fs-util: extra chase_symlink() safety check on "path" parameter

[thirdparty/systemd.git] / src / basic / fs-util.c

/* SPDX-License-Identifier: LGPL-2.1+ */
/***
  This file is part of systemd.

  Copyright 2010 Lennart Poettering

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation; either version 2.1 of the License, or
  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
***/

#include <errno.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <linux/magic.h>
#include <time.h>
#include <unistd.h>

#include "alloc-util.h"
#include "dirent-util.h"
#include "fd-util.h"
#include "fileio.h"
#include "fs-util.h"
#include "log.h"
#include "macro.h"
#include "missing.h"
#include "mkdir.h"
#include "parse-util.h"
#include "path-util.h"
#include "process-util.h"
#include "stat-util.h"
#include "stdio-util.h"
#include "string-util.h"
#include "strv.h"
#include "time-util.h"
#include "user-util.h"
#include "util.h"

int unlink_noerrno(const char *path) {
        PROTECT_ERRNO;
        int r;

        r = unlink(path);
        if (r < 0)
                return -errno;

        return 0;
}

int rmdir_parents(const char *path, const char *stop) {
        size_t l;
        int r = 0;

        assert(path);
        assert(stop);

        l = strlen(path);

        /* Skip trailing slashes */
        while (l > 0 && path[l-1] == '/')
                l--;

        while (l > 0) {
                char *t;

                /* Skip last component */
                while (l > 0 && path[l-1] != '/')
                        l--;

                /* Skip trailing slashes */
                while (l > 0 && path[l-1] == '/')
                        l--;

                if (l <= 0)
                        break;

                t = strndup(path, l);
                if (!t)
                        return -ENOMEM;

                if (path_startswith(stop, t)) {
                        free(t);
                        return 0;
                }

                r = rmdir(t);
                free(t);

                if (r < 0)
                        if (errno != ENOENT)
                                return -errno;
        }

        return 0;
}

int rename_noreplace(int olddirfd, const char *oldpath, int newdirfd, const char *newpath) {
        struct stat buf;
        int ret;

        ret = renameat2(olddirfd, oldpath, newdirfd, newpath, RENAME_NOREPLACE);
        if (ret >= 0)
                return 0;

        /* renameat2() exists since Linux 3.15, btrfs added support for it later.
         * If it is not implemented, fallback to another method. */
        if (!IN_SET(errno, EINVAL, ENOSYS))
                return -errno;

        /* The link()/unlink() fallback does not work on directories. But
         * renameat() without RENAME_NOREPLACE gives the same semantics on
         * directories, except when newpath is an *empty* directory. This is
         * good enough. */
        ret = fstatat(olddirfd, oldpath, &buf, AT_SYMLINK_NOFOLLOW);
        if (ret >= 0 && S_ISDIR(buf.st_mode)) {
                ret = renameat(olddirfd, oldpath, newdirfd, newpath);
                return ret >= 0 ? 0 : -errno;
        }

        /* If it is not a directory, use the link()/unlink() fallback. */
        ret = linkat(olddirfd, oldpath, newdirfd, newpath, 0);
        if (ret < 0)
                return -errno;

        ret = unlinkat(olddirfd, oldpath, 0);
        if (ret < 0) {
                /* backup errno before the following unlinkat() alters it */
                ret = errno;
                (void) unlinkat(newdirfd, newpath, 0);
                errno = ret;
                return -errno;
        }

        return 0;
}

int readlinkat_malloc(int fd, const char *p, char **ret) {
        size_t l = 100;
        int r;

        assert(p);
        assert(ret);

        for (;;) {
                char *c;
                ssize_t n;

                c = new(char, l);
                if (!c)
                        return -ENOMEM;

                n = readlinkat(fd, p, c, l-1);
                if (n < 0) {
                        r = -errno;
                        free(c);
                        return r;
                }

                if ((size_t) n < l-1) {
                        c[n] = 0;
                        *ret = c;
                        return 0;
                }

                free(c);
                l *= 2;
        }
}

int readlink_malloc(const char *p, char **ret) {
        return readlinkat_malloc(AT_FDCWD, p, ret);
}

int readlink_value(const char *p, char **ret) {
        _cleanup_free_ char *link = NULL;
        char *value;
        int r;

        r = readlink_malloc(p, &link);
        if (r < 0)
                return r;

        value = basename(link);
        if (!value)
                return -ENOENT;

        value = strdup(value);
        if (!value)
                return -ENOMEM;

