eCryptfs: Improve statfs reporting

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>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: John Johansen <john.johansen@canonical.com>

4 files changed, 83 insertions, 14 deletions

diff --git a/fs/ecryptfs/crypto.c b/fs/ecryptfs/crypto.c
index 63ab24510649..ea9931281557 100644
--- a/fs/ecryptfs/crypto.c
+++ b/fs/ecryptfs/crypto.c
@@ -1990,6 +1990,17 @@ 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
@@ -2008,13 +2019,7 @@ 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) {
@@ -2239,3 +2244,52 @@ 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 a2362df58ae8..867b64c5d84f 100644
--- a/fs/ecryptfs/ecryptfs_kernel.h
+++ b/fs/ecryptfs/ecryptfs_kernel.h
@@ -162,6 +162,10 @@ 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 + ECRYPTFS_MIN_PKT_LEN_SIZE \
+                                           + ECRYPTFS_SIG_SIZE + 1 + 1)
+#define ECRYPTFS_TAG_70_MAX_METADATA_SIZE (1 + ECRYPTFS_MAX_PKT_LEN_SIZE \
+                                           + ECRYPTFS_SIG_SIZE + 1 + 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."
@@ -701,6 +705,8 @@ 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 8e3b943e330f..2333203a120b 100644
--- a/fs/ecryptfs/keystore.c
+++ b/fs/ecryptfs/keystore.c
@@ -679,10 +679,7 @@ 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;
@@ -934,10 +931,10 @@ 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 9df7fd6e0c39..cf152823bbf4 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;
@@ -102,10 +104,20 @@ 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;
 }
 
 /**