fscrypt_private.h

/* SPDX-License-Identifier: GPL-2.0 */
/*
 * fscrypt_private.h
 *
 * Copyright (C) 2015, Google, Inc.
 *
 * Originally written by Michael Halcrow, Ildar Muslukhov, and Uday Savagaonkar.
 * Heavily modified since then.
 */

#ifndef _FSCRYPT_PRIVATE_H
#define _FSCRYPT_PRIVATE_H

#include <linux/fscrypt.h>
#include <linux/siphash.h>
#include <crypto/hash.h>

#define CONST_STRLEN(str)	(sizeof(str) - 1)

#define FS_KEY_DERIVATION_NONCE_SIZE	16

#define FSCRYPT_MIN_KEY_SIZE		16

#define FSCRYPT_CONTEXT_V1	1
#define FSCRYPT_CONTEXT_V2	2

struct fscrypt_context_v1 {
	u8 version; /* FSCRYPT_CONTEXT_V1 */
	u8 contents_encryption_mode;
	u8 filenames_encryption_mode;
	u8 flags;
	u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
	u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE];
};

struct fscrypt_context_v2 {
	u8 version; /* FSCRYPT_CONTEXT_V2 */
	u8 contents_encryption_mode;
	u8 filenames_encryption_mode;
	u8 flags;
	u8 __reserved[4];
	u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
	u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE];
};

/**
 * fscrypt_context - the encryption context of an inode
 *
 * This is the on-disk equivalent of an fscrypt_policy, stored alongside each
 * encrypted file usually in a hidden extended attribute.  It contains the
 * fields from the fscrypt_policy, in order to identify the encryption algorithm
 * and key with which the file is encrypted.  It also contains a nonce that was
 * randomly generated by fscrypt itself; this is used as KDF input or as a tweak
 * to cause different files to be encrypted differently.
 */
union fscrypt_context {
	u8 version;
	struct fscrypt_context_v1 v1;
	struct fscrypt_context_v2 v2;
};

/*
 * Return the size expected for the given fscrypt_context based on its version
 * number, or 0 if the context version is unrecognized.
 */
static inline int fscrypt_context_size(const union fscrypt_context *ctx)
{
	switch (ctx->version) {
	case FSCRYPT_CONTEXT_V1:
		BUILD_BUG_ON(sizeof(ctx->v1) != 28);
		return sizeof(ctx->v1);
	case FSCRYPT_CONTEXT_V2:
		BUILD_BUG_ON(sizeof(ctx->v2) != 40);
		return sizeof(ctx->v2);
	}
	return 0;
}

#undef fscrypt_policy
union fscrypt_policy {
	u8 version;
	struct fscrypt_policy_v1 v1;
	struct fscrypt_policy_v2 v2;
};

/*
 * Return the size expected for the given fscrypt_policy based on its version
 * number, or 0 if the policy version is unrecognized.
 */
static inline int fscrypt_policy_size(const union fscrypt_policy *policy)
{
	switch (policy->version) {
	case FSCRYPT_POLICY_V1:
		return sizeof(policy->v1);
	case FSCRYPT_POLICY_V2:
		return sizeof(policy->v2);
	}
	return 0;
}

/* Return the contents encryption mode of a valid encryption policy */
static inline u8
fscrypt_policy_contents_mode(const union fscrypt_policy *policy)
{
	switch (policy->version) {
	case FSCRYPT_POLICY_V1:
		return policy->v1.contents_encryption_mode;
	case FSCRYPT_POLICY_V2:
		return policy->v2.contents_encryption_mode;
	}
	BUG();
}

/* Return the filenames encryption mode of a valid encryption policy */
static inline u8
fscrypt_policy_fnames_mode(const union fscrypt_policy *policy)
{
	switch (policy->version) {
	case FSCRYPT_POLICY_V1:
		return policy->v1.filenames_encryption_mode;
	case FSCRYPT_POLICY_V2:
		return policy->v2.filenames_encryption_mode;
	}
	BUG();
}

