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chown-recursive: let's rework the recursive logic to use O_PATH

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That way we can pin a specific inode and analyze it and manipulate it
without it being swapped out beneath our hands.

Fixes a vulnerability originally found by Jann Horn from Google.

CVE-2018-15687
LP: #1796692
https://bugzilla.redhat.com/show_bug.cgi?id=1639076
This commit is contained in:
Lennart Poettering 2018-10-19 11:26:59 +02:00
parent 845702c638
commit 5de6cce58b
1 changed files with 69 additions and 75 deletions
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144
src/core/chown-recursive.c
View file

@ -1,17 +1,19 @@
/* SPDX-License-Identifier: LGPL-2.1+ */
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include "user-util.h"
#include "macro.h"
#include "fd-util.h"
#include "dirent-util.h"
#include "chown-recursive.h"
#include "dirent-util.h"
#include "fd-util.h"
#include "macro.h"
#include "stdio-util.h"
#include "strv.h"
#include "user-util.h"
static int chown_one(int fd, const char *name, const struct stat *st, uid_t uid, gid_t gid) {
int r;
static int chown_one(int fd, const struct stat *st, uid_t uid, gid_t gid) {
char procfs_path[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int) + 1];
assert(fd >= 0);
assert(st);
@ -20,90 +22,82 @@ static int chown_one(int fd, const char *name, const struct stat *st, uid_t uid,
(!gid_is_valid(gid) || st->st_gid == gid))
return 0;
if (name)
r = fchownat(fd, name, uid, gid, AT_SYMLINK_NOFOLLOW);
else
r = fchown(fd, uid, gid);
if (r < 0)
/* We change ownership through the /proc/self/fd/%i path, so that we have a stable reference that works with
* O_PATH. (Note: fchown() and fchmod() do not work with O_PATH, the kernel refuses that. */
xsprintf(procfs_path, "/proc/self/fd/%i", fd);
if (chown(procfs_path, uid, gid) < 0)
return -errno;
/* The linux kernel alters the mode in some cases of chown(). Let's undo this. */
if (name) {
if (!S_ISLNK(st->st_mode))
r = fchmodat(fd, name, st->st_mode, 0);
else /* There's currently no AT_SYMLINK_NOFOLLOW for fchmodat() */
r = 0;
} else
r = fchmod(fd, st->st_mode);
if (r < 0)
return -errno;
/* The linux kernel alters the mode in some cases of chown(). Let's undo this. We do this only for non-symlinks
* however. That's because for symlinks the access mode is ignored anyway and because on some kernels/file
* systems trying to change the access mode will succeed but has no effect while on others it actively
* fails. */
if (!S_ISLNK(st->st_mode))
if (chmod(procfs_path, st->st_mode & 07777) < 0)
return -errno;
return 1;
}
static int chown_recursive_internal(int fd, const struct stat *st, uid_t uid, gid_t gid) {
_cleanup_closedir_ DIR *d = NULL;
bool changed = false;
struct dirent *de;
int r;
assert(fd >= 0);
assert(st);
if (S_ISDIR(st->st_mode)) {
_cleanup_closedir_ DIR *d = NULL;
struct dirent *de;
d = fdopendir(fd);
if (!d) {
safe_close(fd);
return -errno;
}
d = fdopendir(fd);
if (!d) {
r = -errno;
goto finish;
FOREACH_DIRENT_ALL(de, d, return -errno) {
_cleanup_close_ int path_fd = -1;
struct stat fst;
if (dot_or_dot_dot(de->d_name))
continue;
/* Let's pin the child inode we want to fix now with an O_PATH fd, so that it cannot be swapped out
* while we manipulate it. */
path_fd = openat(dirfd(d), de->d_name, O_PATH|O_CLOEXEC|O_NOFOLLOW);
if (path_fd < 0)
return -errno;
if (fstat(path_fd, &fst) < 0)
return -errno;
if (S_ISDIR(fst.st_mode)) {
int subdir_fd;
/* Convert it to a "real" (i.e. non-O_PATH) fd now */
subdir_fd = fd_reopen(path_fd, O_RDONLY|O_CLOEXEC|O_NOATIME);
if (subdir_fd < 0)
return subdir_fd;
r = chown_recursive_internal(subdir_fd, &fst, uid, gid); /* takes possession of subdir_fd even on failure */
if (r < 0)
return r;
if (r > 0)
changed = true;
} else {
r = chown_one(path_fd, &fst, uid, gid);
if (r < 0)
return r;
if (r > 0)
changed = true;
}
fd = -1;
}
FOREACH_DIRENT_ALL(de, d, r = -errno; goto finish) {
struct stat fst;
if (dot_or_dot_dot(de->d_name))
continue;
if (fstatat(dirfd(d), de->d_name, &fst, AT_SYMLINK_NOFOLLOW) < 0) {
r = -errno;
goto finish;
}
if (S_ISDIR(fst.st_mode)) {
int subdir_fd;
subdir_fd = openat(dirfd(d), de->d_name, O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC|O_NOFOLLOW|O_NOATIME);
if (subdir_fd < 0) {
r = -errno;
goto finish;
}
r = chown_recursive_internal(subdir_fd, &fst, uid, gid);
if (r < 0)
goto finish;
if (r > 0)
changed = true;
} else {
r = chown_one(dirfd(d), de->d_name, &fst, uid, gid);
if (r < 0)
goto finish;
if (r > 0)
changed = true;
}
}
r = chown_one(dirfd(d), NULL, st, uid, gid);
} else
r = chown_one(fd, NULL, st, uid, gid);
r = chown_one(dirfd(d), st, uid, gid);
if (r < 0)
goto finish;
return r;
r = r > 0 || changed;
finish:
safe_close(fd);
return r;
return r > 0 || changed;
}
int path_chown_recursive(const char *path, uid_t uid, gid_t gid) {
@ -111,7 +105,7 @@ int path_chown_recursive(const char *path, uid_t uid, gid_t gid) {
struct stat st;
int r;
fd = open(path, O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC|O_NOFOLLOW|O_NOATIME);
fd = open(path, O_RDONLY|O_DIRECTORY|O_CLOEXEC|O_NOFOLLOW|O_NOATIME);
if (fd < 0)
return -errno;