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systemd/src/basic/chase-symlinks.c
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/* 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;
/* 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;
}
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