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fscrypt: introduce helper function for filename matching 

Introduce a helper function fscrypt_match_name() which tests whether a
fscrypt_name matches a directory entry.  Also clean up the magic numbers
and document things properly.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Eric Biggers 2017-04-24 10:00:10 -07:00 committed by Theodore Ts'o
parent 6b06cdee81
commit 17159420a6
4 changed files with 157 additions and 22 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

90
fs/crypto/fname.c
View File

@ -159,6 +159,8 @@ static int fname_decrypt(struct inode *inode,
static const char *lookup_table =
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,";
#define BASE64_CHARS(nbytes) DIV_ROUND_UP((nbytes) * 4, 3)
/**
* digest_encode() -
*
@ -230,11 +232,14 @@ EXPORT_SYMBOL(fscrypt_fname_encrypted_size);
int fscrypt_fname_alloc_buffer(const struct inode *inode,
u32 ilen, struct fscrypt_str *crypto_str)
{
unsigned int olen = fscrypt_fname_encrypted_size(inode, ilen);
u32 olen = fscrypt_fname_encrypted_size(inode, ilen);
const u32 max_encoded_len =
max_t(u32, BASE64_CHARS(FSCRYPT_FNAME_MAX_UNDIGESTED_SIZE),
1 + BASE64_CHARS(sizeof(struct fscrypt_digested_name)));
crypto_str->len = olen;
if (olen < FS_FNAME_CRYPTO_DIGEST_SIZE * 2)
olen = FS_FNAME_CRYPTO_DIGEST_SIZE * 2;
olen = max(olen, max_encoded_len);
/*
* Allocated buffer can hold one more character to null-terminate the
* string
@ -266,6 +271,10 @@ EXPORT_SYMBOL(fscrypt_fname_free_buffer);
*
* The caller must have allocated sufficient memory for the @oname string.
*
* If the key is available, we'll decrypt the disk name; otherwise, we'll encode
* it for presentation. Short names are directly base64-encoded, while long
* names are encoded in fscrypt_digested_name format.
*
* Return: 0 on success, -errno on failure
*/
int fscrypt_fname_disk_to_usr(struct inode *inode,
@ -274,7 +283,7 @@ int fscrypt_fname_disk_to_usr(struct inode *inode,
struct fscrypt_str *oname)
{
const struct qstr qname = FSTR_TO_QSTR(iname);
char buf[24];
struct fscrypt_digested_name digested_name;
if (fscrypt_is_dot_dotdot(&qname)) {
oname->name[0] = '.';
@ -289,20 +298,24 @@ int fscrypt_fname_disk_to_usr(struct inode *inode,
if (inode->i_crypt_info)
return fname_decrypt(inode, iname, oname);
if (iname->len <= FS_FNAME_CRYPTO_DIGEST_SIZE) {
if (iname->len <= FSCRYPT_FNAME_MAX_UNDIGESTED_SIZE) {
oname->len = digest_encode(iname->name, iname->len,
oname->name);
return 0;
}
if (hash) {
memcpy(buf, &hash, 4);
memcpy(buf + 4, &minor_hash, 4);
digested_name.hash = hash;
digested_name.minor_hash = minor_hash;
} else {
memset(buf, 0, 8);
digested_name.hash = 0;
digested_name.minor_hash = 0;
}
memcpy(buf + 8, iname->name + ((iname->len - 17) & ~15), 16);
memcpy(digested_name.digest,
FSCRYPT_FNAME_DIGEST(iname->name, iname->len),
FSCRYPT_FNAME_DIGEST_SIZE);
oname->name[0] = '_';
oname->len = 1 + digest_encode(buf, 24, oname->name + 1);
oname->len = 1 + digest_encode((const char *)&digested_name,
