+++ /dev/null
-From 0a8e4117e7f715d5fbeef398405813ce8e88558b Mon Sep 17 00:00:00 2001
-From: Aleksa Sarai <asarai@suse.de>
-Date: Wed, 9 Jan 2019 13:40:01 +1100
-Subject: [PATCH] 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).
-
-We require memfd_create(2) -- though there is an O_TMPFILE fallback --
-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: Peter Korsgaard <peter@korsgaard.com>
----
- libcontainer/nsenter/cloned_binary.c | 268 +++++++++++++++++++++++++++++++++++
- libcontainer/nsenter/nsexec.c | 11 ++
- 2 files changed, 279 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..c8a42c23
---- /dev/null
-+++ b/libcontainer/nsenter/cloned_binary.c
-@@ -0,0 +1,268 @@
-+/*
-+ * Copyright (C) 2019 Aleksa Sarai <cyphar@cyphar.com>
-+ * 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;
-+}
-diff --git a/libcontainer/nsenter/nsexec.c b/libcontainer/nsenter/nsexec.c
-index 28269dfc..7750af35 100644
---- a/libcontainer/nsenter/nsexec.c
-+++ b/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);
-
---
-2.11.0
-
projects / buildroot.git / commitdiff