        *ret = value;

        return 0;
}

int readlink_and_make_absolute(const char *p, char **r) {
        _cleanup_free_ char *target = NULL;
        char *k;
        int j;

        assert(p);
        assert(r);

        j = readlink_malloc(p, &target);
        if (j < 0)
                return j;

        k = file_in_same_dir(p, target);
        if (!k)
                return -ENOMEM;

        *r = k;
        return 0;
}

int readlink_and_canonicalize(const char *p, const char *root, char **ret) {
        char *t, *s;
        int r;

        assert(p);
        assert(ret);

        r = readlink_and_make_absolute(p, &t);
        if (r < 0)
                return r;

        r = chase_symlinks(t, root, 0, &s);
        if (r < 0)
                /* If we can't follow up, then let's return the original string, slightly cleaned up. */
                *ret = path_kill_slashes(t);
        else {
                *ret = s;
                free(t);
        }

        return 0;
}

int readlink_and_make_absolute_root(const char *root, const char *path, char **ret) {
        _cleanup_free_ char *target = NULL, *t = NULL;
        const char *full;
        int r;

        full = prefix_roota(root, path);
        r = readlink_malloc(full, &target);
        if (r < 0)
                return r;

        t = file_in_same_dir(path, target);
        if (!t)
                return -ENOMEM;

        *ret = t;
        t = NULL;

        return 0;
}

int chmod_and_chown(const char *path, mode_t mode, uid_t uid, gid_t gid) {
        assert(path);

        /* Under the assumption that we are running privileged we
         * first change the access mode and only then hand out
         * ownership to avoid a window where access is too open. */

        if (mode != MODE_INVALID)
                if (chmod(path, mode) < 0)
                        return -errno;

        if (uid != UID_INVALID || gid != GID_INVALID)
                if (chown(path, uid, gid) < 0)
                        return -errno;

        return 0;
}

int fchmod_umask(int fd, mode_t m) {
        mode_t u;
        int r;

        u = umask(0777);
        r = fchmod(fd, m & (~u)) < 0 ? -errno : 0;
        umask(u);

        return r;
}

int fd_warn_permissions(const char *path, int fd) {
        struct stat st;

        if (fstat(fd, &st) < 0)
                return -errno;

        if (st.st_mode & 0111)
                log_warning("Configuration file %s is marked executable. Please remove executable permission bits. Proceeding anyway.", path);

        if (st.st_mode & 0002)
                log_warning("Configuration file %s is marked world-writable. Please remove world writability permission bits. Proceeding anyway.", path);

        if (getpid_cached() == 1 && (st.st_mode & 0044) != 0044)
                log_warning("Configuration file %s is marked world-inaccessible. This has no effect as configuration data is accessible via APIs without restrictions. Proceeding anyway.", path);

        return 0;
}

int touch_file(const char *path, bool parents, usec_t stamp, uid_t uid, gid_t gid, mode_t mode) {
        char fdpath[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int)];
        _cleanup_close_ int fd = -1;
        int r, ret = 0;

        assert(path);

        /* Note that touch_file() does not follow symlinks: if invoked on an existing symlink, then it is the symlink
         * itself which is updated, not its target
         *
         * Returns the first error we encounter, but tries to apply as much as possible. */

        if (parents)
                (void) mkdir_parents(path, 0755);

        /* Initially, we try to open the node with O_PATH, so that we get a reference to the node. This is useful in
         * case the path refers to an existing device or socket node, as we can open it successfully in all cases, and
         * won't trigger any driver magic or so. */
        fd = open(path, O_PATH|O_CLOEXEC|O_NOFOLLOW);
        if (fd < 0) {
                if (errno != ENOENT)
                        return -errno;

