| Message ID | 20170712210035.51534-2-ebiggers3@gmail.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
On Wed, Jul 12, 2017 at 02:00:30PM -0700, Eric Biggers wrote: > From: Eric Biggers <ebiggers@google.com> > > Currently, the fscrypt_context (i.e. the encryption xattr) does not > contain a cryptographically secure identifier for the master key's > payload. Therefore it's not possible to verify that the correct key was > supplied, which is problematic in multi-user scenarios. To make this > possible, define a new fscrypt_context version (v2) which includes a > key_hash field, and allow userspace to opt-in to it when setting an > encryption policy by setting fscrypt_policy.version to 2. For now just > zero the new field; a later patch will start setting it for real. The main concern that comes to mind is potentially blowing past the inline xattr size limit and allocating a new inode block. The security benefit probably outweighs that concern in this case. > Even though we aren't changing the layout of struct fscrypt_policy (i.e. > the struct used by the ioctls), the new context version still has to be > "opt-in" because old kernels will not recognize it, and the keyring key > will now need to be available when setting an encryption policy, which > is an API change. We'll also be taking the opportunity to make another > API change (dropping support for the filesystem-specific key prefixes). > > Previously, the version numbers were 0 in the fscrypt_policy and 1 in > the fscrypt_context. Rather than incrementing them to 1 and 2, make > them both 2 to be consistent with each other. It's not required that > these numbers match, but it should make things less confusing. > > An alternative to adding a key_hash field would have been to reuse > master_key_descriptor. However, master_key_descriptor is only 8 bytes, > which is too short to be a cryptographically secure hash. Thus, > master_key_descriptor would have needed to be lengthened to 16+ bytes, > which would have required defining a fscrypt_policy_v2 structure and > adding a FS_IOC_GET_ENCRYPTION_POLICY_V2 ioctl. It also would have > required userspace to start using a specific hash algorithm to create > the key descriptors, which would have made the API harder to use. > Perhaps it should have been done that way originally, but at this point > it seems better to keep the API simpler. > > Signed-off-by: Eric Biggers <ebiggers@google.com> Acked-by: Michael Halcrow <mhalcrow@google.com> > --- > fs/crypto/fscrypt_private.h | 79 ++++++++++++++++++++++++++++++++++-------- > fs/crypto/keyinfo.c | 14 ++++---- > fs/crypto/policy.c | 67 ++++++++++++++++++++++++++--------- > include/linux/fscrypt_common.h | 2 +- > include/uapi/linux/fs.h | 6 ++++ > 5 files changed, 127 insertions(+), 41 deletions(-) > > diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h > index a1d5021c31ef..ef6909035823 100644 > --- a/fs/crypto/fscrypt_private.h > +++ b/fs/crypto/fscrypt_private.h > @@ -25,39 +25,88 @@ > #define FS_AES_256_XTS_KEY_SIZE 64 > > #define FS_KEY_DERIVATION_NONCE_SIZE 16 I'm seeing tab misalignment from all the other values here. Maybe remove the extra tab while you're at it? > +#define FSCRYPT_KEY_HASH_SIZE 16 > > /** > - * Encryption context for inode > + * fscrypt_context - the encryption context for an inode > * > - * Protector format: > - * 1 byte: Protector format (1 = this version) > - * 1 byte: File contents encryption mode > - * 1 byte: File names encryption mode > - * 1 byte: Flags > - * 8 bytes: Master Key descriptor > - * 16 bytes: Encryption Key derivation nonce > + * Filesystems usually store this in an extended attribute. It identifies the > + * encryption algorithm and key with which the file is encrypted. > */ > struct fscrypt_context { > - u8 format; > + /* v1+ */ > + > + /* Version of this structure */ > + u8 version; > + > + /* Encryption mode for the contents of regular files */ > u8 contents_encryption_mode; > + > + /* Encryption mode for filenames in directories and symlink targets */ > u8 filenames_encryption_mode; > + > + /* Options that affect how encryption is done (e.g. padding amount) */ > u8 flags; > + > + /* Descriptor for this file's master key in the keyring */ > u8 master_key_descriptor[FS_KEY_DESCRIPTOR_SIZE]; > + > + /* > + * A unique value used in combination with the master key to derive the > + * file's actual encryption key > + */ > u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE]; > -} __packed; > > -#define FS_ENCRYPTION_CONTEXT_FORMAT_V1 1 > + /* v2+ */ > + > + /* Cryptographically secure hash of the master key */ > + u8 key_hash[FSCRYPT_KEY_HASH_SIZE]; Please add a comment not to re-order without macro changes below. > +}; > + > +#define FSCRYPT_CONTEXT_V1 1 > +#define FSCRYPT_CONTEXT_V1_SIZE offsetof(struct fscrypt_context, key_hash) > + > +#define FSCRYPT_CONTEXT_V2 2 > +#define FSCRYPT_CONTEXT_V2_SIZE sizeof(struct fscrypt_context) > + > +static inline int fscrypt_context_size(const struct fscrypt_context *ctx) > +{ > + switch (ctx->version) { > + case FSCRYPT_CONTEXT_V1: > + return FSCRYPT_CONTEXT_V1_SIZE; > + case FSCRYPT_CONTEXT_V2: > + return FSCRYPT_CONTEXT_V2_SIZE; > + } > + return 0; > +} > + > +static inline bool > +fscrypt_valid_context_format(const struct fscrypt_context *ctx, int size) > +{ > + return size >= 1 && size == fscrypt_context_size(ctx); > +} > > /* > - * A pointer to this structure is stored in the file system's in-core > - * representation of an inode. > + * 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 transforms needed for encryption and decryption */ > + struct crypto_skcipher *ci_ctfm; > + struct crypto_cipher *ci_essiv_tfm; > + > + /* > + * Cached fields from the fscrypt_context needed for encryption policy > + * inheritance and enforcement > + */ > + u8 ci_context_version; > u8 ci_data_mode; > u8 ci_filename_mode; > u8 ci_flags; > - struct crypto_skcipher *ci_ctfm; > - struct crypto_cipher *ci_essiv_tfm; > u8 ci_master_key[FS_KEY_DESCRIPTOR_SIZE]; > }; > > diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c > index 018c588c7ac3..7e664a11340a 100644 > --- a/fs/crypto/keyinfo.c > +++ b/fs/crypto/keyinfo.c > @@ -272,29 +272,27 @@ int fscrypt_get_encryption_info(struct inode *inode) > return res; > /* Fake up a context for an unencrypted directory */ > memset(&ctx, 0, sizeof(ctx)); > - ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1; > + ctx.version = FSCRYPT_CONTEXT_V1; > ctx.contents_encryption_mode = FS_ENCRYPTION_MODE_AES_256_XTS; > ctx.filenames_encryption_mode = FS_ENCRYPTION_MODE_AES_256_CTS; > memset(ctx.master_key_descriptor, 0x42, FS_KEY_DESCRIPTOR_SIZE); > - } else if (res != sizeof(ctx)) { > - return -EINVAL; > + res = FSCRYPT_CONTEXT_V1_SIZE; > } > > - if (ctx.format != FS_ENCRYPTION_CONTEXT_FORMAT_V1) > + if (!fscrypt_valid_context_format(&ctx, res)) > return -EINVAL; > > if (ctx.flags & ~FS_POLICY_FLAGS_VALID) > return -EINVAL; > > - crypt_info = kmem_cache_alloc(fscrypt_info_cachep, GFP_NOFS); > + crypt_info = kmem_cache_zalloc(fscrypt_info_cachep, GFP_NOFS); > if (!crypt_info) > return -ENOMEM; > > - crypt_info->ci_flags = ctx.flags; > + crypt_info->ci_context_version = ctx.version; > crypt_info->ci_data_mode = ctx.contents_encryption_mode; > crypt_info->ci_filename_mode = ctx.filenames_encryption_mode; > - crypt_info->ci_ctfm = NULL; > - crypt_info->ci_essiv_tfm = NULL; > + crypt_info->ci_flags = ctx.flags; > memcpy(crypt_info->ci_master_key, ctx.master_key_descriptor, > sizeof(crypt_info->ci_master_key)); > > diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c > index ce07a86200f3..044f23fadb5a 100644 > --- a/fs/crypto/policy.c > +++ b/fs/crypto/policy.c > @@ -13,6 +13,28 @@ > #include <linux/mount.h> > #include "fscrypt_private.h" > > +static u8 policy_version_for_context(const struct fscrypt_context *ctx) > +{ > + switch (ctx->version) { > + case FSCRYPT_CONTEXT_V1: > + return FS_POLICY_VERSION_ORIGINAL; > + case FSCRYPT_CONTEXT_V2: > + return FS_POLICY_VERSION_HKDF; > + } > + BUG(); > +} > + > +static u8 context_version_for_policy(const struct fscrypt_policy *policy) > +{ > + switch (policy->version) { > + case FS_POLICY_VERSION_ORIGINAL: > + return FSCRYPT_CONTEXT_V1; > + case FS_POLICY_VERSION_HKDF: > + return FSCRYPT_CONTEXT_V2; > + } > + BUG(); Suggest commenting this function to require that policy be validated prior to passing it here. It's only called from fscrypt_ioctl_set_policy() from what I can see, and that function validates. > +} > + > /* > * check whether an encryption policy is consistent with an encryption context > */ > @@ -20,8 +42,10 @@ static bool is_encryption_context_consistent_with_policy( > const struct fscrypt_context *ctx, > const struct fscrypt_policy *policy) > { > - return memcmp(ctx->master_key_descriptor, policy->master_key_descriptor, > - FS_KEY_DESCRIPTOR_SIZE) == 0 && > + return (ctx->version == context_version_for_policy(policy)) && > + (memcmp(ctx->master_key_descriptor, > + policy->master_key_descriptor, > + FS_KEY_DESCRIPTOR_SIZE) == 0) && > (ctx->flags == policy->flags) && > (ctx->contents_encryption_mode == > policy->contents_encryption_mode) && > @@ -34,10 +58,6 @@ static int create_encryption_context_from_policy(struct inode *inode, > { > struct fscrypt_context ctx; > > - ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1; > - memcpy(ctx.master_key_descriptor, policy->master_key_descriptor, > - FS_KEY_DESCRIPTOR_SIZE); > - > if (!fscrypt_valid_enc_modes(policy->contents_encryption_mode, > policy->filenames_encryption_mode)) > return -EINVAL; > @@ -45,13 +65,20 @@ static int create_encryption_context_from_policy(struct inode *inode, > if (policy->flags & ~FS_POLICY_FLAGS_VALID) > return -EINVAL; > > + ctx.version = context_version_for_policy(policy); > ctx.contents_encryption_mode = policy->contents_encryption_mode; > ctx.filenames_encryption_mode = policy->filenames_encryption_mode; > ctx.flags = policy->flags; > + memcpy(ctx.master_key_descriptor, policy->master_key_descriptor, > + FS_KEY_DESCRIPTOR_SIZE); > BUILD_BUG_ON(sizeof(ctx.nonce) != FS_KEY_DERIVATION_NONCE_SIZE); > get_random_bytes(ctx.nonce, FS_KEY_DERIVATION_NONCE_SIZE); > + if (ctx.version != FSCRYPT_CONTEXT_V1) > + memset(ctx.key_hash, 0, FSCRYPT_KEY_HASH_SIZE); > > - return inode->i_sb->s_cop->set_context(inode, &ctx, sizeof(ctx), NULL); > + return inode->i_sb->s_cop->set_context(inode, &ctx, > + fscrypt_context_size(&ctx), > + NULL); > } > > int fscrypt_ioctl_set_policy(struct file *filp, const void __user *arg) > @@ -67,7 +94,8 @@ int fscrypt_ioctl_set_policy(struct file *filp, const void __user *arg) > if (!inode_owner_or_capable(inode)) > return -EACCES; > > - if (policy.version != 0) > + if (policy.version != FS_POLICY_VERSION_ORIGINAL && > + policy.version != FS_POLICY_VERSION_HKDF) > return -EINVAL; > > ret = mnt_want_write_file(filp); > @@ -85,7 +113,7 @@ int fscrypt_ioctl_set_policy(struct file *filp, const void __user *arg) > else > ret = create_encryption_context_from_policy(inode, > &policy); > - } else if (ret == sizeof(ctx) && > + } else if (ret >= 0 && fscrypt_valid_context_format(&ctx, ret) && > is_encryption_context_consistent_with_policy(&ctx, > &policy)) { > /* The file already uses the same encryption policy. */ > @@ -115,12 +143,10 @@ int fscrypt_ioctl_get_policy(struct file *filp, void __user *arg) > res = inode->i_sb->s_cop->get_context(inode, &ctx, sizeof(ctx)); > if (res < 0 && res != -ERANGE) > return res; > - if (res != sizeof(ctx)) > - return -EINVAL; > - if (ctx.format != FS_ENCRYPTION_CONTEXT_FORMAT_V1) > + if (res < 0 || !fscrypt_valid_context_format(&ctx, res)) > return -EINVAL; This ends up looking like a somewhat convoluted way of writing: "if (res < 0) return res == -ERANGE ? -EINVAL : res;" Followed by the check for context format validity. Although there may be an even less convoluted way to write it. > > - policy.version = 0; > + policy.version = policy_version_for_context(&ctx); > policy.contents_encryption_mode = ctx.contents_encryption_mode; > policy.filenames_encryption_mode = ctx.filenames_encryption_mode; > policy.flags = ctx.flags; > @@ -200,6 +226,8 @@ int fscrypt_has_permitted_context(struct inode *parent, struct inode *child) > if (parent_ci && child_ci) { > return memcmp(parent_ci->ci_master_key, child_ci->ci_master_key, > FS_KEY_DESCRIPTOR_SIZE) == 0 && > + (parent_ci->ci_context_version == > + child_ci->ci_context_version) && > (parent_ci->ci_data_mode == child_ci->ci_data_mode) && > (parent_ci->ci_filename_mode == > child_ci->ci_filename_mode) && > @@ -207,16 +235,17 @@ int fscrypt_has_permitted_context(struct inode *parent, struct inode *child) > } > > res = cops->get_context(parent, &parent_ctx, sizeof(parent_ctx)); > - if (res != sizeof(parent_ctx)) > + if (res < 0 || !fscrypt_valid_context_format(&parent_ctx, res)) > return 0; > > res = cops->get_context(child, &child_ctx, sizeof(child_ctx)); > - if (res != sizeof(child_ctx)) > + if (res < 0 || !fscrypt_valid_context_format(&child_ctx, res)) > return 0; > > return memcmp(parent_ctx.master_key_descriptor, > child_ctx.master_key_descriptor, > FS_KEY_DESCRIPTOR_SIZE) == 0 && > + (parent_ctx.version == child_ctx.version) && > (parent_ctx.contents_encryption_mode == > child_ctx.contents_encryption_mode) && > (parent_ctx.filenames_encryption_mode == > @@ -249,16 +278,20 @@ int fscrypt_inherit_context(struct inode *parent, struct inode *child, > if (ci == NULL) > return -ENOKEY; > > - ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1; > + ctx.version = ci->ci_context_version; > ctx.contents_encryption_mode = ci->ci_data_mode; > ctx.filenames_encryption_mode = ci->ci_filename_mode; > ctx.flags = ci->ci_flags; > memcpy(ctx.master_key_descriptor, ci->ci_master_key, > FS_KEY_DESCRIPTOR_SIZE); > get_random_bytes(ctx.nonce, FS_KEY_DERIVATION_NONCE_SIZE); > + if (ctx.version != FSCRYPT_CONTEXT_V1) > + memset(ctx.key_hash, 0, FSCRYPT_KEY_HASH_SIZE); > + > BUILD_BUG_ON(sizeof(ctx) != FSCRYPT_SET_CONTEXT_MAX_SIZE); > res = parent->i_sb->s_cop->set_context(child, &ctx, > - sizeof(ctx), fs_data); > + fscrypt_context_size(&ctx), > + fs_data); > if (res) > return res; > return preload ? fscrypt_get_encryption_info(child): 0; > diff --git a/include/linux/fscrypt_common.h b/include/linux/fscrypt_common.h > index 97f738628b36..c08e8ae63a02 100644 > --- a/include/linux/fscrypt_common.h > +++ b/include/linux/fscrypt_common.h > @@ -84,7 +84,7 @@ struct fscrypt_operations { > }; > > /* Maximum value for the third parameter of fscrypt_operations.set_context(). */ > -#define FSCRYPT_SET_CONTEXT_MAX_SIZE 28 > +#define FSCRYPT_SET_CONTEXT_MAX_SIZE 44 > > static inline bool fscrypt_dummy_context_enabled(struct inode *inode) > { > diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h > index b7495d05e8de..a5423ddd3b67 100644 > --- a/include/uapi/linux/fs.h > +++ b/include/uapi/linux/fs.h > @@ -257,6 +257,12 @@ struct fsxattr { > * File system encryption support > */ > /* Policy provided via an ioctl on the topmost directory */ > + > +/* original policy version, no key verification (potentially insecure) */ > +#define FS_POLICY_VERSION_ORIGINAL 0 > +/* new version w/ HKDF and key verification (recommended) */ > +#define FS_POLICY_VERSION_HKDF 2 > + > #define FS_KEY_DESCRIPTOR_SIZE 8 > > #define FS_POLICY_FLAGS_PAD_4 0x00 > -- > 2.13.2.932.g7449e964c-goog > -- To unsubscribe from this list: send the line "unsubscribe linux-fscrypt" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Hi Michael, On Thu, Jul 13, 2017 at 03:29:44PM -0700, Michael Halcrow wrote: > On Wed, Jul 12, 2017 at 02:00:30PM -0700, Eric Biggers wrote: > > From: Eric Biggers <ebiggers@google.com> > > > > Currently, the