/* SPDX-License-Identifier: LGPL-2.1-or-later */ #include #include "alloc-util.h" #include "chase-symlinks.h" #include "fd-util.h" #include "fileio.h" #include "fs-util.h" #include "glyph-util.h" #include "log.h" #include "path-util.h" #include "string-util.h" #include "user-util.h" bool unsafe_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 false; return a->st_uid != b->st_uid; /* Otherwise we need to stay within the same UID */ } static int log_unsafe_transition(int a, int b, const char *path, ChaseSymlinksFlags flags) { _cleanup_free_ char *n1 = NULL, *n2 = NULL, *user_a = NULL, *user_b = NULL; struct stat st; if (!FLAGS_SET(flags, CHASE_WARN)) return -ENOLINK; (void) fd_get_path(a, &n1); (void) fd_get_path(b, &n2); if (fstat(a, &st) == 0) user_a = uid_to_name(st.st_uid); if (fstat(b, &st) == 0) user_b = uid_to_name(st.st_uid); return log_warning_errno(SYNTHETIC_ERRNO(ENOLINK), "Detected unsafe path transition %s (owned by %s) %s %s (owned by %s) during canonicalization of %s.", strna(n1), strna(user_a), special_glyph(SPECIAL_GLYPH_ARROW_RIGHT), strna(n2), strna(user_b), path); } static int log_autofs_mount_point(int fd, const char *path, ChaseSymlinksFlags flags) { _cleanup_free_ char *n1 = NULL; if (!FLAGS_SET(flags, CHASE_WARN)) return -EREMOTE; (void) fd_get_path(fd, &n1); return log_warning_errno(SYNTHETIC_ERRNO(EREMOTE), "Detected autofs mount point %s during canonicalization of %s.", strna(n1), path); } int chase_symlinks( const char *path, const char *original_root, ChaseSymlinksFlags flags, char **ret_path, int *ret_fd) { _cleanup_free_ char *buffer = NULL, *done = NULL, *root = NULL; _cleanup_close_ int fd = -1; unsigned max_follow = CHASE_SYMLINKS_MAX; /* how many symlinks to follow before giving up and returning ELOOP */ bool exists = true, append_trail_slash = false; struct stat previous_stat; const 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) && ret_fd) return -EINVAL; if ((flags & CHASE_STEP) && ret_fd) return -EINVAL; if (isempty(path)) return -EINVAL; /* We don't support relative paths in combination with a root directory */ if (FLAGS_SET(flags, CHASE_PREFIX_ROOT) && !path_is_absolute(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 * to 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. * * There are five ways to invoke this function: * * 1. Without CHASE_STEP or ret_fd: in this case the path is resolved and the normalized path is * returned in `ret_path`. The return value is < 0 on error. If CHASE_NONEXISTENT is also set, 0 * is returned if the file doesn't exist, > 0 otherwise. If CHASE_NONEXISTENT is not set, >= 0 is * returned if the destination was found, -ENOENT if it wasn't. * * 2. With ret_fd: in this case the destination is opened after chasing it as O_PATH and this file * descriptor is returned as return value. This is useful to open files relative to some root * directory. Note that the returned O_PATH file descriptors must be converted into a regular one (using * fd_reopen() or such) before it can be used for reading/writing. ret_fd may not be combined with * CHASE_NONEXISTENT. * * 3. With CHASE_STEP: in this case only a single step of the normalization is executed, i.e. only the first * symlink or ".." component of the path is resolved, and the resulting path is returned. This is useful if * a caller wants to trace the path through the file system verbosely. Returns < 0 on error, > 0 if the * path is fully normalized, and == 0 for each normalization step. This may be combined with * CHASE_NONEXISTENT, in which case 1 is returned when a component is not found. * * 4. With CHASE_SAFE: in this case the path must not contain unsafe transitions, i.e. transitions from * unprivileged to privileged files or directories. In such cases the return value is -ENOLINK. If * CHASE_WARN is also set, a warning describing the unsafe transition is emitted. * * 5. With CHASE_NO_AUTOFS: in this case if an autofs mount point is encountered, path normalization * is aborted and -EREMOTE is returned. If CHASE_WARN is also set, a warning showing the path of * the mount point is emitted. */ /* A root directory of "/" or "" is identical to none */ if (empty_or_root(original_root)) original_root = NULL; if (!original_root && !ret_path && !