                /* if the node doesn't exist yet, we create it, but with O_EXCL, so that we only create a regular file
                 * here, and nothing else */
                fd = open(path, O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, IN_SET(mode, 0, MODE_INVALID) ? 0644 : mode);
                if (fd < 0)
                        return -errno;
        }

        /* Let's make a path from the fd, and operate on that. With this logic, we can adjust the access mode,
         * ownership and time of the file node in all cases, even if the fd refers to an O_PATH object — which is
         * something fchown(), fchmod(), futimensat() don't allow. */
        xsprintf(fdpath, "/proc/self/fd/%i", fd);

        if (mode != MODE_INVALID)
                if (chmod(fdpath, mode) < 0)
                        ret = -errno;

        if (uid_is_valid(uid) || gid_is_valid(gid))
                if (chown(fdpath, uid, gid) < 0 && ret >= 0)
                        ret = -errno;

        if (stamp != USEC_INFINITY) {
                struct timespec ts[2];

                timespec_store(&ts[0], stamp);
                ts[1] = ts[0];
                r = utimensat(AT_FDCWD, fdpath, ts, 0);
        } else
                r = utimensat(AT_FDCWD, fdpath, NULL, 0);
        if (r < 0 && ret >= 0)
                return -errno;

        return ret;
}

int touch(const char *path) {
        return touch_file(path, false, USEC_INFINITY, UID_INVALID, GID_INVALID, MODE_INVALID);
}

int symlink_idempotent(const char *from, const char *to) {
        int r;

        assert(from);
        assert(to);

        if (symlink(from, to) < 0) {
                _cleanup_free_ char *p = NULL;

                if (errno != EEXIST)
                        return -errno;

                r = readlink_malloc(to, &p);
                if (r == -EINVAL) /* Not a symlink? In that case return the original error we encountered: -EEXIST */
                        return -EEXIST;
                if (r < 0) /* Any other error? In that case propagate it as is */
                        return r;

                if (!streq(p, from)) /* Not the symlink we want it to be? In that case, propagate the original -EEXIST */
                        return -EEXIST;
        }

        return 0;
}

int symlink_atomic(const char *from, const char *to) {
        _cleanup_free_ char *t = NULL;
        int r;

        assert(from);
        assert(to);

        r = tempfn_random(to, NULL, &t);
        if (r < 0)
                return r;

        if (symlink(from, t) < 0)
                return -errno;

        if (rename(t, to) < 0) {
                unlink_noerrno(t);
                return -errno;
        }

        return 0;
}

int mknod_atomic(const char *path, mode_t mode, dev_t dev) {
        _cleanup_free_ char *t = NULL;
        int r;

        assert(path);

        r = tempfn_random(path, NULL, &t);
        if (r < 0)
                return r;

        if (mknod(t, mode, dev) < 0)
                return -errno;

        if (rename(t, path) < 0) {
                unlink_noerrno(t);
                return -errno;
        }

        return 0;
}

int mkfifo_atomic(const char *path, mode_t mode) {
        _cleanup_free_ char *t = NULL;
        int r;

        assert(path);

        r = tempfn_random(path, NULL, &t);
        if (r < 0)
                return r;

        if (mkfifo(t, mode) < 0)
                return -errno;

        if (rename(t, path) < 0) {
                unlink_noerrno(t);
                return -errno;
        }

        return 0;
}

int get_files_in_directory(const char *path, char ***list) {
        _cleanup_closedir_ DIR *d = NULL;
        struct dirent *de;
        size_t bufsize = 0, n = 0;
        _cleanup_strv_free_ char **l = NULL;

        assert(path);

        /* Returns all files in a directory in *list, and the number
         * of files as return value. If list is NULL returns only the
         * number. */

        d = opendir(path);
        if (!d)
                return -errno;

        FOREACH_DIRENT_ALL(de, d, return -errno) {
                dirent_ensure_type(d, de);

                if (!dirent_is_file(de))
                        continue;

                if (list) {
                        /* one extra slot is needed for the terminating NULL */
                        if (!GREEDY_REALLOC(l, bufsize, n + 2))
                                return -ENOMEM;

                        l[n] = strdup(de->d_name);
                        if (!l[n])
                                return -ENOMEM;