fscrypt_context (i.e. the encryption xattr) does not > > contain a cryptographically secure identifier for the master key's > > payload. Therefore it's not possible to verify that the correct key was > > supplied, which is problematic in multi-user scenarios. To make this > > possible, define a new fscrypt_context version (v2) which includes a > > key_hash field, and allow userspace to opt-in to it when setting an > > encryption policy by setting fscrypt_policy.version to 2. For now just > > zero the new field; a later patch will start setting it for real. > > The main concern that comes to mind is potentially blowing past the > inline xattr size limit and allocating a new inode block. The > security benefit probably outweighs that concern in this case. > The way it adds up now for ext4 is: 128 bytes for base inode + 32 bytes for i_extra fields + 4 bytes for in-inode xattrs header + 20 bytes for encryption xattr header + name + 28 bytes for encryption xattr value ---------------------------------- = 212 bytes total. By adding the 16-byte 'key_hash' field it grows to 228 bytes total. So it still fits in a 256-byte inode, though it's getting closer to the limit. We could save 8 bytes by instead using the design where master_key_descriptor is extended to 16 bytes and redefined as a cryptographically secure hash. But as noted, that has some significant disadvantages. Also note that we don't really have to worry about leaving space for a SELinux xattr anymore because with 256-byte inodes + encryption the SELinux xattr is already being written to an external block, given that it requires about 52-62 bytes (at least when using Android's SELinux policy; different SELinux policies may use different values), and 212 + 52 > 256. So if someone wants both xattrs in-inode they need to use 512-byte inodes already. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fscrypt" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Jul 13, 2017, at 3:58 PM, Eric Biggers <ebiggers3@gmail.com> wrote: > > Hi Michael, > > On Thu, Jul 13, 2017 at 03:29:44PM -0700, Michael Halcrow wrote: >> On Wed, Jul 12, 2017 at 02:00:30PM -0700, Eric Biggers wrote: >>> From: Eric Biggers <ebiggers@google.com> >>> >>> Currently, the fscrypt_context (i.e. the encryption xattr) does not >>> contain a cryptographically secure identifier for the master key's >>> payload. Therefore it's not possible to verify that the correct key was >>> supplied, which is problematic in multi-user scenarios. To make this >>> possible, define a new fscrypt_context version (v2) which includes a >>> key_hash field, and allow userspace to opt-in to it when setting an >>> encryption policy by setting fscrypt_policy.version to 2. For now just >>> zero the new field; a later patch will start setting it for real. >> >> The main concern that comes to mind is potentially blowing past the >> inline xattr size limit and allocating a new inode block. The >> security benefit probably outweighs that concern in this case. >> > > The way it adds up now for ext4 is: > > 128 bytes for base inode > + 32 bytes for i_extra fields > + 4 bytes for in-inode xattrs header > + 20 bytes for encryption xattr header + name > + 28 bytes for encryption xattr value > ---------------------------------- > = 212 bytes total. > > By adding the 16-byte 'key_hash' field it grows to 228 bytes total. So it still > fits in a 256-byte inode, though it's getting closer to the limit. We could > save 8 bytes by instead using the design where master_key_descriptor is extended > to 16 bytes and redefined as a cryptographically secure hash. But as noted, > that has some significant disadvantages. > > Also note that we don't really have to worry about leaving space for a SELinux > xattr anymore because with 256-byte inodes + encryption the SELinux xattr is > already being written to an external block, given that it requires about 52-62 > bytes (at least when using Android's SELinux policy; different SELinux policies > may use different values), and 212 + 52 > 256. So if someone wants both xattrs > in-inode they need to use 512-byte inodes already. It is probably time to consider changing to a default of 512-byte inodes for larger filesystems anyway. In our testing, this affected performance only by a couple of percent under normal usage, and avoided a significant performance drop if the xattrs ever fall out of the inode. Cheers, Andreas