(flags & (CHASE_NONEXISTENT|CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_STEP)) && ret_fd) { /* Shortcut the ret_fd case if the caller isn't interested in the actual path and has no root set * and doesn't care about any of the other special features we provide either. */ r = open(path, O_PATH|O_CLOEXEC|((flags & CHASE_NOFOLLOW) ? O_NOFOLLOW : 0)); if (r < 0) return -errno; *ret_fd = r; return 0; } if (original_root) { r = path_make_absolute_cwd(original_root, &root); if (r < 0) return r; /* Simplify the root directory, so that it has no duplicate slashes and nothing at the * end. While we won't resolve the root path we still simplify it. Note that dropping the * trailing slash should not change behaviour, since when opening it we specify O_DIRECTORY * anyway. Moreover at the end of this function after processing everything we'll always turn * the empty string back to "/". */ delete_trailing_chars(root, "/"); path_simplify(root); if (flags & CHASE_PREFIX_ROOT) { buffer = path_join(root, path); if (!buffer) return -ENOMEM; } } if (!buffer) { r = path_make_absolute_cwd(path, &buffer); if (r < 0) return r; } fd = open(empty_to_root(root), O_CLOEXEC|O_DIRECTORY|O_PATH); if (fd < 0) return -errno; if (flags & CHASE_SAFE) if (fstat(fd, &previous_stat) < 0) return -errno; if (flags & CHASE_TRAIL_SLASH) append_trail_slash = endswith(buffer, "/") || endswith(buffer, "/."); if (root) { /* If we are operating on a root directory, let's take the root directory as it is. */ todo = path_startswith(buffer, root); if (!todo) return log_full_errno(flags & CHASE_WARN ? LOG_WARNING : LOG_DEBUG, SYNTHETIC_ERRNO(ECHRNG), "Specified path '%s' is outside of specified root directory '%s', refusing to resolve.", path, root); done = strdup(root); } else { todo = buffer; done = strdup("/"); } if (!done) return -ENOMEM; for (;;) { _cleanup_free_ char *first = NULL; _cleanup_close_ int child = -1; struct stat st; const char *e; r = path_find_first_component(&todo, /* accept_dot_dot= */ true, &e); if (r < 0) return r; if (r == 0) { /* We reached the end. */ if (append_trail_slash) if (!strextend(&done, "/")) return -ENOMEM; break; } first = strndup(e, r); if (!first) return -ENOMEM; /* Two dots? Then chop off the last bit of what we already found out. */ if (path_equal(first, "..")) { _cleanup_free_ char *parent = NULL; _cleanup_close_ 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 (empty_or_root(done)) continue; r = path_extract_directory(done, &parent); if (r < 0) return r; /* 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); if (flags & CHASE_STEP) goto chased_one; 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 (unsafe_transition(&previous_stat, &st)) return log_unsafe_transition(fd, fd_parent, path, flags); previous_stat = st; } safe_close(fd); fd = TAKE_FD(fd_parent); continue; } /* Otherwise let's see what this is. */ child = openat(fd, first, O_CLOEXEC|O_NOFOLLOW|O_PATH); if (child < 0) { if (errno == ENOENT && (flags & CHASE_NONEXISTENT) && (isempty(todo) || path_is_safe(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 something else weird. */ if (!path_extend(&done, first, todo)) return -ENOMEM; exists = false; break; } return -errno; } if (fstat(child, &st) < 0) return -errno; if ((flags & CHASE_SAFE) && unsafe_transition(&previous_stat, &st)) return log_unsafe_transition(fd, child, path, flags); previous_stat = st; if ((flags & CHASE_NO_AUTOFS) && fd_is_fs_type(child, AUTOFS_SUPER_MAGIC) > 0) return log_autofs_mount_point(child, path, flags); if (S_ISLNK(st.st_mode) && !