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SubjectRe: [PATCH] ext4 crypto: migrate into vfs's crypto engine
On Thu, May 05, 2016 at 08:44:12AM -0400, Theodore Ts'o wrote:
> On Wed, May 04, 2016 at 09:22:48PM -0700, Jaegeuk Kim wrote:
> >
> > Got it. Let me add (*key_prefix(inode)) in fscrypt_operations so that filesystem
> > can give a specific prefix additionally.
> > Once fscrypto supports both of prefixes, does e4crypto have to set "fscrypto"?
> > The "ext4" should work all the time tho.
>
> Well, the question is what userspace tool should be used for the
> non-Android case?
>
> The other thing to consider is the cost of doing lookups in two
> keyrings all of the time. I'm not sure how much overhead there is,
> but if it's significant, it might be time to hang a keyring off the
> superblock, and to start migrating to a model where keys are globally
> available to the file system once they get their (either implicitly
> because they are on a user's keyring, or explicitly via a new ioctl),
> and when they are removed (either implicitly because a key has been
> invalidated, or explicitly via a new ioctl), when a key gets added or
> removed from the file system global keyring, we bump a sequence
> counter so we can invalidate negative or positve dcache entries as
> appropriate.

The below patch implements allowing one more prefix given by fs.

Regarding to the overhead, I measured elapsed time of each function in
get_crypt_info() under x86 server.
And, I could see mostly negligible latencies including double check one.

Especially, the result shows just zero latencies except some cold misses
made by get_context(), derive_key_aes(), and crypto_alloc_skcipher(); those
are to read xattrs or load kernel crypto modules, I think.
After cold misses, I couldn't see any delay from get_crypt_info() actually.

Moreover, when considering prefix check is done by request_key() in early stage,
IMHO, calling it twice would not be a big deal.

Thought?

Thanks,

From 08eb7ef36243728e00d9ffc6c56fa7d088d3d27c Mon Sep 17 00:00:00 2001
From: Jaegeuk Kim <jaegeuk@kernel.org>
Date: Wed, 4 May 2016 22:05:01 -0700
Subject: [PATCH] fscrypto/f2fs: allow fs-specific key prefix for fs encryption

This patch allows fscrypto to handle a second key prefix given by filesystem.
The main reason is to provide backward compatibility, since previously f2fs
used "f2fs:" as a crypto prefix instead of "fscrypt:".
Later, ext4 should also provide key_prefix() to give "ext4:".

One concern decribed by Ted would be kinda double check overhead of prefixes.
In x86, for example, validate_user_key consumes 8 ms after boot-up, which turns
out derive_key_aes() consumed most of the time to load specific crypto module.
After such the cold miss, it shows almost zero latencies, which treats as a
negligible overhead.
Note that request_key() detects wrong prefix in prior to derive_key_aes() even.

Cc: Ted Tso <tytso@mit.edu>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
---
fs/crypto/keyinfo.c | 120 ++++++++++++++++++++++++++++++-----------------
fs/f2fs/f2fs.h | 8 ++++
fs/f2fs/super.c | 13 +++++
include/linux/fscrypto.h | 1 +
4 files changed, 98 insertions(+), 44 deletions(-)

diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c
index 06f5aa4..1ac263e 100644
--- a/fs/crypto/keyinfo.c
+++ b/fs/crypto/keyinfo.c
@@ -78,6 +78,67 @@ out:
return res;
}

