GET_SEALS);

+	close(fd);

+	return seals == OUR_MEMFD_SEALS;

+}

+

+/*

+ * Basic wrapper around mmap(2) that gives you the file length so you can

+ * safely treat it as an ordinary buffer. Only gives you read access.

+ */

+static char *read_file(char *path, size_t *length)

+{

+	int fd;

+	char buf[4096], *copy = NULL;

+

+	if (!length)

+		return NULL;

+

+	fd = open(path, O_RDONLY | O_CLOEXEC);

+	if (fd < 0)

+		return NULL;

+

+	*length = 0;

+	for (;;) {

+		int n;

+

+		n = read(fd, buf, sizeof(buf));

+		if (n < 0)

+			goto error;

+		if (!n)

+			break;

+

+		copy = must_realloc(copy, (*length + n) * sizeof(*copy));

+		memcpy(copy + *length, buf, n);

+		*length += n;

+	}

+	close(fd);

+	return copy;

+

+error:

+	close(fd);

+	free(copy);

+	return NULL;

+}

+

+/*

+ * A poor-man's version of "xargs -0". Basically parses a given block of

+ * NUL-delimited data, within the given length and adds a pointer to each entry

+ * to the array of pointers.

+ */

+static int parse_xargs(char *data, int data_length, char ***output)

+{

+	int num = 0;

+	char *cur = data;

+

+	if (!data || *output != NULL)

+		return -1;

+

+	while (cur < data + data_length) {

+		num++;

+		*output = must_realloc(*output, (num + 1) * sizeof(**output));

+		(*output)[num - 1] = cur;

+		cur += strlen(cur) + 1;

+	}

+	(*output)[num] = NULL;

+	return num;

+}

+

+/*

+ * "Parse" out argv and envp from /proc/self/cmdline and /proc/self/environ.

+ * This is necessary because we are running in a context where we don't have a

+ * main() that we can just get the arguments from.

+ */

+static int fetchve(char ***argv, char ***envp)

+{

+	char *cmdline = NULL, *environ = NULL;

+	size_t cmdline_size, environ_size;

+

+	cmdline = read_file("/proc/self/cmdline", &cmdline_size);

+	if (!cmdline)

+		goto error;

+	environ = read_file("/proc/self/environ", &environ_size);

+	if (!environ)

+		goto error;

+

+	if (parse_xargs(cmdline, cmdline_size, argv) <= 0)

+		goto error;

+	if (parse_xargs(environ, environ_size, envp) <= 0)

+		goto error;

+

+	return 0;

+

+error:

+	free(environ);

+	free(cmdline);

+	return -EINVAL;

+}

+

+#define SENDFILE_MAX 0x7FFFF000 /* sendfile(2) is limited to 2GB. */

+static int clone_binary(void)

+{

+	int binfd, memfd, err;

+	ssize_t sent = 0;

+

+	memfd = memfd_create(OUR_MEMFD_COMMENT, MFD_CLOEXEC | MFD_ALLOW_SEALING);

+	if (memfd < 0)

+		return -ENOTRECOVERABLE;

+

+	binfd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC);

+	if (binfd < 0)

+		goto error;

+

+	sent = sendfile(memfd, binfd, NULL, SENDFILE_MAX);

+	close(binfd);

+	if (sent < 0)

+		goto error;

+

+	err = fcntl(memfd, F_ADD_SEALS, OUR_MEMFD_SEALS);

+	if (err < 0)

+		goto error;

+

+	return memfd;

+

+error:

+	close(memfd);

+	return -EIO;

+}

+

+int ensure_cloned_binary(void)

+{

+	int execfd;

+	char **argv = NULL, **envp = NULL;

+

+	/* Check that we're not self-cloned, and if we are then bail. */

+	int cloned = is_self_cloned();

+	if (cloned > 0 || cloned == -ENOTRECOVERABLE)

+		return cloned;

+

+	if (fetchve(&argv, &envp) < 0)

+		return -EINVAL;

+

+	execfd = clone_binary();

+	if (execfd < 0)

+		return -EIO;

+

+	fexecve(execfd, argv, envp);

+	return -ENOEXEC;

+}

diff --git a/libcontainer/nsenter/nsexec.c b/libcontainer/nsenter/nsexec.c

index cb224314..784fd9b0 100644

--- a/libcontainer/nsenter/nsexec.c

+++ b/libcontainer/nsenter/nsexec.c

@@ -528,6 +528,9 @@ void join_namespaces(char *nslist)

 	free(namespaces);

 }

+/* Defined in cloned_binary.c. */

+int ensure_cloned_binary(void);

+

 void nsexec(void)

 {

 	int pipenum;

@@ -543,6 +546,14 @@ void nsexec(void)

 	if (pipenum == -1)

 		return;

+	/*

+	 * We need to re-exec if we are not in a cloned binary. This is necessary

+	 * to ensure that containers won't be able to access the host binary

+	 * through /proc/self/exe. See CVE-2019-5736.

+	 */

+	if (ensure_cloned_binary() < 0)

+		bail("could not ensure we are a cloned binary");

+

 	/* Parse all of the netlink configuration. */

 	nl_parse(pipenum, &config);

-- 

2.20.1