((flags & CHASE_NOFOLLOW) && isempty(todo))) { _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, &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(empty_to_root(root), O_CLOEXEC|O_DIRECTORY|O_PATH); if (fd < 0) return -errno; if (flags & CHASE_SAFE) { if (fstat(fd, &st) < 0) return -errno; if (unsafe_transition(&previous_stat, &st)) return log_unsafe_transition(child, fd, path, flags); previous_stat = st; } /* Note that we do not revalidate the root, we take it as is. */ r = free_and_strdup(&done, empty_to_root(root)); if (r < 0) return r; } /* Prefix what's left to do with what we just read, and start the loop again, but * remain in the current directory. */ if (!path_extend(&destination, todo)) return -ENOMEM; free_and_replace(buffer, destination); todo = buffer; if (flags & CHASE_STEP) goto chased_one; continue; } /* If this is not a symlink, then let's just add the name we read to what we already verified. */ if (!path_extend(&done, first)) return -ENOMEM; /* And iterate again, but go one directory further down. */ safe_close(fd); fd = TAKE_FD(child); } if (ret_path) *ret_path = TAKE_PTR(done); if (ret_fd) { /* 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); *ret_fd = TAKE_FD(fd); } if (flags & CHASE_STEP) return 1; return exists; chased_one: if (ret_path) { const char *e; /* todo may contain slashes at the beginning. */ r = path_find_first_component(&todo, /* accept_dot_dot= */ true, &e); if (r < 0) return r; if (r == 0) *ret_path = TAKE_PTR(done); else { char *c; c = path_join(done, e); if (!c) return -ENOMEM; *ret_path = c; } } return 0; } int chase_symlinks_and_open( const char *path, const char *root, ChaseSymlinksFlags chase_flags, int open_flags, char **ret_path) { _cleanup_close_ int path_fd = -1; _cleanup_free_ char *p = NULL; int r; if (chase_flags & CHASE_NONEXISTENT) return -EINVAL; if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE)) == 0) { /* Shortcut this call if none of the special features of this call are requested */ r = open(path, open_flags); if (r < 0) return -errno; return r; } r = chase_symlinks(path, root, chase_flags, ret_path ? &p : NULL, &path_fd); if (r < 0) return r; assert(path_fd >= 0); r = fd_reopen(path_fd, open_flags); if (r < 0) return r; if (ret_path) *ret_path = TAKE_PTR(p); return r; } int chase_symlinks_and_opendir( const char *path, const char *root, ChaseSymlinksFlags chase_flags, char **ret_path, DIR **ret_dir) { _cleanup_close_ int path_fd = -1; _cleanup_free_ char *p = NULL; DIR *d; int r; if (!ret_dir) return -EINVAL; if (chase_flags & CHASE_NONEXISTENT) return -EINVAL; if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE)) == 0) { /* Shortcut this call if none of the special features of this call are requested */ d = opendir(path); if (!d) return -errno; *ret_dir = d; return 0; } r = chase_symlinks(path, root, chase_flags, ret_path ? &p : NULL, &path_fd); if (r < 0) return r; assert(path_fd >= 0); d = opendir(FORMAT_PROC_FD_PATH(path_fd)); if (!d) return -errno; if (ret_path) *ret_path = TAKE_PTR(p); *ret_dir = d; return 0; } int chase_symlinks_and_stat( const char *path, const char *root, ChaseSymlinksFlags chase_flags, char **ret_path, struct stat *ret_stat, int *ret_fd) { _cleanup_close_ int path_fd = -1; _cleanup_free_ char *p = NULL; int r; assert(path); assert(ret_stat); if (chase_flags & CHASE_NONEXISTENT) return -EINVAL; if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE)) == 0) { /* Shortcut this call if none of the special features of this call are requested */ if (stat(path, ret_stat) < 0) return -errno; return 1; } r = chase_symlinks(path, root, chase_flags, ret_path ? &p : NULL, &path_fd); if (r < 0) return r; assert(path_fd >= 0); if (fstat(path_fd, ret_stat) < 0) return -errno; if (ret_path) *ret_path = TAKE_PTR(p); if (ret_fd) *ret_fd = TAKE_FD(path_fd); return 1; } int chase_symlinks_and_fopen_unlocked( const char *path, const char *root, ChaseSymlinksFlags chase_flags, const char *open_flags, char **ret_path, FILE **ret_file) { _cleanup_free_ char *final_path = NULL; _cleanup_close_ int fd = -1; int mode_flags, r; assert(path); assert(open_flags); assert(ret_file); mode_flags = fopen_mode_to_flags(open_flags); if (mode_flags < 0) return mode_flags; fd = chase_symlinks_and_open(path, root, chase_flags, mode_flags, ret_path ? &final_path : NULL); if (fd < 0) return fd; r = take_fdopen_unlocked(&fd, open_flags, ret_file); if (r < 0) return r; if (ret_path) *ret_path = TAKE_PTR(final_path); return 0; }