334 lines
8.7 KiB
Diff
334 lines
8.7 KiB
Diff
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>
|
|
---
|
|
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 000000000..c8a42c23f
|
|
--- /dev/null
|
|
+++ 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 28269dfc0..7750af35e 100644
|
|
--- libcontainer/nsenter/nsexec.c
|
|
+++ 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);
|
|
|