/*
* 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/statfs.h>
#include <sys/vfs.h>
#include <sys/mman.h>
#include <sys/mount.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
/* 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)
{
#ifdef SYS_memfd_create
return syscall(SYS_memfd_create, name, flags);
#else
errno = ENOSYS;
return -1;
#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 CLONED_BINARY_ENV "_LIBCONTAINER_CLONED_BINARY"
#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)
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;
struct stat statbuf = {};
struct statfs fsbuf = {};
fd = open("/proc/self/exe", O_RDONLY|O_CLOEXEC);
if (fd < 0)
return -ENOTRECOVERABLE;
/*
* Is the binary a fully-sealed memfd? We don't need CLONED_BINARY_ENV for
* this, because you cannot write to a sealed memfd no matter what (so
* sharing it isn't a bad thing -- and an admin could bind-mount a sealed
* memfd to /usr/bin/runc to allow re-use).
*/
ret = fcntl(fd, F_GET_SEALS);
if (ret >= 0) {
is_cloned = (ret == RUNC_MEMFD_SEALS);
goto out;
}
/*
* All other forms require CLONED_BINARY_ENV, since they are potentially
* writeable (or we can't tell if they're fully safe) and thus we must
* check the environment as an extra layer of defence.
*/
if (!getenv(CLONED_BINARY_ENV)) {
is_cloned = false;
goto out;
}
/*
* Is the binary on a read-only filesystem? We can't detect bind-mounts in
* particular (in-kernel they are identical to regular mounts) but we can
* at least be sure that it's read-only. In addition, to make sure that
* it's *our* bind-mount we check CLONED_BINARY_ENV.
*/
if (fstatfs(fd, &fsbuf) >= 0)
is_cloned |= (fsbuf.f_flags & MS_RDONLY);
/*
* Okay, we're a tmpfile -- or we're currently running on RHEL <=7.6
* which appears to have a borked backport of F_GET_SEALS. Either way,
* having a file which has no hardlinks indicates that we aren't using
* a host-side "runc" binary and this is something that a container
* cannot fake (because unlinking requires being able to resolve the
* path that you want to unlink).
*/
if (fstat(fd, &statbuf) >= 0)
is_cloned |= (statbuf.st_nlink == 0);
out:
close(fd);
return is_cloned;
}
/* Read a given file into a new buffer, and providing the length. */
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 (;;) {
ssize_t 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 from /proc/self/cmdline.
* 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 *cmdline = NULL;
size_t cmdline_size;
cmdline = read_file("/proc/self/cmdline", &cmdline_size);
if (!cmdline)
goto error;
if (parse_xargs(cmdline, cmdline_size, argv) <= 0)
goto error;
return 0;
error:
free(cmdline);
return -EINVAL;
}
enum {
EFD_NONE = 0,
EFD_MEMFD,
EFD_FILE,
};
/*
* This comes from <linux/fcntl.h>. We can't hard-code __O_TMPFILE because it
* changes depending on the architecture. If we don't have O_TMPFILE we always
* have the mkostemp(3) fallback.
*/
#ifndef O_TMPFILE
# if defined(__O_TMPFILE) && defined(O_DIRECTORY)
# define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
# endif
#endif
static int make_execfd(int *fdtype)
{
int fd = -1;
char template[PATH_MAX] = {0};
char *prefix = getenv("_LIBCONTAINER_STATEDIR");
if (!prefix || *prefix != '/')
prefix = "/tmp";
if (snprintf(template, sizeof(template), "%s/runc.XXXXXX", prefix) < 0)
return -1;
/*
* Now try memfd, it's much nicer than actually creating a file in STATEDIR
* since it's easily detected thanks to sealing and also doesn't require
* assumptions about STATEDIR.
*/
*fdtype = EFD_MEMFD;
fd = memfd_create(RUNC_MEMFD_COMMENT, MFD_CLOEXEC | MFD_ALLOW_SEALING);
if (fd >= 0)
return fd;
if (errno != ENOSYS && errno != EINVAL)
goto error;
#ifdef O_TMPFILE
/*
* Try O_TMPFILE to avoid races where someone might snatch our file. Note
* that O_EXCL isn't actually a security measure here (since you can just
* fd re-open it and clear O_EXCL).
*/
*fdtype = EFD_FILE;
fd = open(prefix, O_TMPFILE | O_EXCL | O_RDWR | O_CLOEXEC, 0700);
if (fd >= 0) {
struct stat statbuf = {};
bool working_otmpfile = false;
/*
* open(2) ignores unknown O_* flags -- yeah, I was surprised when I
* found this out too. As a result we can't check for EINVAL. However,
* if we get nlink != 0 (or EISDIR) then we know that this kernel
* doesn't support O_TMPFILE.
*/
if (fstat(fd, &statbuf) >= 0)
working_otmpfile = (statbuf.st_nlink == 0);
if (working_otmpfile)
return fd;
/* Pretend that we got EISDIR since O_TMPFILE failed. */
close(fd);
errno = EISDIR;
}
if (errno != EISDIR)
goto error;
#endif /* defined(O_TMPFILE) */
/*
* Our final option is to create a temporary file the old-school way, and
* then unlink it so that nothing else sees it by accident.
