diff options
Diffstat (limited to 'src/basic/chase-symlinks.c')
| -rw-r--r-- | src/basic/chase-symlinks.c | 603 |
1 files changed, 603 insertions, 0 deletions
diff --git a/src/basic/chase-symlinks.c b/src/basic/chase-symlinks.c new file mode 100644 index 0000000..ac55311 --- /dev/null +++ b/src/basic/chase-symlinks.c @@ -0,0 +1,603 @@ +/* SPDX-License-Identifier: LGPL-2.1-or-later */ + +#include <linux/magic.h> + +#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; + + /* 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. CHASE_WARN cannot + * be used in PID 1. + * + * 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. CHASE_WARN cannot be used in PID 1. + */ + + /* 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; + } + + close_and_replace(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. */ + close_and_replace(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|CHASE_STEP)) + 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 | (FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? O_NOFOLLOW : 0)); + 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|CHASE_STEP)) + 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 = xopendirat(path_fd, ".", O_NOFOLLOW); + 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|CHASE_STEP)) + return -EINVAL; + + if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE)) == 0 && !ret_fd) { + /* Shortcut this call if none of the special features of this call are requested */ + + if (fstatat(AT_FDCWD, path, ret_stat, FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0) < 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_access( + const char *path, + const char *root, + ChaseSymlinksFlags chase_flags, + int access_mode, + char **ret_path, + int *ret_fd) { + + _cleanup_close_ int path_fd = -1; + _cleanup_free_ char *p = NULL; + int r; + + assert(path); + + if (chase_flags & (CHASE_NONEXISTENT|CHASE_STEP)) + return -EINVAL; + + if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE)) == 0 && !ret_fd) { + /* Shortcut this call if none of the special features of this call are requested */ + + if (faccessat(AT_FDCWD, path, access_mode, FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0) < 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); + + r = access_fd(path_fd, access_mode); + if (r < 0) + return r; + + 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; +} |