/* Return the flags (FSCRYPT_POLICY_FLAG*) of a valid encryption policy */
static inline u8
fscrypt_policy_flags(const union fscrypt_policy *policy)
{
	switch (policy->version) {
	case FSCRYPT_POLICY_V1:
		return policy->v1.flags;
	case FSCRYPT_POLICY_V2:
		return policy->v2.flags;
	}
	BUG();
}

/**
 * For encrypted symlinks, the ciphertext length is stored at the beginning
 * of the string in little-endian format.
 */
struct fscrypt_symlink_data {
	__le16 len;
	char encrypted_path[1];
} __packed;

/*
 * fscrypt_info - the "encryption key" for an inode
 *
 * When an encrypted file's key is made available, an instance of this struct is
 * allocated and stored in ->i_crypt_info.  Once created, it remains until the
 * inode is evicted.
 */
struct fscrypt_info {

	/* The actual crypto transform used for encryption and decryption */
	struct crypto_skcipher *ci_ctfm;

	/* True if the key should be freed when this fscrypt_info is freed */
	bool ci_owns_key;

	/*
	 * Encryption mode used for this inode.  It corresponds to either the
	 * contents or filenames encryption mode, depending on the inode type.
	 */
	struct fscrypt_mode *ci_mode;

	/* Back-pointer to the inode */
	struct inode *ci_inode;

	/*
	 * The master key with which this inode was unlocked (decrypted).  This
	 * will be NULL if the master key was found in a process-subscribed
	 * keyring rather than in the filesystem-level keyring.
	 */
	struct key *ci_master_key;

	/*
	 * Link in list of inodes that were unlocked with the master key.
	 * Only used when ->ci_master_key is set.
	 */
	struct list_head ci_master_key_link;

	/*
	 * If non-NULL, then encryption is done using the master key directly
	 * and ci_ctfm will equal ci_direct_key->dk_ctfm.
	 */
	struct fscrypt_direct_key *ci_direct_key;

	/*
	 * This inode's hash key for filenames.  This is a 128-bit SipHash-2-4
	 * key.  This is only set for directories that use a keyed dirhash over
	 * the plaintext filenames -- currently just casefolded directories.
	 */
	siphash_key_t ci_dirhash_key;
	bool ci_dirhash_key_initialized;

	/* The encryption policy used by this inode */
	union fscrypt_policy ci_policy;

	/* This inode's nonce, copied from the fscrypt_context */
	u8 ci_nonce[FS_KEY_DERIVATION_NONCE_SIZE];
};

typedef enum {
	FS_DECRYPT = 0,
	FS_ENCRYPT,
} fscrypt_direction_t;

/* crypto.c */
extern struct kmem_cache *fscrypt_info_cachep;
extern int fscrypt_initialize(unsigned int cop_flags);
extern int fscrypt_crypt_block(const struct inode *inode,
			       fscrypt_direction_t rw, u64 lblk_num,
			       struct page *src_page, struct page *dest_page,
			       unsigned int len, unsigned int offs,
			       gfp_t gfp_flags);
extern struct page *fscrypt_alloc_bounce_page(gfp_t gfp_flags);

extern void __printf(3, 4) __cold
fscrypt_msg(const struct inode *inode, const char *level, const char *fmt, ...);

#define fscrypt_warn(inode, fmt, ...)		\
	fscrypt_msg((inode), KERN_WARNING, fmt, ##__VA_ARGS__)
#define fscrypt_err(inode, fmt, ...)		\
	fscrypt_msg((inode), KERN_ERR, fmt, ##__VA_ARGS__)

#define FSCRYPT_MAX_IV_SIZE	32

union fscrypt_iv {
	struct {
		/* logical block number within the file */
		__le64 lblk_num;

		/* per-file nonce; only set in DIRECT_KEY mode */
		u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE];
	};
	u8 raw[FSCRYPT_MAX_IV_SIZE];
};

void fscrypt_generate_iv(union fscrypt_iv *iv, u64 lblk_num,
			 const struct fscrypt_info *ci);

