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merge branch 'cve-2019-5736'
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  nsenter: clone /proc/self/exe to avoid exposing host binary to container

Fixes: CVE-2019-5736
LGTMs: @cyphar @crosbymichael
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@cyphar
cyphar committed Feb 8, 2019
2 parents dd023c4 + 0a8e411 commit 6635b4f
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268 changes: 268 additions & 0 deletions libcontainer/nsenter/cloned_binary.c
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@@ -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;
}
11 changes: 11 additions & 0 deletions libcontainer/nsenter/nsexec.c
View file
Expand Up @@ -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;
Expand All @@ -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);

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