                        l[++n] = NULL;
                } else
                        n++;
        }

        if (list) {
                *list = l;
                l = NULL; /* avoid freeing */
        }

        return n;
}

static int getenv_tmp_dir(const char **ret_path) {
        const char *n;
        int r, ret = 0;

        assert(ret_path);

        /* We use the same order of environment variables python uses in tempfile.gettempdir():
         * https://docs.python.org/3/library/tempfile.html#tempfile.gettempdir */
        FOREACH_STRING(n, "TMPDIR", "TEMP", "TMP") {
                const char *e;

                e = secure_getenv(n);
                if (!e)
                        continue;
                if (!path_is_absolute(e)) {
                        r = -ENOTDIR;
                        goto next;
                }
                if (!path_is_normalized(e)) {
                        r = -EPERM;
                        goto next;
                }

                r = is_dir(e, true);
                if (r < 0)
                        goto next;
                if (r == 0) {
                        r = -ENOTDIR;
                        goto next;
                }

                *ret_path = e;
                return 1;

        next:
                /* Remember first error, to make this more debuggable */
                if (ret >= 0)
                        ret = r;
        }

        if (ret < 0)
                return ret;

        *ret_path = NULL;
        return ret;
}

static int tmp_dir_internal(const char *def, const char **ret) {
        const char *e;
        int r, k;

        assert(def);
        assert(ret);

        r = getenv_tmp_dir(&e);
        if (r > 0) {
                *ret = e;
                return 0;
        }

        k = is_dir(def, true);
        if (k == 0)
                k = -ENOTDIR;
        if (k < 0)
                return r < 0 ? r : k;

        *ret = def;
        return 0;
}

int var_tmp_dir(const char **ret) {

        /* Returns the location for "larger" temporary files, that is backed by physical storage if available, and thus
         * even might survive a boot: /var/tmp. If $TMPDIR (or related environment variables) are set, its value is
         * returned preferably however. Note that both this function and tmp_dir() below are affected by $TMPDIR,
         * making it a variable that overrides all temporary file storage locations. */

        return tmp_dir_internal("/var/tmp", ret);
}

int tmp_dir(const char **ret) {

        /* Similar to var_tmp_dir() above, but returns the location for "smaller" temporary files, which is usually
         * backed by an in-memory file system: /tmp. */

        return tmp_dir_internal("/tmp", ret);
}

int inotify_add_watch_fd(int fd, int what, uint32_t mask) {
        char path[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int) + 1];
        int r;

        /* This is like inotify_add_watch(), except that the file to watch is not referenced by a path, but by an fd */
        xsprintf(path, "/proc/self/fd/%i", what);

        r = inotify_add_watch(fd, path, mask);
        if (r < 0)
                return -errno;

        return r;
}

static bool safe_transition(const struct stat *a, const struct stat *b) {
        /* Returns true if the transition from a to b is safe, i.e. that we never transition from unprivileged to
         * privileged files or directories. Why bother? So that unprivileged code can't symlink to privileged files
         * making us believe we read something safe even though it isn't safe in the specific context we open it in. */

        if (a->st_uid == 0) /* Transitioning from privileged to unprivileged is always fine */
                return true;

        return a->st_uid == b->st_uid; /* Otherwise we need to stay within the same UID */
}

int chase_symlinks(const char *path, const char *original_root, unsigned flags, char **ret) {
        _cleanup_free_ char *buffer = NULL, *done = NULL, *root = NULL;
        _cleanup_close_ int fd = -1;
        unsigned max_follow = 32; /* how many symlinks to follow before giving up and returning ELOOP */
        struct stat previous_stat;
        bool exists = true;
        char *todo;
        int r;

        assert(path);

        /* Either the file may be missing, or we return an fd to the final object, but both make no sense */
        if ((flags & (CHASE_NONEXISTENT|CHASE_OPEN)) == (CHASE_NONEXISTENT|CHASE_OPEN))
                return -EINVAL;

        if (isempty(path))
                return -EINVAL;

