Re: [PATCH v2] eCryptfs: Improve statfs reporting

[John Johansen <john.johansen@canonical.com>]

On 12/19/2011 04:31 PM, Tyler Hicks wrote:
> statfs() calls on eCryptfs files returned the wrong filesystem type and,
> when using filename encryption, the wrong maximum filename length.
> 
> If mount-wide filename encryption is enabled, the cipher block size and
> the lower filesystem's max filename length will determine the max
> eCryptfs filename length. Pre-tested, known good lengths are used when
> the lower filesystem's namelen is 255 and a cipher with 8 or 16 byte
> block sizes is used. In other, less common cases, we fall back to a safe
> rounded-down estimate when determining the eCryptfs namelen.
> 
> https://launchpad.net/bugs/885744
> 
> Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
> Reported-by: Kees Cook <keescook@chromium.org>

So this looks good
Reviewed-by: John Johansen <john.johansen@canonical.com>

> ---
> Changes from v1:
>  - provide more clarity in the commit message
>  - unify code paths for setting namelen when lower_namelen == NAME_MAX and
>    cipher block size is 8 or 16 bytes
>  - change ecryptfs_set_f_namelen() parameter types from size_t to long, to
>    match the type of struct kstatfs.f_namelen
>  - check if (*namelen) is less than zero after going through the estimation
>    algorithm and, if so, set it to 0
> 
>  fs/ecryptfs/crypto.c          |   68 ++++++++++++++++++++++++++++++++++++----
>  fs/ecryptfs/ecryptfs_kernel.h |    5 +++
>  fs/ecryptfs/keystore.c        |    9 ++---
>  fs/ecryptfs/super.c           |   14 ++++++++-
>  4 files changed, 82 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/ecryptfs/crypto.c b/fs/ecryptfs/crypto.c
> index 2a83425..d3c8776 100644
> --- a/fs/ecryptfs/crypto.c
> +++ b/fs/ecryptfs/crypto.c
> @@ -2026,6 +2026,17 @@ out:
>  	return;
>  }
>  
> +static size_t ecryptfs_max_decoded_size(size_t encoded_size)
> +{
> +	/* Not exact; conservatively long. Every block of 4
> +	 * encoded characters decodes into a block of 3
> +	 * decoded characters. This segment of code provides
> +	 * the caller with the maximum amount of allocated
> +	 * space that @dst will need to point to in a
> +	 * subsequent call. */
> +	return ((encoded_size + 1) * 3) / 4;
> +}
> +
>  /**
>   * ecryptfs_decode_from_filename
>   * @dst: If NULL, this function only sets @dst_size and returns. If
> @@ -2044,13 +2055,7 @@ ecryptfs_decode_from_filename(unsigned char *dst, size_t *dst_size,
>  	size_t dst_byte_offset = 0;
>  
>  	if (dst == NULL) {
> -		/* Not exact; conservatively long. Every block of 4
> -		 * encoded characters decodes into a block of 3
> -		 * decoded characters. This segment of code provides
> -		 * the caller with the maximum amount of allocated
> -		 * space that @dst will need to point to in a
> -		 * subsequent call. */
> -		(*dst_size) = (((src_size + 1) * 3) / 4);
> +		(*dst_size) = ecryptfs_max_decoded_size(src_size);
>  		goto out;
>  	}
>  	while (src_byte_offset < src_size) {
> @@ -2275,3 +2280,52 @@ out_free:
>  out:
>  	return rc;
>  }
> +
> +#define ENC_NAME_MAX_BLOCKLEN_8_OR_16	143
> +
> +int ecryptfs_set_f_namelen(long *namelen, long lower_namelen,
> +			   struct ecryptfs_mount_crypt_stat *mount_crypt_stat)
> +{
> +	struct blkcipher_desc desc;
> +	struct mutex *tfm_mutex;
> +	size_t cipher_blocksize;
> +	int rc;
> +
> +	if (!(mount_crypt_stat->flags & ECRYPTFS_GLOBAL_ENCRYPT_FILENAMES)) {
> +		(*namelen) = lower_namelen;
> +		return 0;
> +	}
> +
> +	rc = ecryptfs_get_tfm_and_mutex_for_cipher_name(&desc.tfm, &tfm_mutex,
> +			mount_crypt_stat->global_default_fn_cipher_name);
> +	if (unlikely(rc)) {
> +		(*namelen) = 0;
> +		return rc;
> +	}
> +
> +	mutex_lock(tfm_mutex);
> +	cipher_blocksize = crypto_blkcipher_blocksize(desc.tfm);
> +	mutex_unlock(tfm_mutex);
> +
> +	/* Return an exact amount for the common cases */
> +	if (lower_namelen == NAME_MAX
> +	    && (cipher_blocksize == 8 || cipher_blocksize == 16)) {
> +		(*namelen) = ENC_NAME_MAX_BLOCKLEN_8_OR_16;
> +		return 0;
> +	}
> +
> +	/* Return a safe estimate for the uncommon cases */
> +	(*namelen) = lower_namelen;
> +	(*namelen) -= ECRYPTFS_FNEK_ENCRYPTED_FILENAME_PREFIX_SIZE;
