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evm: re-release EVM protects a file's security extended attributes(xattrs) against integrity attacks. This patchset provides the framework and an initial method. The initial method maintains an HMAC-sha1 value across the security extended attributes, storing the HMAC value as the extended attribute 'security.evm'. Other methods of validating the integrity of a file's metadata will be posted separately (eg. EVM-digital-signatures). While this patchset does authenticate the security xattrs, and cryptographically binds them to the inode, coming extensions will bind other directory and inode metadata for more complete protection. To help simplify the review and upstreaming process, each extension will be posted separately (eg. IMA-appraisal, IMA-appraisal-directory). For a general overview of the proposed Linux integrity subsystem, refer to Dave Safford's whitepaper: http://downloads.sf.net/project/linux-ima/linux-ima/Integrity_overview.pdf. EVM depends on the Kernel Key Retention System to provide it with a trusted/encrypted key for the HMAC-sha1 operation. The key is loaded onto the root's keyring using keyctl. Until EVM receives notification that the key has been successfully loaded onto the keyring (echo 1 > <securityfs>/evm), EVM can not create or validate the 'security.evm' xattr, but returns INTEGRITY_UNKNOWN. Loading the key and signaling EVM should be done as early as possible. Normally this is done in the initramfs, which has already been measured as part of the trusted boot. For more information on creating and loading existing trusted/encrypted keys, refer to Documentation/keys-trusted-encrypted.txt. A sample dracut patch, which loads the trusted/encrypted key and enables EVM, is available from http://linux-ima.sourceforge.net/#EVM. Based on the LSMs enabled, the set of EVM protected security xattrs is defined at compile. EVM adds the following three calls to the existing security hooks: evm_inode_setxattr(), evm_inode_post_setxattr(), and evm_inode_removexattr. To initialize and update the 'security.evm' extended attribute, EVM defines three calls: evm_inode_post_init(), evm_inode_post_setattr() and evm_inode_post_removexattr() hooks. To verify the integrity of a security xattr, EVM exports evm_verifyxattr(). Changelog v7: - Fixed URL in EVM ABI documentation Changelog v6: (based on Serge Hallyn's review) - fix URL in patch description - remove evm_hmac_size definition - use SHA1_DIGEST_SIZE (removed both MAX_DIGEST_SIZE and evm_hmac_size) - moved linux include before other includes - test for crypto_hash_setkey failure - fail earlier for invalid key - clear entire encrypted key, even on failure - check xattr name length before comparing xattr names Changelog: - locking based on i_mutex, remove evm_mutex - using trusted/encrypted keys for storing the EVM key used in the HMAC-sha1 operation. - replaced crypto hash with shash (Dmitry Kasatkin) - support for additional methods of verifying the security xattrs (Dmitry Kasatkin) - iint not allocated for all regular files, but only for those appraised - Use cap_sys_admin in lieu of cap_mac_admin - Use __vfs_setxattr_noperm(), without permission checks, from EVM Signed-off-by: Mimi Zohar <zohar@us.ibm.com> Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
2011-03-16 05:12:09 +09:00
What: security/evm
Date: March 2011
Contact: Mimi Zohar <zohar@us.ibm.com>
Description:
EVM protects a file's security extended attributes(xattrs)
against integrity attacks. The initial method maintains an
HMAC-sha1 value across the extended attributes, storing the
value as the extended attribute 'security.evm'.