        /* This is a lot like canonicalize_file_name(), but takes an additional "root" parameter, that allows following
         * symlinks relative to a root directory, instead of the root of the host.
         *
         * Note that "root" primarily matters if we encounter an absolute symlink. It is also used when following
         * relative symlinks to ensure they cannot be used to "escape" the root directory. The path parameter passed is
         * assumed to be already prefixed by it, except if the CHASE_PREFIX_ROOT flag is set, in which case it is first
         * prefixed accordingly.
         *
         * Algorithmically this operates on two path buffers: "done" are the components of the path we already
         * processed and resolved symlinks, "." and ".." of. "todo" are the components of the path we still need to
         * process. On each iteration, we move one component from "todo" to "done", processing it's special meaning
         * each time. The "todo" path always starts with at least one slash, the "done" path always ends in no
         * slash. We always keep an O_PATH fd to the component we are currently processing, thus keeping lookup races
         * at a minimum.
         *
         * Suggested usage: whenever you want to canonicalize a path, use this function. Pass the absolute path you got
         * as-is: fully qualified and relative to your host's root. Optionally, specify the root parameter to tell this
         * function what to do when encountering a symlink with an absolute path as directory: prefix it by the
         * specified path. */

        if (original_root) {
                if (isempty(original_root)) /* What's this even supposed to mean? */
                        return -EINVAL;

                if (path_equal(original_root, "/")) /* A root directory of "/" is identical to none */
                        original_root = NULL;
        }

        if (original_root) {
                r = path_make_absolute_cwd(original_root, &root);
                if (r < 0)
                        return r;

                if (flags & CHASE_PREFIX_ROOT)
                        path = prefix_roota(root, path);
        }

        r = path_make_absolute_cwd(path, &buffer);
        if (r < 0)
                return r;

        fd = open("/", O_CLOEXEC|O_NOFOLLOW|O_PATH);
        if (fd < 0)
                return -errno;

        if (flags & CHASE_SAFE) {
                if (fstat(fd, &previous_stat) < 0)
                        return -errno;
        }

        todo = buffer;
        for (;;) {
                _cleanup_free_ char *first = NULL;
                _cleanup_close_ int child = -1;
                struct stat st;
                size_t n, m;

                /* Determine length of first component in the path */
                n = strspn(todo, "/");                  /* The slashes */
                m = n + strcspn(todo + n, "/");         /* The entire length of the component */

                /* Extract the first component. */
                first = strndup(todo, m);
                if (!first)
                        return -ENOMEM;

                todo += m;

                /* Empty? Then we reached the end. */
                if (isempty(first))
                        break;

                /* Just a single slash? Then we reached the end. */
                if (path_equal(first, "/")) {
                        /* Preserve the trailing slash */
                        if (!strextend(&done, "/", NULL))
                                return -ENOMEM;

                        break;
                }

                /* Just a dot? Then let's eat this up. */
                if (path_equal(first, "/."))
                        continue;

                /* Two dots? Then chop off the last bit of what we already found out. */
                if (path_equal(first, "/..")) {
                        _cleanup_free_ char *parent = NULL;
                        int fd_parent = -1;

                        /* If we already are at the top, then going up will not change anything. This is in-line with
                         * how the kernel handles this. */
                        if (isempty(done) || path_equal(done, "/"))
                                continue;

                        parent = dirname_malloc(done);
                        if (!parent)
                                return -ENOMEM;

                        /* Don't allow this to leave the root dir.  */
                        if (root &&
                            path_startswith(done, root) &&
                            !path_startswith(parent, root))
                                continue;

                        free_and_replace(done, parent);

                        fd_parent = openat(fd, "..", O_CLOEXEC|O_NOFOLLOW|O_PATH);
                        if (fd_parent < 0)
                                return -errno;

                        if (flags & CHASE_SAFE) {
                                if (fstat(fd_parent, &st) < 0)
                                        return -errno;

                                if (!safe_transition(&previous_stat, &st))
                                        return -EPERM;

                                previous_stat = st;
                        }

                        safe_close(fd);
                        fd = fd_parent;

                        continue;
                }

                /* Otherwise let's see what this is. */
                child = openat(fd, first + n, O_CLOEXEC|O_NOFOLLOW|O_PATH);
                if (child < 0) {