sizeof(digested_name), oname->name + 1);
return 0;
}
EXPORT_SYMBOL(fscrypt_fname_disk_to_usr);
@ -336,10 +349,35 @@ int fscrypt_fname_usr_to_disk(struct inode *inode,
}
EXPORT_SYMBOL(fscrypt_fname_usr_to_disk);
/**
* fscrypt_setup_filename() - prepare to search a possibly encrypted directory
* @dir: the directory that will be searched
* @iname: the user-provided filename being searched for
* @lookup: 1 if we're allowed to proceed without the key because it's
* ->lookup() or we're finding the dir_entry for deletion; 0 if we cannot
* proceed without the key because we're going to create the dir_entry.
* @fname: the filename information to be filled in
*
* Given a user-provided filename @iname, this function sets @fname->disk_name
* to the name that would be stored in the on-disk directory entry, if possible.
* If the directory is unencrypted this is simply @iname. Else, if we have the
* directory's encryption key, then @iname is the plaintext, so we encrypt it to
* get the disk_name.
*
* Else, for keyless @lookup operations, @iname is the presented ciphertext, so
* we decode it to get either the ciphertext disk_name (for short names) or the
* fscrypt_digested_name (for long names). Non-@lookup operations will be
* impossible in this case, so we fail them with ENOKEY.
*
* If successful, fscrypt_free_filename() must be called later to clean up.
*
* Return: 0 on success, -errno on failure
*/
int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
int lookup, struct fscrypt_name *fname)
{
int ret = 0, bigname = 0;
int ret;
int digested;
memset(fname, 0, sizeof(struct fscrypt_name));
fname->usr_fname = iname;
@ -373,25 +411,37 @@ int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
* We don't have the key and we are doing a lookup; decode the
* user-supplied name
*/
if (iname->name[0] == '_')
bigname = 1;
if ((bigname && (iname->len != 33)) || (!bigname && (iname->len > 43)))
return -ENOENT;
if (iname->name[0] == '_') {
if (iname->len !=
1 + BASE64_CHARS(sizeof(struct fscrypt_digested_name)))
return -ENOENT;
digested = 1;
} else {
if (iname->len >
BASE64_CHARS(FSCRYPT_FNAME_MAX_UNDIGESTED_SIZE))
return -ENOENT;
digested = 0;
}
fname->crypto_buf.name = kmalloc(32, GFP_KERNEL);
fname->crypto_buf.name =
kmalloc(max_t(size_t, FSCRYPT_FNAME_MAX_UNDIGESTED_SIZE,
sizeof(struct fscrypt_digested_name)),
GFP_KERNEL);
if (fname->crypto_buf.name == NULL)
return -ENOMEM;
ret = digest_decode(iname->name + bigname, iname->len - bigname,
ret = digest_decode(iname->name + digested, iname->len - digested,
fname->crypto_buf.name);
if (ret < 0) {
ret = -ENOENT;
goto errout;
}
fname->crypto_buf.len = ret;
if (bigname) {
memcpy(&fname->hash, fname->crypto_buf.name, 4);
memcpy(&fname->minor_hash, fname->crypto_buf.name + 4, 4);
if (digested) {
const struct fscrypt_digested_name *n =
(const void *)fname->crypto_buf.name;
fname->hash = n->hash;
fname->minor_hash = n->minor_hash;
} else {
fname->disk_name.name = fname->crypto_buf.name;
fname->disk_name.len = fname->crypto_buf.len;

