Re: [RFC PATCH v2 16/18] ceph: add support to readdir for encrypted filenames

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Sep 04, 2020 at 12:05:35PM -0400, Jeff Layton wrote:
> diff --git a/fs/ceph/crypto.h b/fs/ceph/crypto.h
> index 1b11e9af165e..88c672ccdcf8 100644
> --- a/fs/ceph/crypto.h
> +++ b/fs/ceph/crypto.h
> @@ -6,6 +6,8 @@
>  #ifndef _CEPH_CRYPTO_H
>  #define _CEPH_CRYPTO_H
>  
> +#include <linux/fscrypt.h>
> +
>  #ifdef CONFIG_FS_ENCRYPTION
>  
>  #define	CEPH_XATTR_NAME_ENCRYPTION_CONTEXT	"encryption.ctx"
> @@ -16,6 +18,29 @@ int ceph_fscrypt_set_ops(struct super_block *sb);
>  int ceph_fscrypt_prepare_context(struct inode *dir, struct inode *inode,
>  				 struct ceph_acl_sec_ctx *as);
>  
> +static inline int ceph_fname_alloc_buffer(struct inode *parent, struct fscrypt_str *fname)
> +{
> +	if (!IS_ENCRYPTED(parent))
> +		return 0;
> +	return fscrypt_fname_alloc_buffer(NAME_MAX, fname);
> +}
> +
> +static inline void ceph_fname_free_buffer(struct inode *parent, struct fscrypt_str *fname)
> +{
> +	if (IS_ENCRYPTED(parent))
> +		fscrypt_fname_free_buffer(fname);
> +}
> +
> +static inline int ceph_get_encryption_info(struct inode *inode)
> +{
> +	if (!IS_ENCRYPTED(inode))
> +		return 0;
> +	return fscrypt_get_encryption_info(inode);
> +}
> +
> +int ceph_fname_to_usr(struct inode *parent, char *name, u32 len,
> +			struct fscrypt_str *tname, struct fscrypt_str *oname);
> +
>  #else /* CONFIG_FS_ENCRYPTION */
>  
>  #define DUMMY_ENCRYPTION_ENABLED(fsc) (0)
> @@ -31,6 +56,28 @@ static inline int ceph_fscrypt_prepare_context(struct inode *dir, struct inode *
>  	return 0;
>  }
>  
> +static inline int ceph_fname_alloc_buffer(struct inode *parent, struct fscrypt_str *fname)
> +{
> +	return 0;
> +}
> +
> +static inline void ceph_fname_free_buffer(struct inode *parent, struct fscrypt_str *fname)
> +{
> +}
> +
> +static inline int ceph_get_encryption_info(struct inode *inode)
> +{
> +	return 0;
> +}

This makes it so that readdir will succeed on encrypted directories when
!CONFIG_FS_ENCRYPTION.  The other filesystems instead return an error code,
which seems much better.  Can you check what the other filesystems handle
readdir?

> +static bool fscrypt_key_status_change(struct dentry *dentry)
> +{
> +	struct inode *dir;
> +	bool encrypted_name, have_key;
> +
> +	lockdep_assert_held(&dentry->d_lock);
> +
> +	dir = d_inode(dentry->d_parent);
> +	if (!IS_ENCRYPTED(dir))
> +		return false;
> +
> +	encrypted_name = dentry->d_flags & DCACHE_ENCRYPTED_NAME;
> +	have_key = fscrypt_has_encryption_key(dir);
> +
> +	if (encrypted_name == have_key)
> +		ceph_dir_clear_complete(dir);
> +
> +	dout("%s encrypted_name=%d have_key=%d\n", __func__, encrypted_name, have_key);
> +	return encrypted_name == have_key;
> +}
> +

Only the no-key => key case needs to be handled, not key => no-key.
Also, the caller already has 'dir', so there's no need to use ->d_parent.

What's wrong with just:

                di = ceph_dentry(dentry);
                if (d_unhashed(dentry) ||
                    d_really_is_negative(dentry) ||
                    di->lease_shared_gen != shared_gen ||
+                   ((dentry->d_flags & DCACHE_ENCRYPTED_NAME) &&
+                    fscrypt_has_encryption_key(dir)))  {
                        spin_unlock(&dentry->d_lock);
                        dput(dentry);
                        err = -EAGAIN;
                        goto out;
                }
>  /*
>   * When possible, we try to satisfy a readdir by peeking at the
>   * dcache.  We make this work by carefully ordering dentries on
> @@ -238,11 +261,11 @@ static int __dcache_readdir(struct file *file,  struct dir_context *ctx,
>  			goto out;
>  		}
>  
> -		spin_lock(&dentry->d_lock);

Why delete this spin_lock()?

> +			if (IS_ENCRYPTED(inode) && !fscrypt_has_encryption_key(inode)) {
> +				spin_lock(&dn->d_lock);
> +				dn->d_flags |= DCACHE_ENCRYPTED_NAME;
> +				spin_unlock(&dn->d_lock);
> +			}

This is racy because fscrypt_has_encryption_key() could have been false when the
dentry was created, then true here.

Take a look at how __fscrypt_prepare_lookup() solves this problem.

- Eric