EVM: Allow userspace to signal an RSA key has been loaded EVM will only perform validation once a key has been loaded. This key may either be a symmetric trusted key (for HMAC validation and creation) or the public half of an asymmetric key (for digital signature validation). The /sys/kernel/security/evm interface allows userland to signal that a symmetric key has been loaded, but does not allow userland to signal that an asymmetric public key has been loaded. This patch extends the interface to permit userspace to pass a bitmask of loaded key types. It also allows userspace to block loading of a symmetric key in order to avoid a compromised system from being able to load an additional key type later. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-10-12 04:10:14 +09:00
EVM supports two classes of security.evm. The first is
an HMAC-sha1 generated locally with a
trusted/encrypted key stored in the Kernel Key
Retention System. The second is a digital signature
generated either locally or remotely using an
asymmetric key. These keys are loaded onto root's
keyring using keyctl, and EVM is then enabled by
EVM: Allow userland to permit modification of EVM-protected metadata When EVM is enabled it forbids modification of metadata protected by EVM unless there is already a valid EVM signature. If any modification is made, the kernel will then generate a new EVM HMAC. However, this does not map well on use cases which use only asymmetric EVM signatures, as in this scenario the kernel is unable to generate new signatures. This patch extends the /sys/kernel/security/evm interface to allow userland to request that modification of these xattrs be permitted. This is only permitted if no keys have already been loaded. In this configuration, modifying the metadata will invalidate the EVM appraisal on the file in question. This allows packaging systems to write out new files, set the relevant extended attributes and then move them into place. There's also some refactoring of the use of evm_initialized in order to avoid heading down codepaths that assume there's a key available. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-11-08 00:18:35 +09:00
echoing a value to <securityfs>/evm made up of the
following bits:
EVM: Allow userspace to signal an RSA key has been loaded EVM will only perform validation once a key has been loaded. This key may either be a symmetric trusted key (for HMAC validation and creation) or the public half of an asymmetric key (for digital signature validation). The /sys/kernel/security/evm interface allows userland to signal that a symmetric key has been loaded, but does not allow userland to signal that an asymmetric public key has been loaded. This patch extends the interface to permit userspace to pass a bitmask of loaded key types. It also allows userspace to block loading of a symmetric key in order to avoid a compromised system from being able to load an additional key type later. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-10-12 04:10:14 +09:00
EVM: Allow userland to permit modification of EVM-protected metadata When EVM is enabled it forbids modification of metadata protected by EVM unless there is already a valid EVM signature. If any modification is made, the kernel will then generate a new EVM HMAC. However, this does not map well on use cases which use only asymmetric EVM signatures, as in this scenario the kernel is unable to generate new signatures. This patch extends the /sys/kernel/security/evm interface to allow userland to request that modification of these xattrs be permitted. This is only permitted if no keys have already been loaded. In this configuration, modifying the metadata will invalidate the EVM appraisal on the file in question. This allows packaging systems to write out new files, set the relevant extended attributes and then move them into place. There's also some refactoring of the use of evm_initialized in order to avoid heading down codepaths that assume there's a key available. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-11-08 00:18:35 +09:00
Bit Effect
0 Enable HMAC validation and creation
1 Enable digital signature validation
2 Permit modification of EVM-protected metadata at
runtime. Not supported if HMAC validation and
creation is enabled.