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h index a1d5021c31ef..ef6909035823 100644 --- a/fs/crypto/fscrypt_private.h +++ b/fs/crypto/fscrypt_private.h @@ -25,39 +25,88 @@ #define FS_AES_256_XTS_KEY_SIZE 64 #define FS_KEY_DERIVATION_NONCE_SIZE 16 +#define FSCRYPT_KEY_HASH_SIZE 16 /** - * Encryption context for inode + * fscrypt_context - the encryption context for an inode * - * Protector format: - * 1 byte: Protector format (1 = this version) - * 1 byte: File contents encryption mode - * 1 byte: File names encryption mode - * 1 byte: Flags - * 8 bytes: Master Key descriptor - * 16 bytes: Encryption Key derivation nonce + * Filesystems usually store this in an extended attribute. It identifies the + * encryption algorithm and key with which the file is encrypted. */ struct fscrypt_context { - u8 format; + /* v1+ */ + + /* Version of this structure */ + u8 version; + + /* Encryption mode for the contents of regular files */ u8 contents_encryption_mode; + + /* Encryption mode for filenames in directories and symlink targets */ u8 filenames_encryption_mode; + + /* Options that affect how encryption is done (e.g. padding amount) */ u8 flags; + + /* Descriptor for this file's master key in the keyring */ u8 master_key_descriptor[FS_KEY_DESCRIPTOR_SIZE]; + + /* + * A unique value used in combination with the master key to derive the + * file's actual encryption key + */ u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE]; -} __packed; -#define FS_ENCRYPTION_CONTEXT_FORMAT_V1 1 + /* v2+ */ + + /* Cryptographically secure hash of the master key */ + u8 key_hash[FSCRYPT_KEY_HASH_SIZE]; +}; + +#define FSCRYPT_CONTEXT_V1 1 +#define FSCRYPT_CONTEXT_V1_SIZE offsetof(struct fscrypt_context, key_hash) + +#define FSCRYPT_CONTEXT_V2 2 +#define FSCRYPT_CONTEXT_V2_SIZE sizeof(struct fscrypt_context) + +static inline int fscrypt_context_size(const struct fscrypt_context *ctx) +{ + switch (ctx->version) { + case FSCRYPT_CONTEXT_V1: + return FSCRYPT_CONTEXT_V1_SIZE; + case FSCRYPT_CONTEXT_V2: + return FSCRYPT_CONTEXT_V2_SIZE; + } + return 0; +} + +static inline bool +fscrypt_valid_context_format(const struct fscrypt_context *ctx, int size) +{ + return size >= 1 && size == fscrypt_context_size(ctx); +} /* - * A pointer to this structure is stored in the file system's in-core - * representation of an inode. + * 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 transforms needed for encryption and decryption */ + struct crypto_skcipher *ci_ctfm; + struct crypto_cipher *ci_essiv_tfm; + + /* + * Cached fields from the fscrypt_context needed for encryption policy + * inheritance and enforcement + */ + u8 ci_context_version; u8 ci_data_mode; u8 ci_filename_mode; u8 ci_flags; - struct crypto_skcipher *ci_ctfm; - struct crypto_cipher *ci_essiv_tfm; u8 ci_master_key[FS_KEY_DESCRIPTOR_SIZE]; }; diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c index 018c588c7ac3..7e664a11340a 100644 --- a/fs/crypto/keyinfo.c +++ b/fs/crypto/keyinfo.c @@ -272,29 +272,27 @@ int fscrypt_get_encryption_info(struct inode *inode) return res; /* Fake up a context for an unencrypted directory */ memset(&ctx, 0, sizeof(ctx)); - ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1; + ctx.version = FSCRYPT_CONTEXT_V1; ctx.contents_encryption_mode = FS_ENCRYPTION_MODE_AES_256_XTS; ctx.filenames_encryption_mode = FS_ENCRYPTION_MODE_AES_256_CTS; memset(ctx.master_key_descriptor, 