From: Shengjing Zhu Date: Sun, 10 Mar 2019 17:47:46 +0800 Subject: CVE-2019-5736 Backport upstream patches for CVE-2019-5736 Fix in nsexec.c is adjusted to current version Include commits: 2d4a37b427167907ef2402586a8e8e2931a22490 nsenter: cloned_binary: userspace copy fallback if sendfile fails 16612d74de5f84977e50a9c8ead7f0e9e13b8628 nsenter: cloned_binary: try to ro-bind /proc/self/exe before copying af9da0a45082783f6005b252488943b5ee2e2138 nsenter: cloned_binary: use the runc statedir for O_TMPFILE 2429d59352b81f6b9cc79b5ed26780c5fe6ba4ec nsenter: cloned_binary: expand and add pre-3.11 fallbacks 5b775bf297c47a6bc50e36da89d1ec74a6fa01dc nsenter: cloned_binary: detect and handle short copies bb7d8b1f41f7bf0399204d54009d6da57c3cc775 nsexec (CVE-2019-5736): avoid parsing environ 0a8e4117e7f715d5fbeef398405813ce8e88558b nsenter: clone /proc/self/exe to avoid exposing host binary to container Bug-Debian: https://bugs.debian.org/922050 --- libcontainer/nsenter/cloned_binary.c | 516 +++++++++++++++++++++++++++++++++++ libcontainer/nsenter/nsexec.c | 8 + 2 files changed, 524 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 0000000..b410e29 --- /dev/null +++ b/libcontainer/nsenter/cloned_binary.c @@ -0,0 +1,516 @@ +/* + * Copyright (C) 2019 Aleksa Sarai + * 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 +#include +#include +#include +#include +#include +#include +#include + +#include +#include +#include +#include +#include +#include +#include +#include + +/* 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 . */ +#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 . */ +#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 . 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 = secure_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 = secure_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; +} diff --git a/libcontainer/nsenter/nsexec.c b/libcontainer/nsenter/nsexec.c index 3c74c63..a6ec725 100644 --- a/libcontainer/nsenter/nsexec.c +++ b/libcontainer/nsenter/nsexec.c @@ -353,6 +353,9 @@ static struct nsenter_config process_nl_attributes(int pipenum, char *data, int return config; } +/* Defined in cloned_binary.c. */ +extern int ensure_cloned_binary(void); + void nsexec(void) { int pipenum; @@ -364,6 +367,11 @@ void nsexec(void) return; } + if (ensure_cloned_binary() < 0) { + pr_perror("could not ensure we are a cloned binary"); + exit(1); + } + /* make the process non-dumpable */ if (prctl(PR_SET_DUMPABLE, 0, 0, 0, 0) != 0) { pr_perror("Failed to set process as non-dumpable");