31 Disable further runtime modification of EVM policy
EVM: Allow userspace to signal an RSA key has been loaded EVM will only perform validation once a key has been loaded. This key may either be a symmetric trusted key (for HMAC validation and creation) or the public half of an asymmetric key (for digital signature validation). The /sys/kernel/security/evm interface allows userland to signal that a symmetric key has been loaded, but does not allow userland to signal that an asymmetric public key has been loaded. This patch extends the interface to permit userspace to pass a bitmask of loaded key types. It also allows userspace to block loading of a symmetric key in order to avoid a compromised system from being able to load an additional key type later. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-10-12 04:10:14 +09:00
EVM: Allow userland to permit modification of EVM-protected metadata When EVM is enabled it forbids modification of metadata protected by EVM unless there is already a valid EVM signature. If any modification is made, the kernel will then generate a new EVM HMAC. However, this does not map well on use cases which use only asymmetric EVM signatures, as in this scenario the kernel is unable to generate new signatures. This patch extends the /sys/kernel/security/evm interface to allow userland to request that modification of these xattrs be permitted. This is only permitted if no keys have already been loaded. In this configuration, modifying the metadata will invalidate the EVM appraisal on the file in question. This allows packaging systems to write out new files, set the relevant extended attributes and then move them into place. There's also some refactoring of the use of evm_initialized in order to avoid heading down codepaths that assume there's a key available. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-11-08 00:18:35 +09:00
For example:
EVM: Allow userspace to signal an RSA key has been loaded EVM will only perform validation once a key has been loaded. This key may either be a symmetric trusted key (for HMAC validation and creation) or the public half of an asymmetric key (for digital signature validation). The /sys/kernel/security/evm interface allows userland to signal that a symmetric key has been loaded, but does not allow userland to signal that an asymmetric public key has been loaded. This patch extends the interface to permit userspace to pass a bitmask of loaded key types. It also allows userspace to block loading of a symmetric key in order to avoid a compromised system from being able to load an additional key type later. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-10-12 04:10:14 +09:00
EVM: Allow userland to permit modification of EVM-protected metadata When EVM is enabled it forbids modification of metadata protected by EVM unless there is already a valid EVM signature. If any modification is made, the kernel will then generate a new EVM HMAC. However, this does not map well on use cases which use only asymmetric EVM signatures, as in this scenario the kernel is unable to generate new signatures. This patch extends the /sys/kernel/security/evm interface to allow userland to request that modification of these xattrs be permitted. This is only permitted if no keys have already been loaded. In this configuration, modifying the metadata will invalidate the EVM appraisal on the file in question. This allows packaging systems to write out new files, set the relevant extended attributes and then move them into place. There's also some refactoring of the use of evm_initialized in order to avoid heading down codepaths that assume there's a key available. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-11-08 00:18:35 +09:00
echo 1 ><securityfs>/evm
EVM: Allow userspace to signal an RSA key has been loaded EVM will only perform validation once a key has been loaded. This key may either be a symmetric trusted key (for HMAC validation and creation) or the public half of an asymmetric key (for digital signature validation). The /sys/kernel/security/evm interface allows userland to signal that a symmetric key has been loaded, but does not allow userland to signal that an asymmetric public key has been loaded. This patch extends the interface to permit userspace to pass a bitmask of loaded key types. It also allows userspace to block loading of a symmetric key in order to avoid a compromised system from being able to load an additional key type later. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-10-12 04:10:14 +09:00
EVM: Allow userland to permit modification of EVM-protected metadata When EVM is enabled it forbids modification of metadata protected by EVM unless there is already a valid EVM signature. If any modification is made, the kernel will then generate a new EVM HMAC. However, this does not map well on use cases which use only asymmetric EVM signatures, as in this scenario the kernel is unable to generate new signatures. This patch extends the /sys/kernel/security/evm interface to allow userland to request that modification of these xattrs be permitted. This is only permitted if no keys have already been loaded. In this configuration, modifying the metadata will invalidate the EVM appraisal on the file in question. This allows packaging systems to write out new files, set the relevant extended attributes and then move them into place. There's also some refactoring of the use of evm_initialized in order to avoid heading down codepaths that assume there's a key available. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-11-08 00:18:35 +09:00
will enable HMAC validation and creation
