Commit e98ad464 authored by Eric Biggers's avatar Eric Biggers
Browse files

fscrypt: add FS_IOC_GET_ENCRYPTION_NONCE ioctl 

Add an ioctl FS_IOC_GET_ENCRYPTION_NONCE which retrieves the nonce from
an encrypted file or directory.  The nonce is the 16-byte random value
stored in the inode's encryption xattr.  It is normally used together
with the master key to derive the inode's actual encryption key.

The nonces are needed by automated tests that verify the correctness of
the ciphertext on-disk.  Except for the IV_INO_LBLK_64 case, there's no
way to replicate a file's ciphertext without knowing that file's nonce.

The nonces aren't secret, and the existing ciphertext verification tests
in xfstests retrieve them from disk using debugfs or dump.f2fs.  But in
environments that lack these debugging tools, getting the nonces by
manually parsing the filesystem structure would be very hard.

To make this important type of testing much easier, let's just add an
ioctl that retrieves the nonce.

Link: https://lore.kernel.org/r/20200314205052.93294-2-ebiggers@kernel.org


Reviewed-by: default avatarTheodore Ts'o <tytso@mit.edu>
Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
parent 98d54f81

Documentation/filesystems/fscrypt.rst

+11 −0
Original line number Diff line number Diff line
@@ -633,6 +633,17 @@ from a passphrase or other low-entropy user credential. 
FS_IOC_GET_ENCRYPTION_PWSALT is deprecated.  Instead, prefer to

generate and manage any needed salt(s) in userspace.



Getting a file's encryption nonce

---------------------------------



Since Linux v5.7, the ioctl FS_IOC_GET_ENCRYPTION_NONCE is supported.

On encrypted files and directories it gets the inode's 16-byte nonce.

On unencrypted files and directories, it fails with ENODATA.



This ioctl can be useful for automated tests which verify that the

encryption is being done correctly.  It is not needed for normal use

of fscrypt.



Adding keys

-----------

fs/crypto/fscrypt_private.h

+20 −0
Original line number Diff line number Diff line
@@ -76,6 +76,26 @@ static inline int fscrypt_context_size(const union fscrypt_context *ctx) 
	return 0;

}



/* Check whether an fscrypt_context has a recognized version number and size */

static inline bool fscrypt_context_is_valid(const union fscrypt_context *ctx,

					    int ctx_size)

{

	return ctx_size >= 1 && ctx_size == fscrypt_context_size(ctx);

}



/* Retrieve the context's nonce, assuming the context was already validated */

static inline const u8 *fscrypt_context_nonce(const union fscrypt_context *ctx)

{

	switch (ctx->version) {

	case FSCRYPT_CONTEXT_V1:

		return ctx->v1.nonce;

	case FSCRYPT_CONTEXT_V2:

		return ctx->v2.nonce;

	}

	WARN_ON(1);

	return NULL;

}



#undef fscrypt_policy

union fscrypt_policy {

	u8 version;

fs/crypto/keysetup.c

+2 −14
Original line number Diff line number Diff line
@@ -425,20 +425,8 @@ int fscrypt_get_encryption_info(struct inode *inode) 
		goto out;

	}



	switch (ctx.version) {

	case FSCRYPT_CONTEXT_V1:

		memcpy(crypt_info->ci_nonce, ctx.v1.nonce,

	memcpy(crypt_info->ci_nonce, fscrypt_context_nonce(&ctx),

	       FS_KEY_DERIVATION_NONCE_SIZE);

		break;

	case FSCRYPT_CONTEXT_V2:

		memcpy(crypt_info->ci_nonce, ctx.v2.nonce,

		       FS_KEY_DERIVATION_NONCE_SIZE);

		break;

	default:

		WARN_ON(1);

		res = -EINVAL;

		goto out;

	}



	if (!fscrypt_supported_policy(&crypt_info->ci_policy, inode)) {

		res = -EINVAL;

fs/crypto/policy.c

+20 −1
Original line number Diff line number Diff line
@@ -258,7 +258,7 @@ int fscrypt_policy_from_context(union fscrypt_policy *policy_u, 
{

	memset(policy_u, 0, sizeof(*policy_u));



	if (ctx_size <= 0 || ctx_size != fscrypt_context_size(ctx_u))

	if (!fscrypt_context_is_valid(ctx_u, ctx_size))

		return -EINVAL;



	switch (ctx_u->version) {
@@ -481,6 +481,25 @@ int fscrypt_ioctl_get_policy_ex(struct file *filp, void __user *uarg) 
}

EXPORT_SYMBOL_GPL(fscrypt_ioctl_get_policy_ex);



/* FS_IOC_GET_ENCRYPTION_NONCE: retrieve file's encryption nonce for testing */

int fscrypt_ioctl_get_nonce(struct file *filp, void __user *arg)

{

	struct inode *inode = file_inode(filp);

	union fscrypt_context ctx;

	int ret;



	ret = inode->i_sb->s_cop->get_context(inode, &ctx, sizeof(ctx));

	if (ret < 0)

		return ret;

	if (!fscrypt_context_is_valid(&ctx, ret))

		return -EINVAL;

	if (copy_to_user(arg, fscrypt_context_nonce(&ctx),

			 FS_KEY_DERIVATION_NONCE_SIZE))

		return -EFAULT;

	return 0;

}

EXPORT_SYMBOL_GPL(fscrypt_ioctl_get_nonce);



/**

 * fscrypt_has_permitted_context() - is a file's encryption policy permitted

 *				     within its directory?

include/linux/fscrypt.h

+6 −0
Original line number Diff line number Diff line
@@ -139,6 +139,7 @@ extern void fscrypt_free_bounce_page(struct page *bounce_page); 
extern int fscrypt_ioctl_set_policy(struct file *, const void __user *);

extern int fscrypt_ioctl_get_policy(struct file *, void __user *);

extern int fscrypt_ioctl_get_policy_ex(struct file *, void __user *);

extern int fscrypt_ioctl_get_nonce(struct file *filp, void __user *arg);

extern int fscrypt_has_permitted_context(struct inode *, struct inode *);

extern int fscrypt_inherit_context(struct inode *, struct inode *,

					void *, bool);
@@ -300,6 +301,11 @@ static inline int fscrypt_ioctl_get_policy_ex(struct file *filp, 
	return -EOPNOTSUPP;

}



static inline int fscrypt_ioctl_get_nonce(struct file *filp, void __user *arg)

{

	return -EOPNOTSUPP;

}



static inline int fscrypt_has_permitted_context(struct inode *parent,

						struct inode *child)

{

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