/* fname.c */
extern int fscrypt_fname_encrypt(const struct inode *inode,
				 const struct qstr *iname,
				 u8 *out, unsigned int olen);
extern bool fscrypt_fname_encrypted_size(const struct inode *inode,
					 u32 orig_len, u32 max_len,
					 u32 *encrypted_len_ret);
extern const struct dentry_operations fscrypt_d_ops;

/* hkdf.c */

struct fscrypt_hkdf {
	struct crypto_shash *hmac_tfm;
};

extern int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
			     unsigned int master_key_size);

/*
 * The list of contexts in which fscrypt uses HKDF.  These values are used as
 * the first byte of the HKDF application-specific info string to guarantee that
 * info strings are never repeated between contexts.  This ensures that all HKDF
 * outputs are unique and cryptographically isolated, i.e. knowledge of one
 * output doesn't reveal another.
 */
#define HKDF_CONTEXT_KEY_IDENTIFIER	1
#define HKDF_CONTEXT_PER_FILE_ENC_KEY	2
#define HKDF_CONTEXT_DIRECT_KEY		3
#define HKDF_CONTEXT_IV_INO_LBLK_64_KEY	4
#define HKDF_CONTEXT_DIRHASH_KEY	5

extern int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
			       const u8 *info, unsigned int infolen,
			       u8 *okm, unsigned int okmlen);

extern void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf);

/* keyring.c */

/*
 * fscrypt_master_key_secret - secret key material of an in-use master key
 */
struct fscrypt_master_key_secret {

	/*
	 * For v2 policy keys: HKDF context keyed by this master key.
	 * For v1 policy keys: not set (hkdf.hmac_tfm == NULL).
	 */
	struct fscrypt_hkdf	hkdf;

	/* Size of the raw key in bytes.  Set even if ->raw isn't set. */
	u32			size;

	/* For v1 policy keys: the raw key.  Wiped for v2 policy keys. */
	u8			raw[FSCRYPT_MAX_KEY_SIZE];

} __randomize_layout;

/*
 * fscrypt_master_key - an in-use master key
 *
 * This represents a master encryption key which has been added to the
 * filesystem and can be used to "unlock" the encrypted files which were
 * encrypted with it.
 */
struct fscrypt_master_key {

	/*
	 * The secret key material.  After FS_IOC_REMOVE_ENCRYPTION_KEY is
	 * executed, this is wiped and no new inodes can be unlocked with this
	 * key; however, there may still be inodes in ->mk_decrypted_inodes
	 * which could not be evicted.  As long as some inodes still remain,
	 * FS_IOC_REMOVE_ENCRYPTION_KEY can be retried, or
	 * FS_IOC_ADD_ENCRYPTION_KEY can add the secret again.
	 *
	 * Locking: protected by key->sem (outer) and mk_secret_sem (inner).
	 * The reason for two locks is that key->sem also protects modifying
	 * mk_users, which ranks it above the semaphore for the keyring key
	 * type, which is in turn above page faults (via keyring_read).  But
	 * sometimes filesystems call fscrypt_get_encryption_info() from within
	 * a transaction, which ranks it below page faults.  So we need a
	 * separate lock which protects mk_secret but not also mk_users.
	 */
	struct fscrypt_master_key_secret	mk_secret;
	struct rw_semaphore			mk_secret_sem;

	/*
	 * For v1 policy keys: an arbitrary key descriptor which was assigned by
	 * userspace (->descriptor).
	 *
	 * For v2 policy keys: a cryptographic hash of this key (->identifier).
	 */
	struct fscrypt_key_specifier		mk_spec;

	/*
	 * Keyring which contains a key of type 'key_type_fscrypt_user' for each
	 * user who has added this key.  Normally each key will be added by just
	 * one user, but it's possible that multiple users share a key, and in
	 * that case we need to keep track of those users so that one user can't
	 * remove the key before the others want it removed too.
	 *
	 * This is NULL for v1 policy keys; those can only be added by root.
	 *
	 * Locking: in addition to this keyrings own semaphore, this is
	 * protected by the master key's key->sem, so we can do atomic
	 * search+insert.  It can also be searched without taking any locks, but
	 * in that case the returned key may have already been removed.
	 */
	struct key		*mk_users;