+static int validate_user_key(struct fscrypt_info *crypt_info,
+ struct fscrypt_context *ctx, u8 *raw_key,
+ u8 *prefix, int prefix_size)
+{
+ u8 *full_key_descriptor;
+ struct key *keyring_key;
+ struct fscrypt_key *master_key;
+ const struct user_key_payload *ukp;
+ int full_key_len = prefix_size + (FS_KEY_DESCRIPTOR_SIZE * 2) + 1;
+ int res;
+
+ full_key_descriptor = kmalloc(full_key_len, GFP_NOFS);
+ if (!full_key_descriptor)
+ return -ENOMEM;
+
+ memcpy(full_key_descriptor, prefix, prefix_size);
+ sprintf(full_key_descriptor + prefix_size,
+ "%*phN", FS_KEY_DESCRIPTOR_SIZE,
+ ctx->master_key_descriptor);
+ full_key_descriptor[full_key_len - 1] = '\0';
+ keyring_key = request_key(&key_type_logon, full_key_descriptor, NULL);
+ kfree(full_key_descriptor);
+ if (IS_ERR(keyring_key))
+ return PTR_ERR(keyring_key);
+
+ if (keyring_key->type != &key_type_logon) {
+ printk_once(KERN_WARNING
+ "%s: key type must be logon\n", __func__);
+ res = -ENOKEY;
+ goto out;
+ }
+ down_read(&keyring_key->sem);
+ ukp = user_key_payload(keyring_key);
+ if (ukp->datalen != sizeof(struct fscrypt_key)) {
+ res = -EINVAL;
+ up_read(&keyring_key->sem);
+ goto out;
+ }
+ master_key = (struct fscrypt_key *)ukp->data;
+ BUILD_BUG_ON(FS_AES_128_ECB_KEY_SIZE != FS_KEY_DERIVATION_NONCE_SIZE);
+
+ if (master_key->size != FS_AES_256_XTS_KEY_SIZE) {
+ printk_once(KERN_WARNING
+ "%s: key size incorrect: %d\n",
+ __func__, master_key->size);
+ res = -ENOKEY;
+ up_read(&keyring_key->sem);
+ goto out;
+ }
+ res = derive_key_aes(ctx->nonce, master_key->raw, raw_key);
+ up_read(&keyring_key->sem);
+ if (res)
+ goto out;
+
+ crypt_info->ci_keyring_key = keyring_key;
+ return 0;
+out:
+ key_put(keyring_key);
+ return res;
+}
+
static void put_crypt_info(struct fscrypt_info *ci)
{
if (!ci)
@@ -91,12 +152,7 @@ static void put_crypt_info(struct fscrypt_info *ci)
int get_crypt_info(struct inode *inode)
{
struct fscrypt_info *crypt_info;
- u8 full_key_descriptor[FS_KEY_DESC_PREFIX_SIZE +
- (FS_KEY_DESCRIPTOR_SIZE * 2) + 1];
- struct key *keyring_key = NULL;
- struct fscrypt_key *master_key;
struct fscrypt_context ctx;
- const struct user_key_payload *ukp;
struct crypto_skcipher *ctfm;
const char *cipher_str;
u8 raw_key[FS_MAX_KEY_SIZE];
@@ -167,48 +223,24 @@ retry:
memset(raw_key, 0x42, FS_AES_256_XTS_KEY_SIZE);
goto got_key;
}
- memcpy(full_key_descriptor, FS_KEY_DESC_PREFIX,
- FS_KEY_DESC_PREFIX_SIZE);
- sprintf(full_key_descriptor + FS_KEY_DESC_PREFIX_SIZE,
- "%*phN", FS_KEY_DESCRIPTOR_SIZE,
- ctx.master_key_descriptor);
- full_key_descriptor[FS_KEY_DESC_PREFIX_SIZE +
- (2 * FS_KEY_DESCRIPTOR_SIZE)] = '\0';
- keyring_key = request_key(&key_type_logon, full_key_descriptor, NULL);
- if (IS_ERR(keyring_key)) {
- res = PTR_ERR(keyring_key);
- keyring_key = NULL;
- goto out;
- }
- crypt_info->ci_keyring_key = keyring_key;
- if (keyring_key->type != &key_type_logon) {
- printk_once(KERN_WARNING
- "%s: key type must be logon\n", __func__);
- res = -ENOKEY;
- goto out;
- }
- down_read(&keyring_key->sem);
- ukp = user_key_payload(keyring_key);
- if (ukp->datalen != sizeof(struct fscrypt_key)) {
- res = -EINVAL;
- up_read(&keyring_key->sem);
- goto out;
- }
- master_key = (struct fscrypt_key *)ukp->data;
- BUILD_BUG_ON(FS_AES_128_ECB_KEY_SIZE != FS_KEY_DERIVATION_NONCE_SIZE);

