From 107d75ca9394481bd045385fc45f2ee65b30ad16 Mon Sep 17 00:00:00 2001

From: Lennart Poettering <lennart@poettering.net>

Date: Fri, 19 Oct 2018 11:26:59 +0200

Subject: [PATCH] chown-recursive: let's rework the recursive logic to use

 O_PATH

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

(cherry-picked from commit 5de6cce58b3e8b79239b6e83653459d91af6e57c)

Resolves: #1643368

---

 src/core/chown-recursive.c | 146 ++++++++++++++++++-------------------

 1 file changed, 70 insertions(+), 76 deletions(-)

diff --git a/src/core/chown-recursive.c b/src/core/chown-recursive.c

index c4794501c2..27c64489b5 100644

--- a/src/core/chown-recursive.c

+++ b/src/core/chown-recursive.c

@@ -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)

-                return -errno;

+        /* 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);

-        /* 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)

+        if (chown(procfs_path, uid, gid) < 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) {

-                        r = -errno;

-                        goto finish;

-                }

-                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;

-                        }

+        d = fdopendir(fd);

+        if (!d) {

+                safe_close(fd);

+                return -errno;

+        }

+

+        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;

                 }

+        }

-                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;