EVM: Allow userspace to signal an RSA key has been loaded EVM will only perform validation once a key has been loaded. This key may either be a symmetric trusted key (for HMAC validation and creation) or the public half of an asymmetric key (for digital signature validation). The /sys/kernel/security/evm interface allows userland to signal that a symmetric key has been loaded, but does not allow userland to signal that an asymmetric public key has been loaded. This patch extends the interface to permit userspace to pass a bitmask of loaded key types. It also allows userspace to block loading of a symmetric key in order to avoid a compromised system from being able to load an additional key type later. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-10-12 04:10:14 +09:00
EVM: Allow userland to permit modification of EVM-protected metadata When EVM is enabled it forbids modification of metadata protected by EVM unless there is already a valid EVM signature. If any modification is made, the kernel will then generate a new EVM HMAC. However, this does not map well on use cases which use only asymmetric EVM signatures, as in this scenario the kernel is unable to generate new signatures. This patch extends the /sys/kernel/security/evm interface to allow userland to request that modification of these xattrs be permitted. This is only permitted if no keys have already been loaded. In this configuration, modifying the metadata will invalidate the EVM appraisal on the file in question. This allows packaging systems to write out new files, set the relevant extended attributes and then move them into place. There's also some refactoring of the use of evm_initialized in order to avoid heading down codepaths that assume there's a key available. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-11-08 00:18:35 +09:00
echo 0x80000003 ><securityfs>/evm
Merge branch 'next-integrity' of git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security Pull security subsystem integrity updates from James Morris: "There is a mixture of bug fixes, code cleanup, preparatory code for new functionality and new functionality. Commit 26ddabfe96bb ("evm: enable EVM when X509 certificate is loaded") enabled EVM without loading a symmetric key, but was limited to defining the x509 certificate pathname at build. Included in this set of patches is the ability of enabling EVM, without loading the EVM symmetric key, from userspace. New is the ability to prevent the loading of an EVM symmetric key." * 'next-integrity' of git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security: ima: Remove redundant conditional operator ima: Fix bool initialization/comparison ima: check signature enforcement against cmdline param instead of CONFIG module: export module signature enforcement status ima: fix hash algorithm initialization EVM: Only complain about a missing HMAC key once EVM: Allow userspace to signal an RSA key has been loaded EVM: Include security.apparmor in EVM measurements ima: call ima_file_free() prior to calling fasync integrity: use kernel_read_file_from_path() to read x509 certs ima: always measure and audit files in policy ima: don't remove the securityfs policy file vfs: fix mounting a filesystem with i_version
2017-11-14 03:41:25 +09:00
EVM: Allow userland to permit modification of EVM-protected metadata When EVM is enabled it forbids modification of metadata protected by EVM unless there is already a valid EVM signature. If any modification is made, the kernel will then generate a new EVM HMAC. However, this does not map well on use cases which use only asymmetric EVM signatures, as in this scenario the kernel is unable to generate new signatures. This patch extends the /sys/kernel/security/evm interface to allow userland to request that modification of these xattrs be permitted. This is only permitted if no keys have already been loaded. In this configuration, modifying the metadata will invalidate the EVM appraisal on the file in question. This allows packaging systems to write out new files, set the relevant extended attributes and then move them into place. There's also some refactoring of the use of evm_initialized in order to avoid heading down codepaths that assume there's a key available. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-11-08 00:18:35 +09:00
will enable HMAC and digital signature validation and
HMAC creation and disable all further modification of policy.
echo 0x80000006 ><securityfs>/evm
will enable digital signature validation, permit
modification of EVM-protected metadata and
disable all further modification of policy
evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded commit 9acc89d31f0c94c8e573ed61f3e4340bbd526d0c upstream. EVM_ALLOW_METADATA_WRITES is an EVM initialization flag that can be set to temporarily disable metadata verification until all xattrs/attrs necessary to verify an EVM portable signature are copied to the file. This flag is cleared when EVM is initialized with an HMAC key, to avoid that the HMAC is calculated on unverified xattrs/attrs. Currently EVM unnecessarily denies setting this flag if EVM is initialized with a public key, which is not a concern as it cannot be used to trust xattrs/attrs updates. This patch removes this limitation. Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of EVM-protected metadata") Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com> Cc: stable@vger.kernel.org # 4.16.x Signed-off-by: Mimi Zohar <zohar@linux.ibm.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2021-05-15 00:27:44 +09:00
Echoing a value is additive, the new value is added to the
existing initialization flags.
For example, after::
echo 2 ><securityfs>/evm
another echo can be performed::
echo 1 ><securityfs>/evm
and the resulting value will be 3.
