merge branch 'cve-2019-5736'

  nsenter: clone /proc/self/exe to avoid exposing host binary to container

Fixes: CVE-2019-5736
LGTMs: @cyphar @crosbymichael

libcontainer/nsenter/cloned_binary.c

@@ -0,0 +1,268 @@
+/*
+ * Copyright (C) 2019 Aleksa Sarai <[email protected]>
+ * Copyright (C) 2019 SUSE LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#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>
+
+/* Use our own wrapper for memfd_create. */
+#if !defined(SYS_memfd_create) && defined(__NR_memfd_create)
+#  define SYS_memfd_create __NR_memfd_create
+#endif
+#ifdef SYS_memfd_create
+#  define HAVE_MEMFD_CREATE
+/* memfd_create(2) flags -- copied from <linux/memfd.h>. */
+#  ifndef MFD_CLOEXEC
+#    define MFD_CLOEXEC       0x0001U
+#    define MFD_ALLOW_SEALING 0x0002U
+#  endif
+int memfd_create(const char *name, unsigned int flags)
+{
+	return syscall(SYS_memfd_create, name, flags);
+}
+#endif
+
+/* 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 RUNC_SENDFILE_MAX 0x7FFFF000 /* sendfile(2) is limited to 2GB. */
+#ifdef HAVE_MEMFD_CREATE
+#  define RUNC_MEMFD_COMMENT "runc_cloned:/proc/self/exe"
+#  define RUNC_MEMFD_SEALS \
+	(F_SEAL_SEAL | F_SEAL_SHRINK | F_SEAL_GROW | F_SEAL_WRITE)
+#endif
+
+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, ret, is_cloned = 0;
+
+	fd = open("/proc/self/exe", O_RDONLY|O_CLOEXEC);
+	if (fd < 0)
+		return -ENOTRECOVERABLE;
+
+#ifdef HAVE_MEMFD_CREATE
+	ret = fcntl(fd, F_GET_SEALS);
+	is_cloned = (ret == RUNC_MEMFD_SEALS);
+#else
+	struct stat statbuf = {0};
+	ret = fstat(fd, &statbuf);
+	if (ret >= 0)
+		is_cloned = (statbuf.st_nlink == 0);
+#endif
+	close(fd);
+	return is_cloned;
+}
+
+/*
+ * 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;
+}
+
+static int clone_binary(void)
+{
+	int binfd, memfd;
+	ssize_t sent = 0;
+
+#ifdef HAVE_MEMFD_CREATE
+	memfd = memfd_create(RUNC_MEMFD_COMMENT, MFD_CLOEXEC | MFD_ALLOW_SEALING);
+#else
+	memfd = open("/tmp", O_TMPFILE | O_EXCL | O_RDWR | O_CLOEXEC, 0711);
+#endif
+	if (memfd < 0)
+		return -ENOTRECOVERABLE;
+
+	binfd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC);
+	if (binfd < 0)
+		goto error;
+
+	sent = sendfile(memfd, binfd, NULL, RUNC_SENDFILE_MAX);
+	close(binfd);
+	if (sent < 0)
+		goto error;
+
+#ifdef HAVE_MEMFD_CREATE
+	int err = fcntl(memfd, F_ADD_SEALS, RUNC_MEMFD_SEALS);
+	if (err < 0)
+		goto error;
+#else
+	/* Need to re-open "memfd" as read-only to avoid execve(2) giving -EXTBUSY. */
+	int newfd;
+	char *fdpath = NULL;
+
+	if (asprintf(&fdpath, "/proc/self/fd/%d", memfd) < 0)
+		goto error;
+	newfd = open(fdpath, O_RDONLY | O_CLOEXEC);
+	free(fdpath);
+	if (newfd < 0)
+		goto error;
+
+	close(memfd);
+	memfd = newfd;
+#endif
+	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;
+}

libcontainer/nsenter/nsexec.c

@@ -534,6 +534,9 @@ void join_namespaces(char *nslist)
 	free(namespaces);
 }
 
+/* Defined in cloned_binary.c. */
+extern int ensure_cloned_binary(void);
+
 void nsexec(void)
 {
 	int pipenum;
@@ -549,6 +552,14 @@ 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);