*/
*fdtype = EFD_FILE;
fd = mkostemp(template, O_CLOEXEC);
if (fd >= 0) {
if (unlink(template) >= 0)
return fd;
close(fd);
}
error:
*fdtype = EFD_NONE;
return -1;
}
static int seal_execfd(int *fd, int fdtype)
{
switch (fdtype) {
case EFD_MEMFD:
return fcntl(*fd, F_ADD_SEALS, RUNC_MEMFD_SEALS);
case EFD_FILE: {
/* Need to re-open our pseudo-memfd as an O_PATH to avoid execve(2) giving -ETXTBSY. */
int newfd;
char fdpath[PATH_MAX] = {0};
if (fchmod(*fd, 0100) < 0)
return -1;
if (snprintf(fdpath, sizeof(fdpath), "/proc/self/fd/%d", *fd) < 0)
return -1;
newfd = open(fdpath, O_PATH | O_CLOEXEC);
if (newfd < 0)
return -1;
close(*fd);
*fd = newfd;
return 0;
}
default:
break;
}
return -1;
}
static int try_bindfd(void)
{
int fd, ret = -1;
char template[PATH_MAX] = {0};
char *prefix = getenv("_LIBCONTAINER_STATEDIR");
if (!prefix || *prefix != '/')
prefix = "/tmp";
if (snprintf(template, sizeof(template), "%s/runc.XXXXXX", prefix) < 0)
return ret;
/*
* We need somewhere to mount it, mounting anything over /proc/self is a
* BAD idea on the host -- even if we do it temporarily.
*/
fd = mkstemp(template);
if (fd < 0)
return ret;
close(fd);
/*
* For obvious reasons this won't work in rootless mode because we haven't
* created a userns+mntns -- but getting that to work will be a bit
* complicated and it's only worth doing if someone actually needs it.
*/
ret = -EPERM;
if (mount("/proc/self/exe", template, "", MS_BIND, "") < 0)
goto out;
if (mount("", template, "", MS_REMOUNT | MS_BIND | MS_RDONLY, "") < 0)
goto out_umount;
/* Get read-only handle that we're sure can't be made read-write. */
ret = open(template, O_PATH | O_CLOEXEC);
out_umount:
/*
* Make sure the MNT_DETACH works, otherwise we could get remounted
* read-write and that would be quite bad (the fd would be made read-write
* too, invalidating the protection).
*/
if (umount2(template, MNT_DETACH) < 0) {
if (ret >= 0)
close(ret);
ret = -ENOTRECOVERABLE;
}
out:
/*
* We don't care about unlink errors, the worst that happens is that
* there's an empty file left around in STATEDIR.
*/
unlink(template);
return ret;
}
static ssize_t fd_to_fd(int outfd, int infd)
{
ssize_t total = 0;
char buffer[4096];
for (;;) {
ssize_t nread, nwritten = 0;
nread = read(infd, buffer, sizeof(buffer));
if (nread < 0)
return -1;
if (!nread)
break;
do {
ssize_t n = write(outfd, buffer + nwritten, nread - nwritten);
if (n < 0)
return -1;
nwritten += n;
} while(nwritten < nread);
total += nwritten;
}
return total;
}
static int clone_binary(void)
{
int binfd, execfd;
struct stat statbuf = {};
size_t sent = 0;
int fdtype = EFD_NONE;
/*
* Before we resort to copying, let's try creating an ro-binfd in one shot
* by getting a handle for a read-only bind-mount of the execfd.
*/
execfd = try_bindfd();
if (execfd >= 0)
return execfd;
/*
* Dammit, that didn't work -- time to copy the binary to a safe place we
* can seal the contents.
*/
execfd = make_execfd(&fdtype);
if (execfd < 0 || fdtype == EFD_NONE)
return -ENOTRECOVERABLE;
binfd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC);
if (binfd < 0)
goto error;
if (fstat(binfd, &statbuf) < 0)
goto error_binfd;
while (sent < statbuf.st_size) {
int n = sendfile(execfd, binfd, NULL, statbuf.st_size - sent);
if (n < 0) {
/* sendfile can fail so we fallback to a dumb user-space copy. */
n = fd_to_fd(execfd, binfd);
if (n < 0)
goto error_binfd;
}
sent += n;
}
close(binfd);
if (sent != statbuf.st_size)
goto error;
if (seal_execfd(&execfd, fdtype) < 0)
goto error;
return execfd;
error_binfd:
close(binfd);
error:
close(execfd);
return -EIO;
}
/* Get cheap access to the environment. */
extern char **environ;
int ensure_cloned_binary(void)
{
int execfd;
char **argv = 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) < 0)
return -EINVAL;
execfd = clone_binary();
if (execfd < 0)
return -EIO;
if (putenv(CLONED_BINARY_ENV "=1"))
goto error;
fexecve(execfd, argv, environ);
error:
close(execfd);
return -ENOEXEC;
}