	/*
	 * Length of ->mk_decrypted_inodes, plus one if mk_secret is present.
	 * Once this goes to 0, the master key is removed from ->s_master_keys.
	 * The 'struct fscrypt_master_key' will continue to live as long as the
	 * 'struct key' whose payload it is, but we won't let this reference
	 * count rise again.
	 */
	refcount_t		mk_refcount;

	/*
	 * List of inodes that were unlocked using this key.  This allows the
	 * inodes to be evicted efficiently if the key is removed.
	 */
	struct list_head	mk_decrypted_inodes;
	spinlock_t		mk_decrypted_inodes_lock;

	/* Crypto API transforms for DIRECT_KEY policies, allocated on-demand */
	struct crypto_skcipher	*mk_direct_tfms[__FSCRYPT_MODE_MAX + 1];

	/*
	 * Crypto API transforms for filesystem-layer implementation of
	 * IV_INO_LBLK_64 policies, allocated on-demand.
	 */
	struct crypto_skcipher	*mk_iv_ino_lblk_64_tfms[__FSCRYPT_MODE_MAX + 1];

} __randomize_layout;

static inline bool
is_master_key_secret_present(const struct fscrypt_master_key_secret *secret)
{
	/*
	 * The READ_ONCE() is only necessary for fscrypt_drop_inode() and
	 * fscrypt_key_describe().  These run in atomic context, so they can't
	 * take ->mk_secret_sem and thus 'secret' can change concurrently which
	 * would be a data race.  But they only need to know whether the secret
	 * *was* present at the time of check, so READ_ONCE() suffices.
	 */
	return READ_ONCE(secret->size) != 0;
}

static inline const char *master_key_spec_type(
				const struct fscrypt_key_specifier *spec)
{
	switch (spec->type) {
	case FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR:
		return "descriptor";
	case FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER:
		return "identifier";
	}
	return "[unknown]";
}

static inline int master_key_spec_len(const struct fscrypt_key_specifier *spec)
{
	switch (spec->type) {
	case FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR:
		return FSCRYPT_KEY_DESCRIPTOR_SIZE;
	case FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER:
		return FSCRYPT_KEY_IDENTIFIER_SIZE;
	}
	return 0;
}

extern struct key *
fscrypt_find_master_key(struct super_block *sb,
			const struct fscrypt_key_specifier *mk_spec);

extern int fscrypt_verify_key_added(struct super_block *sb,
				    const u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]);

extern int __init fscrypt_init_keyring(void);

/* keysetup.c */

struct fscrypt_mode {
	const char *friendly_name;
	const char *cipher_str;
	int keysize;
	int ivsize;
	int logged_impl_name;
};

extern struct fscrypt_mode fscrypt_modes[];

extern struct crypto_skcipher *
fscrypt_allocate_skcipher(struct fscrypt_mode *mode, const u8 *raw_key,
			  const struct inode *inode);

extern int fscrypt_set_per_file_enc_key(struct fscrypt_info *ci,
					const u8 *raw_key);

extern int fscrypt_derive_dirhash_key(struct fscrypt_info *ci,
				      const struct fscrypt_master_key *mk);

/* keysetup_v1.c */

extern void fscrypt_put_direct_key(struct fscrypt_direct_key *dk);

extern int fscrypt_setup_v1_file_key(struct fscrypt_info *ci,
				     const u8 *raw_master_key);

extern int fscrypt_setup_v1_file_key_via_subscribed_keyrings(
					struct fscrypt_info *ci);
/* policy.c */

extern bool fscrypt_policies_equal(const union fscrypt_policy *policy1,
				   const union fscrypt_policy *policy2);
extern bool fscrypt_supported_policy(const union fscrypt_policy *policy_u,
				     const struct inode *inode);
extern int fscrypt_policy_from_context(union fscrypt_policy *policy_u,
				       const union fscrypt_context *ctx_u,
				       int ctx_size);

#endif /* _FSCRYPT_PRIVATE_H */