2
fs/crypto/fscrypt_private.h
View File

@ -13,8 +13,6 @@
#include <linux/fscrypt_supp.h>
#define FS_FNAME_CRYPTO_DIGEST_SIZE 32
/* Encryption parameters */
#define FS_XTS_TWEAK_SIZE 16
#define FS_AES_128_ECB_KEY_SIZE 16

9
include/linux/fscrypt_notsupp.h
View File

@ -147,6 +147,15 @@ static inline int fscrypt_fname_usr_to_disk(struct inode *inode,
return -EOPNOTSUPP;
}
static inline bool fscrypt_match_name(const struct fscrypt_name *fname,
const u8 *de_name, u32 de_name_len)
{
/* Encryption support disabled; use standard comparison */
if (de_name_len != fname->disk_name.len)
return false;
return !memcmp(de_name, fname->disk_name.name, fname->disk_name.len);
}
/* bio.c */
static inline void fscrypt_decrypt_bio_pages(struct fscrypt_ctx *ctx,
struct bio *bio)

78
include/linux/fscrypt_supp.h
View File

@ -57,6 +57,84 @@ extern int fscrypt_fname_disk_to_usr(struct inode *, u32, u32,
extern int fscrypt_fname_usr_to_disk(struct inode *, const struct qstr *,
struct fscrypt_str *);
#define FSCRYPT_FNAME_MAX_UNDIGESTED_SIZE 32
/* Extracts the second-to-last ciphertext block; see explanation below */
#define FSCRYPT_FNAME_DIGEST(name, len) \
((name) + round_down((len) - FS_CRYPTO_BLOCK_SIZE - 1, \
FS_CRYPTO_BLOCK_SIZE))
#define FSCRYPT_FNAME_DIGEST_SIZE FS_CRYPTO_BLOCK_SIZE
/**
* fscrypt_digested_name - alternate identifier for an on-disk filename
*
* When userspace lists an encrypted directory without access to the key,
* filenames whose ciphertext is longer than FSCRYPT_FNAME_MAX_UNDIGESTED_SIZE
* bytes are shown in this abbreviated form (base64-encoded) rather than as the
* full ciphertext (base64-encoded). This is necessary to allow supporting
* filenames up to NAME_MAX bytes, since base64 encoding expands the length.
*
* To make it possible for filesystems to still find the correct directory entry
* despite not knowing the full on-disk name, we encode any filesystem-specific
* 'hash' and/or 'minor_hash' which the filesystem may need for its lookups,
* followed by the second-to-last ciphertext block of the filename. Due to the
* use of the CBC-CTS encryption mode, the second-to-last ciphertext block
* depends on the full plaintext. (Note that ciphertext stealing causes the
* last two blocks to appear "flipped".) This makes collisions very unlikely:
* just a 1 in 2^128 chance for two filenames to collide even if they share the
* same filesystem-specific hashes.
*
* This scheme isn't strictly immune to intentional collisions because it's
* basically like a CBC-MAC, which isn't secure on variable-length inputs.
* However, generating a CBC-MAC collision requires the ability to choose
* arbitrary ciphertext, which won't normally be possible with filename
* encryption since it would require write access to the raw disk.
*
* Taking a real cryptographic hash like SHA-256 over the full ciphertext would
* be better in theory but would be less efficient and more complicated to
* implement, especially since the filesystem would need to calculate it for
* each directory entry examined during a search.
*/
struct fscrypt_digested_name {
u32 hash;
u32 minor_hash;
u8 digest[FSCRYPT_FNAME_DIGEST_SIZE];
};
/**
* fscrypt_match_name() - test whether the given name matches a directory entry
* @fname: the name being searched for
* @de_name: the name from the directory entry
* @de_name_len: the length of @de_name in bytes
*
* Normally @fname->disk_name will be set, and in that case we simply compare
* that to the name stored in the directory entry. The only exception is that
* if we don't have the key for an encrypted directory and a filename in it is
* very long, then we won't have the full disk_name and we'll instead need to
* match against the fscrypt_digested_name.
*
* Return: %true if the name matches, otherwise %false.
*/
static inline bool fscrypt_match_name(const struct fscrypt_name *fname,
const u8 *de_name, u32 de_name_len)
{
if (unlikely(!fname->disk_name.name)) {
const struct fscrypt_digested_name *n =
(const void *)fname->crypto_buf.name;
if (WARN_ON_ONCE(fname->usr_fname->name[0] != '_'))
return false;
if (de_name_len <= FSCRYPT_FNAME_MAX_UNDIGESTED_SIZE)
return false;
return !memcmp(FSCRYPT_FNAME_DIGEST(de_name, de_name_len),
n->digest, FSCRYPT_FNAME_DIGEST_SIZE);
}
if (de_name_len != fname->disk_name.len)
return false;
return !memcmp(de_name, fname->disk_name.name, fname->disk_name.len);
}
/* bio.c */
extern void fscrypt_decrypt_bio_pages(struct fscrypt_ctx *, struct bio *);
extern void fscrypt_pullback_bio_page(struct page **, bool);