Commit bd0d97b7 authored by Eric Biggers's avatar Eric Biggers
Browse files

fscrypt: switch fscrypt_do_sha256() to use the SHA-256 library 

fscrypt_do_sha256() is only used for hashing encrypted filenames to
create no-key tokens, which isn't performance-critical.  Therefore a C
implementation of SHA-256 is sufficient.

Also, the logic to create no-key tokens is always potentially needed.
This differs from fscrypt's other dependencies on crypto API algorithms,
which are conditionally needed depending on what encryption policies
userspace is using.  Therefore, for fscrypt there isn't much benefit to
allowing SHA-256 to be a loadable module.

So, make fscrypt_do_sha256() use the SHA-256 library instead of the
crypto_shash API.  This is much simpler, since it avoids having to
implement one-time-init (which is hard to do correctly, and in fact was
implemented incorrectly) and handle failures to allocate the
crypto_shash object.

Fixes: edc440e3 ("fscrypt: improve format of no-key names")
Cc: Daniel Rosenberg <drosen@google.com>
Link: https://lore.kernel.org/r/20200721225920.114347-2-ebiggers@kernel.org


Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
parent f000223c

fs/crypto/Kconfig

+1 −1
Original line number Diff line number Diff line
@@ -4,6 +4,7 @@ config FS_ENCRYPTION 
	select CRYPTO

	select CRYPTO_HASH

	select CRYPTO_SKCIPHER

	select CRYPTO_LIB_SHA256

	select KEYS

	help

	  Enable encryption of files and directories.  This
@@ -21,7 +22,6 @@ config FS_ENCRYPTION_ALGS 
	select CRYPTO_CTS

	select CRYPTO_ECB

	select CRYPTO_HMAC

	select CRYPTO_SHA256

	select CRYPTO_SHA512

	select CRYPTO_XTS

fs/crypto/fname.c

+10 −31
Original line number Diff line number Diff line
@@ -61,30 +61,13 @@ struct fscrypt_nokey_name { 
 */

#define FSCRYPT_NOKEY_NAME_MAX	offsetofend(struct fscrypt_nokey_name, sha256)



static struct crypto_shash *sha256_hash_tfm;



static int fscrypt_do_sha256(const u8 *data, unsigned int data_len, u8 *result)

static void fscrypt_do_sha256(const u8 *data, unsigned int data_len, u8 *result)

{

	struct crypto_shash *tfm = READ_ONCE(sha256_hash_tfm);



	if (unlikely(!tfm)) {

		struct crypto_shash *prev_tfm;

	struct sha256_state sctx;



		tfm = crypto_alloc_shash("sha256", 0, 0);

		if (IS_ERR(tfm)) {

			fscrypt_err(NULL,

				    "Error allocating SHA-256 transform: %ld",

				    PTR_ERR(tfm));

			return PTR_ERR(tfm);

		}

		prev_tfm = cmpxchg(&sha256_hash_tfm, NULL, tfm);

		if (prev_tfm) {

			crypto_free_shash(tfm);

			tfm = prev_tfm;

		}

	}



	return crypto_shash_tfm_digest(tfm, data, data_len, result);

	sha256_init(&sctx);

	sha256_update(&sctx, data, data_len);

	sha256_final(&sctx, result);

}



static inline bool fscrypt_is_dot_dotdot(const struct qstr *str)
@@ -349,7 +332,6 @@ int fscrypt_fname_disk_to_usr(const struct inode *inode, 
	const struct qstr qname = FSTR_TO_QSTR(iname);

	struct fscrypt_nokey_name nokey_name;

	u32 size; /* size of the unencoded no-key name */

	int err;



	if (fscrypt_is_dot_dotdot(&qname)) {

		oname->name[0] = '.';
@@ -387,11 +369,9 @@ int fscrypt_fname_disk_to_usr(const struct inode *inode, 
	} else {

		memcpy(nokey_name.bytes, iname->name, sizeof(nokey_name.bytes));

		/* Compute strong hash of remaining part of name. */

		err = fscrypt_do_sha256(&iname->name[sizeof(nokey_name.bytes)],

		fscrypt_do_sha256(&iname->name[sizeof(nokey_name.bytes)],

				  iname->len - sizeof(nokey_name.bytes),

				  nokey_name.sha256);

		if (err)

			return err;

		size = FSCRYPT_NOKEY_NAME_MAX;

	}

	oname->len = base64_encode((const u8 *)&nokey_name, size, oname->name);
@@ -530,9 +510,8 @@ bool fscrypt_match_name(const struct fscrypt_name *fname, 
		return false;

	if (memcmp(de_name, nokey_name->bytes, sizeof(nokey_name->bytes)))

		return false;

	if (fscrypt_do_sha256(&de_name[sizeof(nokey_name->bytes)],

			      de_name_len - sizeof(nokey_name->bytes), sha256))

		return false;

	fscrypt_do_sha256(&de_name[sizeof(nokey_name->bytes)],

			  de_name_len - sizeof(nokey_name->bytes), sha256);

	return !memcmp(sha256, nokey_name->sha256, sizeof(sha256));

}

EXPORT_SYMBOL_GPL(fscrypt_match_name);

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