Commit message: vfs: read file_handle only once in handle_to_path

We used to read file_handle twice.  Once to get the amount of extra
bytes, and once to fetch the entire structure.

This may be problematic since we do size verifications only after the
first read, so if the number of extra bytes changes in userspace between
the first and second calls, we'll have an incoherent view of
file_handle.

Instead, read the constant size once, and copy that over to the final
structure without having to re-read it again.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2015-1420
CWE: CWE-362
Vulnerability description: Race condition in the handle_to_path function in fs/fhandle.c in the Linux kernel through 3.19.1 allows local users to bypass intended size restrictions and trigger read operations on additional memory locations by changing the handle_bytes value of a file handle during the execution of this function.

Project: linux
File: fhandle.c

SYSCALL_DEFINE3(open_by_handle_at, int, mountdirfd,
		struct file_handle __user *, handle,
		int, flags)
{
	long ret;

	if (force_o_largefile())
		flags |= O_LARGEFILE;

	ret = do_handle_open(mountdirfd, handle, flags);
	return ret;
}

Project: linux
File: fhandle.c

SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
		struct file_handle __user *, handle, int __user *, mnt_id,
		int, flag)
{
	struct path path;
	int lookup_flags;
	int err;

	if ((flag & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH)) != 0)
		return -EINVAL;

	lookup_flags = (flag & AT_SYMLINK_FOLLOW) ? LOOKUP_FOLLOW : 0;
	if (flag & AT_EMPTY_PATH)
		lookup_flags |= LOOKUP_EMPTY;
	err = user_path_at(dfd, name, lookup_flags, &path);
	if (!err) {
		err = do_sys_name_to_handle(&path, handle, mnt_id);
		path_put(&path);
	}
	return err;
}

Project: linux
File: fhandle.c

static int vfs_dentry_acceptable(void *context, struct dentry *dentry)
{
	return 1;
}

Project: linux
File: fhandle.c

static int do_handle_to_path(int mountdirfd, struct file_handle *handle,
			     struct path *path)
{
	int retval = 0;
	int handle_dwords;

	path->mnt = get_vfsmount_from_fd(mountdirfd);
	if (IS_ERR(path->mnt)) {
		retval = PTR_ERR(path->mnt);
		goto out_err;
	}
	/* change the handle size to multiple of sizeof(u32) */
	handle_dwords = handle->handle_bytes >> 2;
	path->dentry = exportfs_decode_fh(path->mnt,
					  (struct fid *)handle->f_handle,
					  handle_dwords, handle->handle_type,
					  vfs_dentry_acceptable, NULL);
	if (IS_ERR(path->dentry)) {
		retval = PTR_ERR(path->dentry);
		goto out_mnt;
	}
	return 0;
out_mnt:
	mntput(path->mnt);
out_err:
	return retval;
}

Project: linux
File: fhandle.c

static long do_sys_name_to_handle(struct path *path,
				  struct file_handle __user *ufh,
				  int __user *mnt_id)
{
	long retval;
	struct file_handle f_handle;
	int handle_dwords, handle_bytes;
	struct file_handle *handle = NULL;

	/*
	 * We need to make sure whether the file system
	 * support decoding of the file handle
	 */
	if (!path->dentry->d_sb->s_export_op ||
	    !path->dentry->d_sb->s_export_op->fh_to_dentry)
		return -EOPNOTSUPP;

	if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle)))
		return -EFAULT;

	if (f_handle.handle_bytes > MAX_HANDLE_SZ)
		return -EINVAL;

	handle = kmalloc(sizeof(struct file_handle) + f_handle.handle_bytes,
			 GFP_KERNEL);
	if (!handle)
		return -ENOMEM;

	/* convert handle size to multiple of sizeof(u32) */
	handle_dwords = f_handle.handle_bytes >> 2;

	/* we ask for a non connected handle */
	retval = exportfs_encode_fh(path->dentry,
				    (struct fid *)handle->f_handle,
				    &handle_dwords,  0);
	handle->handle_type = retval;
	/* convert handle size to bytes */
	handle_bytes = handle_dwords * sizeof(u32);
	handle->handle_bytes = handle_bytes;
	if ((handle->handle_bytes > f_handle.handle_bytes) ||
	    (retval == FILEID_INVALID) || (retval == -ENOSPC)) {
		/* As per old exportfs_encode_fh documentation
		 * we could return ENOSPC to indicate overflow
		 * But file system returned 255 always. So handle
		 * both the values
		 */
		/*
		 * set the handle size to zero so we copy only
		 * non variable part of the file_handle
		 */
		handle_bytes = 0;
		retval = -EOVERFLOW;
	} else
		retval = 0;
	/* copy the mount id */
	if (copy_to_user(mnt_id, &real_mount(path->mnt)->mnt_id,
			 sizeof(*mnt_id)) ||
	    copy_to_user(ufh, handle,
			 sizeof(struct file_handle) + handle_bytes))
		retval = -EFAULT;
	kfree(handle);
	return retval;
}

Project: linux
File: fhandle.c

long do_handle_open(int mountdirfd,
		    struct file_handle __user *ufh, int open_flag)
{
	long retval = 0;
	struct path path;
	struct file *file;
	int fd;

	retval = handle_to_path(mountdirfd, ufh, &path);
	if (retval)
		return retval;

	fd = get_unused_fd_flags(open_flag);
	if (fd < 0) {
		path_put(&path);
		return fd;
	}
	file = file_open_root(path.dentry, path.mnt, "", open_flag);
	if (IS_ERR(file)) {
		put_unused_fd(fd);
		retval =  PTR_ERR(file);
	} else {
		retval = fd;
		fsnotify_open(file);
		fd_install(fd, file);
	}
	path_put(&path);
	return retval;
}

Project: linux
File: fhandle.c

static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
		   struct path *path)
{
	int retval = 0;
	struct file_handle f_handle;
	struct file_handle *handle = NULL;

	/*
	 * With handle we don't look at the execute bit on the
	 * the directory. Ideally we would like CAP_DAC_SEARCH.
	 * But we don't have that
	 */
	if (!capable(CAP_DAC_READ_SEARCH)) {
		retval = -EPERM;
		goto out_err;
	}
	if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle))) {
		retval = -EFAULT;
		goto out_err;
	}
	if ((f_handle.handle_bytes > MAX_HANDLE_SZ) ||
	    (f_handle.handle_bytes == 0)) {
		retval = -EINVAL;
		goto out_err;
	}
	handle = kmalloc(sizeof(struct file_handle) + f_handle.handle_bytes,
			 GFP_KERNEL);
	if (!handle) {
		retval = -ENOMEM;
		goto out_err;
	}
	/* copy the full handle */
	*handle = f_handle;
	if (copy_from_user(&handle->f_handle,
			   &ufh->f_handle,
			   f_handle.handle_bytes)) {
		retval = -EFAULT;
		goto out_handle;
	}

	retval = do_handle_to_path(mountdirfd, handle, path);

out_handle:
	kfree(handle);
out_err:
	return retval;
}

Project: linux
File: fhandle.c

static struct vfsmount *get_vfsmount_from_fd(int fd)
{
	struct vfsmount *mnt;

	if (fd == AT_FDCWD) {
		struct fs_struct *fs = current->fs;
		spin_lock(&fs->lock);
		mnt = mntget(fs->pwd.mnt);
		spin_unlock(&fs->lock);
	} else {
		struct fd f = fdget(fd);
		if (!f.file)
			return ERR_PTR(-EBADF);
		mnt = mntget(f.file->f_path.mnt);
		fdput(f);
	}
	return mnt;
}