0x42, FS_KEY_DESCRIPTOR_SIZE); - } else if (res != sizeof(ctx)) { - return -EINVAL; + res = FSCRYPT_CONTEXT_V1_SIZE; } - if (ctx.format != FS_ENCRYPTION_CONTEXT_FORMAT_V1) + if (!fscrypt_valid_context_format(&ctx, res)) return -EINVAL; if (ctx.flags & ~FS_POLICY_FLAGS_VALID) return -EINVAL; - crypt_info = kmem_cache_alloc(fscrypt_info_cachep, GFP_NOFS); + crypt_info = kmem_cache_zalloc(fscrypt_info_cachep, GFP_NOFS); if (!crypt_info) return -ENOMEM; - crypt_info->ci_flags = ctx.flags; + crypt_info->ci_context_version = ctx.version; crypt_info->ci_data_mode = ctx.contents_encryption_mode; crypt_info->ci_filename_mode = ctx.filenames_encryption_mode; - crypt_info->ci_ctfm = NULL; - crypt_info->ci_essiv_tfm = NULL; + crypt_info->ci_flags = ctx.flags; memcpy(crypt_info->ci_master_key, ctx.master_key_descriptor, sizeof(crypt_info->ci_master_key)); diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c index ce07a86200f3..044f23fadb5a 100644 --- a/fs/crypto/policy.c +++ b/fs/crypto/policy.c @@ -13,6 +13,28 @@ #include <linux/mount.h> #include "fscrypt_private.h" +static u8 policy_version_for_context(const struct fscrypt_context *ctx) +{ + switch (ctx->version) { + case FSCRYPT_CONTEXT_V1: + return FS_POLICY_VERSION_ORIGINAL; + case FSCRYPT_CONTEXT_V2: + return FS_POLICY_VERSION_HKDF; + } + BUG(); +} + +static u8 context_version_for_policy(const struct fscrypt_policy *policy) +{ + switch (policy->version) { + case FS_POLICY_VERSION_ORIGINAL: + return FSCRYPT_CONTEXT_V1; + case FS_POLICY_VERSION_HKDF: + return FSCRYPT_CONTEXT_V2; + } + BUG(); +} + /* * check whether an encryption policy is consistent with an encryption context */ @@ -20,8 +42,10 @@ static bool is_encryption_context_consistent_with_policy( const struct fscrypt_context *ctx, const struct fscrypt_policy *policy) { - return memcmp(ctx->master_key_descriptor, policy->master_key_descriptor, - FS_KEY_DESCRIPTOR_SIZE) == 0 && + return (ctx->version == context_version_for_policy(policy)) && + (memcmp(ctx->master_key_descriptor, + policy->master_key_descriptor, + FS_KEY_DESCRIPTOR_SIZE) == 0) && (ctx->flags == policy->flags) && (ctx->contents_encryption_mode == policy->contents_encryption_mode) && @@ -34,10 +58,6 @@ static int create_encryption_context_from_policy(struct inode *inode, { struct fscrypt_context ctx; - ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1; - memcpy(ctx.master_key_descriptor, policy->master_key_descriptor, - FS_KEY_DESCRIPTOR_SIZE); - if (!fscrypt_valid_enc_modes(policy->contents_encryption_mode, policy->filenames_encryption_mode)) return -EINVAL; @@ -45,13 +65,20 @@ static int create_encryption_context_from_policy(struct inode *inode, if (policy->flags & ~FS_POLICY_FLAGS_VALID) return -EINVAL; + ctx.version = context_version_for_policy(policy); ctx.contents_encryption_mode = policy->contents_encryption_mode; ctx.filenames_encryption_mode = policy->filenames_encryption_mode; ctx.flags = policy->flags; + memcpy(ctx.master_key_descriptor, policy->master_key_descriptor, + FS_KEY_DESCRIPTOR_SIZE); BUILD_BUG_ON(sizeof(ctx.nonce) != FS_KEY_DERIVATION_NONCE_SIZE); get_random_bytes(ctx.nonce, FS_KEY_DERIVATION_NONCE_SIZE); + if (ctx.version != FSCRYPT_CONTEXT_V1) + memset(ctx.key_hash, 0, FSCRYPT_KEY_HASH_SIZE); - return inode->i_sb->s_cop->set_context(inode, &ctx, sizeof(ctx), NULL); + return inode->i_sb->s_cop->set_context(inode, &ctx, + fscrypt_context_size(&ctx), + NULL); } int fscrypt_ioctl_set_policy(struct file *filp, const void __user *arg) @@ -67,7 +94,8 @@ int fscrypt_ioctl_set_policy(struct file *filp, const void __user *arg) if (!inode_owner_or_capable(inode)) return -EACCES; - if (policy.version != 0) + if (policy.version != FS_POLICY_VERSION_ORIGINAL && + policy.version != FS_POLICY_VERSION_HKDF) return -EINVAL; ret = mnt_want_write_file(filp); @@ -85,7 +113,7 @@ int fscrypt_ioctl_set_policy(struct file *filp, const void __user *arg) else