Note that once an HMAC key has been loaded, it will no longer
be possible to enable metadata modification. Signaling that an
HMAC key has been loaded will clear the corresponding flag.
For example, if the current value is 6 (2 and 4 set)::
echo 1 ><securityfs>/evm
will set the new value to 3 (4 cleared).
Loading an HMAC key is the only way to disable metadata
modification.
EVM: Allow userland to permit modification of EVM-protected metadata When EVM is enabled it forbids modification of metadata protected by EVM unless there is already a valid EVM signature. If any modification is made, the kernel will then generate a new EVM HMAC. However, this does not map well on use cases which use only asymmetric EVM signatures, as in this scenario the kernel is unable to generate new signatures. This patch extends the /sys/kernel/security/evm interface to allow userland to request that modification of these xattrs be permitted. This is only permitted if no keys have already been loaded. In this configuration, modifying the metadata will invalidate the EVM appraisal on the file in question. This allows packaging systems to write out new files, set the relevant extended attributes and then move them into place. There's also some refactoring of the use of evm_initialized in order to avoid heading down codepaths that assume there's a key available. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-11-08 00:18:35 +09:00
Until key loading has been signaled EVM can not create
or validate the 'security.evm' xattr, but returns
INTEGRITY_UNKNOWN. Loading keys and signaling EVM
should be done as early as possible. Normally this is
done in the initramfs, which has already been measured
as part of the trusted boot. For more information on
creating and loading existing trusted/encrypted keys,
refer to:
Documentation/security/keys/trusted-encrypted.rst. Both
dracut (via 97masterkey and 98integrity) and systemd (via
EVM: Allow userspace to signal an RSA key has been loaded EVM will only perform validation once a key has been loaded. This key may either be a symmetric trusted key (for HMAC validation and creation) or the public half of an asymmetric key (for digital signature validation). The /sys/kernel/security/evm interface allows userland to signal that a symmetric key has been loaded, but does not allow userland to signal that an asymmetric public key has been loaded. This patch extends the interface to permit userspace to pass a bitmask of loaded key types. It also allows userspace to block loading of a symmetric key in order to avoid a compromised system from being able to load an additional key type later. Signed-off-by: Matthew Garrett <mjg59@google.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2017-10-12 04:10:14 +09:00
core/ima-setup) have support for loading keys at boot
time.
EVM: Allow runtime modification of the set of verified xattrs Sites may wish to provide additional metadata alongside files in order to make more fine-grained security decisions[1]. The security of this is enhanced if this metadata is protected, something that EVM makes possible. However, the kernel cannot know about the set of extended attributes that local admins may wish to protect, and hardcoding this policy in the kernel makes it difficult to change over time and less convenient for distributions to enable. This patch adds a new /sys/kernel/security/integrity/evm/evm_xattrs node, which can be read to obtain the current set of EVM-protected extended attributes or written to in order to add new entries. Extending this list will not change the validity of any existing signatures provided that the file in question does not have any of the additional extended attributes - missing xattrs are skipped when calculating the EVM hash. [1] For instance, a package manager could install information about the package uploader in an additional extended attribute. Local LSM policy could then be associated with that extended attribute in order to restrict the privileges available to packages from less trusted uploaders. Signed-off-by: Matthew Garrett <mjg59@google.com> Reviewed-by: James Morris <james.morris@microsoft.com> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
2018-05-16 02:38:26 +09:00
What: security/integrity/evm/evm_xattrs
Date: April 2018
Contact: Matthew Garrett <mjg59@google.com>
Description:
Shows the set of extended attributes used to calculate or
validate the EVM signature, and allows additional attributes
to be added at runtime. Any signatures generated after
additional attributes are added (and on files posessing those
additional attributes) will only be valid if the same
additional attributes are configured on system boot. Writing
a single period (.) will lock the xattr list from any further
modification.