                        if (errno == ENOENT &&
                            (flags & CHASE_NONEXISTENT) &&
                            (isempty(todo) || path_is_normalized(todo))) {

                                /* If CHASE_NONEXISTENT is set, and the path does not exist, then that's OK, return
                                 * what we got so far. But don't allow this if the remaining path contains "../ or "./"
                                 * or something else weird. */

                                /* If done is "/", as first also contains slash at the head, then remove this redundant slash. */
                                if (streq_ptr(done, "/"))
                                        *done = '\0';

                                if (!strextend(&done, first, todo, NULL))
                                        return -ENOMEM;

                                exists = false;
                                break;
                        }

                        return -errno;
                }

                if (fstat(child, &st) < 0)
                        return -errno;
                if ((flags & CHASE_SAFE) &&
                    !safe_transition(&previous_stat, &st))
                        return -EPERM;

                previous_stat = st;

                if ((flags & CHASE_NO_AUTOFS) &&
                    fd_is_fs_type(child, AUTOFS_SUPER_MAGIC) > 0)
                        return -EREMOTE;

                if (S_ISLNK(st.st_mode)) {
                        char *joined;

                        _cleanup_free_ char *destination = NULL;

                        /* This is a symlink, in this case read the destination. But let's make sure we don't follow
                         * symlinks without bounds. */
                        if (--max_follow <= 0)
                                return -ELOOP;

                        r = readlinkat_malloc(fd, first + n, &destination);
                        if (r < 0)
                                return r;
                        if (isempty(destination))
                                return -EINVAL;

                        if (path_is_absolute(destination)) {

                                /* An absolute destination. Start the loop from the beginning, but use the root
                                 * directory as base. */

                                safe_close(fd);
                                fd = open(root ?: "/", O_CLOEXEC|O_NOFOLLOW|O_PATH);
                                if (fd < 0)
                                        return -errno;

                                free(done);

                                if (flags & CHASE_SAFE) {
                                        if (fstat(fd, &st) < 0)
                                                return -errno;

                                        if (!safe_transition(&previous_stat, &st))
                                                return -EPERM;

                                        previous_stat = st;
                                }

                                /* Note that we do not revalidate the root, we take it as is. */
                                if (isempty(root))
                                        done = NULL;
                                else {
                                        done = strdup(root);
                                        if (!done)
                                                return -ENOMEM;
                                }

                                /* Prefix what's left to do with what we just read, and start the loop again, but
                                 * remain in the current directory. */
                                joined = strjoin(destination, todo);
                        } else
                                joined = strjoin("/", destination, todo);
                        if (!joined)
                                return -ENOMEM;

                        free(buffer);
                        todo = buffer = joined;

                        continue;
                }

                /* If this is not a symlink, then let's just add the name we read to what we already verified. */
                if (!done) {
                        done = first;
                        first = NULL;
                } else {
                        /* If done is "/", as first also contains slash at the head, then remove this redundant slash. */
                        if (streq(done, "/"))
                                *done = '\0';

                        if (!strextend(&done, first, NULL))
                                return -ENOMEM;
                }

                /* And iterate again, but go one directory further down. */
                safe_close(fd);
                fd = child;
                child = -1;
        }

        if (!done) {
                /* Special case, turn the empty string into "/", to indicate the root directory. */
                done = strdup("/");
                if (!done)
                        return -ENOMEM;
        }

        if (ret) {
                *ret = done;
                done = NULL;
        }

        if (flags & CHASE_OPEN) {
                int q;

                /* Return the O_PATH fd we currently are looking to the caller. It can translate it to a proper fd by
                 * opening /proc/self/fd/xyz. */

                assert(fd >= 0);
                q = fd;
                fd = -1;

                return q;
        }

        return exists;
}

int access_fd(int fd, int mode) {
        char p[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(fd) + 1];
        int r;

        /* Like access() but operates on an already open fd */

        xsprintf(p, "/proc/self/fd/%i", fd);

        r = access(p, mode);
        if (r < 0)
                r = -errno;

        return r;
}