ACKED: [PATCH] eCryptfs: Improve statfs reporting
Andy Whitcroft
apw at canonical.com
Fri Dec 23 11:36:55 UTC 2011
On Thu, Dec 22, 2011 at 03:22:28PM -0600, 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.
I assume the point here is that userspace cannot do the right thing and
use filenames which fit if we lie to it about how large a filename may
be. So that makes sense.
> BugLink: https://launchpad.net/bugs/885744
>
> Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
> Reported-by: Kees Cook <keescook at chromium.org>
> Reviewed-by: Kees Cook <keescook at chromium.org>
> Reviewed-by: John Johansen <john.johansen at canonical.com>
> ---
> 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;
This is outside the encrypted part.
> + /* 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;
These are inside the encrypted part.
> + /* Worst case is that the filename is padded nearly a full block size */
> + (*namelen) -= cipher_blocksize - 1;
Ouch that is painful, we can waste most of an encoded block and that
waste tip us into a new cipher block as well which is then wasted.
Gah. That seems like the worst case calculation.
I suspect that there is a slightly more accuract way to calculate this
as rather than taking the mad decode block-1 and cypher_blocksize-1 off.
Those presumably are aligned with respect to their starts and we might
be able to use that to get closer. Something like this (untested,
approximate, ish, just to illustrate whats in my mind):
(*namelen) = ecryptfs_max_decoded_size(*namelen + 3) & ~3;
and
(*namelen) -= (*namelen + cipher_blocksize - 1) % cipher_blocksize;
Anyhow, that isn't going to make this patch any less valid as an under
estimate is significantly better that what we have now, this would be
an optimisation.
Other than that perhaps personally I would have calculated the namelen in a
local variable assigning it once if valid, it looks to do what is intended.
> +
> + 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);
So before we just handed over the stat call to the lower filesystem and
assumed that was good.
> +
> + 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);
Now we update the fstype and the namelen. Seems much more sensible.
The only other question is can we make some estimate of our overhead on
size and reduce the maximum capacity perhaps; likely impossible to get a
reasonable value and out of scope for this patch.
> +
> + return rc;
> }
>
> /**
Based on the above:
Acked-by: Andy Whitcroft <apw at canonical.com>
-apw
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