> +	/* Since this is the max decoded size, subtract 1 "decoded block" len */
> +	(*namelen) = ecryptfs_max_decoded_size(*namelen) - 3;
> +	(*namelen) -= ECRYPTFS_TAG_70_MAX_METADATA_SIZE;
> +	(*namelen) -= ECRYPTFS_FILENAME_MIN_RANDOM_PREPEND_BYTES;
> +	/* Worst case is that the filename is padded nearly a full block size */
> +	(*namelen) -= cipher_blocksize - 1;
> +
> +	if ((*namelen) < 0)
> +		(*namelen) = 0;
> +
> +	return 0;
> +}
> diff --git a/fs/ecryptfs/ecryptfs_kernel.h b/fs/ecryptfs/ecryptfs_kernel.h
> index a9f29b1..7cad6b0 100644
> --- a/fs/ecryptfs/ecryptfs_kernel.h
> +++ b/fs/ecryptfs/ecryptfs_kernel.h
> @@ -157,6 +157,9 @@ ecryptfs_get_key_payload_data(struct key *key)
>  #define ECRYPTFS_NON_NULL 0x42 /* A reasonable substitute for NULL */
>  #define MD5_DIGEST_SIZE 16
>  #define ECRYPTFS_TAG_70_DIGEST_SIZE MD5_DIGEST_SIZE
> +#define ECRYPTFS_TAG_70_MIN_METADATA_SIZE (1 + 1 + ECRYPTFS_SIG_SIZE + 1 + 1)
> +#define ECRYPTFS_TAG_70_MAX_METADATA_SIZE (ECRYPTFS_TAG_70_MIN_METADATA_SIZE \
> +					   + 1)
>  #define ECRYPTFS_FEK_ENCRYPTED_FILENAME_PREFIX "ECRYPTFS_FEK_ENCRYPTED."
>  #define ECRYPTFS_FEK_ENCRYPTED_FILENAME_PREFIX_SIZE 23
>  #define ECRYPTFS_FNEK_ENCRYPTED_FILENAME_PREFIX "ECRYPTFS_FNEK_ENCRYPTED."
> @@ -696,6 +699,8 @@ ecryptfs_parse_tag_70_packet(char **filename, size_t *filename_size,
>  			     size_t *packet_size,
>  			     struct ecryptfs_mount_crypt_stat *mount_crypt_stat,
>  			     char *data, size_t max_packet_size);
> +int ecryptfs_set_f_namelen(long *namelen, long lower_namelen,
> +			   struct ecryptfs_mount_crypt_stat *mount_crypt_stat);
>  int ecryptfs_derive_iv(char *iv, struct ecryptfs_crypt_stat *crypt_stat,
>  		       loff_t offset);
>  
> diff --git a/fs/ecryptfs/keystore.c b/fs/ecryptfs/keystore.c
> index ac1ad48..06aab59 100644
> --- a/fs/ecryptfs/keystore.c
> +++ b/fs/ecryptfs/keystore.c
> @@ -678,10 +678,7 @@ ecryptfs_write_tag_70_packet(char *dest, size_t *remaining_bytes,
>  	 * Octets N3-N4: Block-aligned encrypted filename
>  	 *  - Consists of a minimum number of random characters, a \0
>  	 *    separator, and then the filename */
> -	s->max_packet_size = (1                   /* Tag 70 identifier */
> -			      + 3                 /* Max Tag 70 packet size */
> -			      + ECRYPTFS_SIG_SIZE /* FNEK sig */
> -			      + 1                 /* Cipher identifier */
> +	s->max_packet_size = (ECRYPTFS_TAG_70_MAX_METADATA_SIZE
>  			      + s->block_aligned_filename_size);
>  	if (dest == NULL) {
>  		(*packet_size) = s->max_packet_size;
> @@ -933,10 +930,10 @@ ecryptfs_parse_tag_70_packet(char **filename, size_t *filename_size,
>  		goto out;
>  	}
>  	s->desc.flags = CRYPTO_TFM_REQ_MAY_SLEEP;
> -	if (max_packet_size < (1 + 1 + ECRYPTFS_SIG_SIZE + 1 + 1)) {
> +	if (max_packet_size < ECRYPTFS_TAG_70_MIN_METADATA_SIZE) {
>  		printk(KERN_WARNING "%s: max_packet_size is [%zd]; it must be "
>  		       "at least [%d]\n", __func__, max_packet_size,
> -			(1 + 1 + ECRYPTFS_SIG_SIZE + 1 + 1));
> +		       ECRYPTFS_TAG_70_MIN_METADATA_SIZE);
>  		rc = -EINVAL;
>  		goto out;
>  	}
> diff --git a/fs/ecryptfs/super.c b/fs/ecryptfs/super.c
> index dbd52d40..a04d9ef 100644
> --- a/fs/ecryptfs/super.c
> +++ b/fs/ecryptfs/super.c
> @@ -30,6 +30,8 @@
>  #include <linux/seq_file.h>
>  #include <linux/file.h>
>  #include <linux/crypto.h>
> +#include <linux/statfs.h>
> +#include <linux/magic.h>
>  #include "ecryptfs_kernel.h"
>  
>  struct kmem_cache *ecryptfs_inode_info_cache;
> @@ -103,10 +105,20 @@ static void ecryptfs_destroy_inode(struct inode *inode)
>  static int ecryptfs_statfs(struct dentry *dentry, struct kstatfs *buf)
>  {
>  	struct dentry *lower_dentry = ecryptfs_dentry_to_lower(dentry);
> +	int rc;
>  
>  	if (!lower_dentry->d_sb->s_op->statfs)
>  		return -ENOSYS;
> -	return lower_dentry->d_sb->s_op->statfs(lower_dentry, buf);
> +
> +	rc = lower_dentry->d_sb->s_op->statfs(lower_dentry, buf);
> +	if (rc)
> +		return rc;
> +
> +	buf->f_type = ECRYPTFS_SUPER_MAGIC;
> +	rc = ecryptfs_set_f_namelen(&buf->f_namelen, buf->f_namelen,
> +	       &ecryptfs_superblock_to_private(dentry->d_sb)->mount_crypt_stat);
> +
> +	return rc;
>  }
>  
>  /**