- if (master_key->size != FS_AES_256_XTS_KEY_SIZE) {
- printk_once(KERN_WARNING
- "%s: key size incorrect: %d\n",
- __func__, master_key->size);
- res = -ENOKEY;
- up_read(&keyring_key->sem);
+ res = validate_user_key(crypt_info, &ctx, raw_key,
+ FS_KEY_DESC_PREFIX, FS_KEY_DESC_PREFIX_SIZE);
+ if (res && inode->i_sb->s_cop->key_prefix) {
+ u8 *prefix = NULL;
+ int prefix_size, res2;
+
+ prefix_size = inode->i_sb->s_cop->key_prefix(inode, &prefix);
+ res2 = validate_user_key(crypt_info, &ctx, raw_key,
+ prefix, prefix_size);
+ if (res2) {
+ if (res2 == -ENOKEY)
+ res = -ENOKEY;
+ goto out;
+ }
+ } else if (res) {
goto out;
}
- res = derive_key_aes(ctx.nonce, master_key->raw, raw_key);
- up_read(&keyring_key->sem);
- if (res)
- goto out;
got_key:
ctfm = crypto_alloc_skcipher(cipher_str, 0, 0);
if (!ctfm || IS_ERR(ctfm)) {
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index fd1b4a5..052f5a8 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -711,6 +711,10 @@ enum {
MAX_TIME,
};

+#ifdef CONFIG_F2FS_FS_ENCRYPTION
+#define F2FS_KEY_DESC_PREFIX "f2fs:"
+#define F2FS_KEY_DESC_PREFIX_SIZE 5
+#endif
struct f2fs_sb_info {
struct super_block *sb; /* pointer to VFS super block */
struct proc_dir_entry *s_proc; /* proc entry */
@@ -718,6 +722,10 @@ struct f2fs_sb_info {
int valid_super_block; /* valid super block no */
int s_flag; /* flags for sbi */

+#ifdef CONFIG_F2FS_FS_ENCRYPTION
+ u8 key_prefix[F2FS_KEY_DESC_PREFIX_SIZE];
+ u8 key_prefix_size;
+#endif
/* for node-related operations */
struct f2fs_nm_info *nm_info; /* node manager */
struct inode *node_inode; /* cache node blocks */
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 8a28f79..28c8992 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -964,6 +964,12 @@ static int f2fs_get_context(struct inode *inode, void *ctx, size_t len)
ctx, len, NULL);
}

+static int f2fs_key_prefix(struct inode *inode, u8 **key)
+{
+ *key = F2FS_I_SB(inode)->key_prefix;
+ return F2FS_I_SB(inode)->key_prefix_size;
+}
+
static int f2fs_set_context(struct inode *inode, const void *ctx, size_t len,
void *fs_data)
{
@@ -980,6 +986,7 @@ static unsigned f2fs_max_namelen(struct inode *inode)

static struct fscrypt_operations f2fs_cryptops = {
.get_context = f2fs_get_context,
+ .key_prefix = f2fs_key_prefix,
.set_context = f2fs_set_context,
.is_encrypted = f2fs_encrypted_inode,
.empty_dir = f2fs_empty_dir,
@@ -1305,6 +1312,12 @@ static void init_sb_info(struct f2fs_sb_info *sbi)

INIT_LIST_HEAD(&sbi->s_list);
mutex_init(&sbi->umount_mutex);
+
+#ifdef CONFIG_F2FS_FS_ENCRYPTION
+ memcpy(sbi->key_prefix, F2FS_KEY_DESC_PREFIX,
+ F2FS_KEY_DESC_PREFIX_SIZE);
+ sbi->key_prefix_size = F2FS_KEY_DESC_PREFIX_SIZE;
+#endif
}

/*
diff --git a/include/linux/fscrypto.h b/include/linux/fscrypto.h
index 6027f6b..cfa6cde 100644
--- a/include/linux/fscrypto.h
+++ b/include/linux/fscrypto.h
@@ -175,6 +175,7 @@ struct fscrypt_name {
*/
struct fscrypt_operations {
int (*get_context)(struct inode *, void *, size_t);
+ int (*key_prefix)(struct inode *, u8 **);
int (*prepare_context)(struct inode *);
int (*set_context)(struct inode *, const void *, size_t, void *);
int (*dummy_context)(struct inode *);
--
2.6.3
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