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>

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
+		(*dst_size) = ecryptfs_max_decoded_size(src_size);
-		 * 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);
 		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;
+}

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);
 

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 */
+	s->max_packet_size = (ECRYPTFS_TAG_70_MAX_METADATA_SIZE
-			      + 3                 /* Max Tag 70 packet size */
-			      + ECRYPTFS_SIG_SIZE /* FNEK sig */
-			      + 1                 /* Cipher identifier */
 			      + 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;
 	}

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;
 }
 
 /**