get rid of unprotected dereferencing of mnt->mnt_ns

It's safe only under namespace_sem or vfsmount_lock; all places
in fs/namespace.c that want mnt->mnt_ns->user_ns actually want to use
current->nsproxy->mnt_ns->user_ns (note the calls of check_mnt() in
there).

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namespace.c b/fs/namespace.c
index 55605c552787cae1188ef93ee522ddd958e8cd83..edac42c6eff22b556c63bd17d386f6998d6f6141 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1237,6 +1237,14 @@ static int do_umount(struct mount *mnt, int flags)
        return retval;
 }
 
+/* 
+ * Is the caller allowed to modify his namespace?
+ */
+static inline bool may_mount(void)
+{
+       return ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN);
+}
+
 /*
  * Now umount can handle mount points as well as block devices.
  * This is important for filesystems which use unnamed block devices.
@@ -1255,6 +1263,9 @@ SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
        if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
                return -EINVAL;
 
+       if (!may_mount())
+               return -EPERM;
+
        if (!(flags & UMOUNT_NOFOLLOW))
                lookup_flags |= LOOKUP_FOLLOW;
 
@@ -1268,10 +1279,6 @@ SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
        if (!check_mnt(mnt))
                goto dput_and_out;
 
-       retval = -EPERM;
-       if (!ns_capable(mnt->mnt_ns->user_ns, CAP_SYS_ADMIN))
-               goto dput_and_out;
-
        retval = do_umount(mnt, flags);
 dput_and_out:
        /* we mustn't call path_put() as that would clear mnt_expiry_mark */
@@ -1295,7 +1302,7 @@ SYSCALL_DEFINE1(oldumount, char __user *, name)
 
 static int mount_is_safe(struct path *path)
 {
-       if (ns_capable(real_mount(path->mnt)->mnt_ns->user_ns, CAP_SYS_ADMIN))
+       if (may_mount())
                return 0;
        return -EPERM;
 #ifdef notyet
@@ -1633,7 +1640,7 @@ static int do_change_type(struct path *path, int flag)
        int type;
        int err = 0;
 
-       if (!ns_capable(mnt->mnt_ns->user_ns, CAP_SYS_ADMIN))
+       if (!may_mount())
                return -EPERM;
 
        if (path->dentry != path->mnt->mnt_root)
@@ -1797,7 +1804,7 @@ static int do_move_mount(struct path *path, const char *old_name)
        struct mount *p;
        struct mount *old;
        int err = 0;
-       if (!ns_capable(real_mount(path->mnt)->mnt_ns->user_ns, CAP_SYS_ADMIN))
+       if (!may_mount())
                return -EPERM;
        if (!old_name || !*old_name)
                return -EINVAL;
@@ -1933,16 +1940,14 @@ static int do_new_mount(struct path *path, const char *fstype, int flags,
                        int mnt_flags, const char *name, void *data)
 {
        struct file_system_type *type;
-       struct user_namespace *user_ns;
+       struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns;
        struct vfsmount *mnt;
        int err;
 
        if (!fstype)
                return -EINVAL;
 
-       /* we need capabilities... */
-       user_ns = real_mount(path->mnt)->mnt_ns->user_ns;
-       if (!ns_capable(user_ns, CAP_SYS_ADMIN))
+       if (!may_mount())
                return -EPERM;
 
        type = get_fs_type(fstype);
@@ -2567,7 +2572,7 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
        struct mount *new_mnt, *root_mnt;
        int error;
 
-       if (!ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN))
+       if (!may_mount())
                return -EPERM;
 
        error = user_path_dir(new_root, &new);