ret = create_encryption_context_from_policy(inode, &policy); - } else if (ret == sizeof(ctx) && + } else if (ret >= 0 && fscrypt_valid_context_format(&ctx, ret) && is_encryption_context_consistent_with_policy(&ctx, &policy)) { /* The file already uses the same encryption policy. */ @@ -115,12 +143,10 @@ int fscrypt_ioctl_get_policy(struct file *filp, void __user *arg) res = inode->i_sb->s_cop->get_context(inode, &ctx, sizeof(ctx)); if (res < 0 && res != -ERANGE) return res; - if (res != sizeof(ctx)) - return -EINVAL; - if (ctx.format != FS_ENCRYPTION_CONTEXT_FORMAT_V1) + if (res < 0 || !fscrypt_valid_context_format(&ctx, res)) return -EINVAL; - policy.version = 0; + policy.version = policy_version_for_context(&ctx); policy.contents_encryption_mode = ctx.contents_encryption_mode; policy.filenames_encryption_mode = ctx.filenames_encryption_mode; policy.flags = ctx.flags; @@ -200,6 +226,8 @@ int fscrypt_has_permitted_context(struct inode *parent, struct inode *child) if (parent_ci && child_ci) { return memcmp(parent_ci->ci_master_key, child_ci->ci_master_key, FS_KEY_DESCRIPTOR_SIZE) == 0 && + (parent_ci->ci_context_version == + child_ci->ci_context_version) && (parent_ci->ci_data_mode == child_ci->ci_data_mode) && (parent_ci->ci_filename_mode == child_ci->ci_filename_mode) && @@ -207,16 +235,17 @@ int fscrypt_has_permitted_context(struct inode *parent, struct inode *child) } res = cops->get_context(parent, &parent_ctx, sizeof(parent_ctx)); - if (res != sizeof(parent_ctx)) + if (res < 0 || !fscrypt_valid_context_format(&parent_ctx, res)) return 0; res = cops->get_context(child, &child_ctx, sizeof(child_ctx)); - if (res != sizeof(child_ctx)) + if (res < 0 || !fscrypt_valid_context_format(&child_ctx, res)) return 0; return memcmp(parent_ctx.master_key_descriptor, child_ctx.master_key_descriptor, FS_KEY_DESCRIPTOR_SIZE) == 0 && + (parent_ctx.version == child_ctx.version) && (parent_ctx.contents_encryption_mode == child_ctx.contents_encryption_mode) && (parent_ctx.filenames_encryption_mode == @@ -249,16 +278,20 @@ int fscrypt_inherit_context(struct inode *parent, struct inode *child, if (ci == NULL) return -ENOKEY; - ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1; + ctx.version = ci->ci_context_version; ctx.contents_encryption_mode = ci->ci_data_mode; ctx.filenames_encryption_mode = ci->ci_filename_mode; ctx.flags = ci->ci_flags; memcpy(ctx.master_key_descriptor, ci->ci_master_key, FS_KEY_DESCRIPTOR_SIZE); get_random_bytes(ctx.nonce, FS_KEY_DERIVATION_NONCE_SIZE); + if (ctx.version != FSCRYPT_CONTEXT_V1) + memset(ctx.key_hash, 0, FSCRYPT_KEY_HASH_SIZE); + BUILD_BUG_ON(sizeof(ctx) != FSCRYPT_SET_CONTEXT_MAX_SIZE); res = parent->i_sb->s_cop->set_context(child, &ctx, - sizeof(ctx), fs_data); + fscrypt_context_size(&ctx), + fs_data); if (res) return res; return preload ? fscrypt_get_encryption_info(child): 0; diff --git a/include/linux/fscrypt_common.h b/include/linux/fscrypt_common.h index 97f738628b36..c08e8ae63a02 100644 --- a/include/linux/fscrypt_common.h +++ b/include/linux/fscrypt_common.h @@ -84,7 +84,7 @@ struct fscrypt_operations { }; /* Maximum value for the third parameter of fscrypt_operations.set_context(). */ -#define FSCRYPT_SET_CONTEXT_MAX_SIZE 28 +#define FSCRYPT_SET_CONTEXT_MAX_SIZE 44 static inline bool fscrypt_dummy_context_enabled(struct inode *inode) { diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h index b7495d05e8de..a5423ddd3b67 100644 --- a/include/uapi/linux/fs.h +++ b/include/uapi/linux/fs.h @@ -257,6 +257,12 @@ struct fsxattr { * File system encryption support */ /* Policy provided via an ioctl on the topmost directory */ + +/* original policy version, no key verification (potentially insecure) */ +#define FS_POLICY_VERSION_ORIGINAL 0 +/* new version w/ HKDF and key verification (recommended) */ +#define FS_POLICY_VERSION_HKDF 2 + #define FS_KEY_DESCRIPTOR_SIZE 8 #define FS_POLICY_FLAGS_PAD_4 0x00