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 an asymmetric key in order to avoid a compromised system from being able to load an additional key type later. Signed-off-by: Matthew Garrett <mjg59@xxxxxxxxxx> --- Documentation/ABI/testing/evm | 48 +++++++++++++++++++++++++++----------- security/integrity/evm/evm.h | 3 +++ security/integrity/evm/evm_secfs.c | 29 +++++++++++++---------- 3 files changed, 54 insertions(+), 26 deletions(-) diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm index 8374d4557e5d..d2782afb0d96 100644 --- a/Documentation/ABI/testing/evm +++ b/Documentation/ABI/testing/evm @@ -7,17 +7,37 @@ Description: HMAC-sha1 value across the extended attributes, storing the value as the extended attribute 'security.evm'. - 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.) + 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 + echoing a value to <securityfs>/evm: + + 1: enable HMAC validation and creation + 2: enable digital signature validation + 3: enable HMAC and digital signature validation and HMAC + creation + + Further writes will be blocked if HMAC support is enabled or + if bit 32 is set: + + echo 0x80000002 ><securityfs>/evm + + will enable digital signature validation and block + further writes to <securityfs>/evm. + + Until this is done, 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/keys-trusted-encrypted.txt. Both dracut + (via 97masterkey and 98integrity) and systemd (via + core/ima-setup) have support for loading keys at boot + time. + diff --git a/security/integrity/evm/evm.h b/security/integrity/evm/evm.h index 2ff02459fcfd..946efffcc389 100644 --- a/security/integrity/evm/evm.h +++ b/security/integrity/evm/evm.h @@ -23,6 +23,9 @@ #define EVM_INIT_HMAC 0x0001 #define EVM_INIT_X509 0x0002 +#define EVM_SETUP 0x80000000 /* userland has signaled key load */ + +#define EVM_INIT_MASK (EVM_INIT_HMAC | EVM_INIT_X509 | EVM_SETUP) extern int evm_initialized; extern char *evm_hmac; diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c index c8dccd54d501..319cf16d6603 100644 --- a/security/integrity/evm/evm_secfs.c +++ b/security/integrity/evm/evm_secfs.c @@ -40,7 +40,7 @@ static ssize_t evm_read_key(struct file *filp, char __user *buf, if (*ppos != 0) return 0; - sprintf(temp, "%d", evm_initialized); + sprintf(temp, "%d", (evm_initialized & ~EVM_SETUP)); rc = simple_read_from_buffer(buf, count, ppos, temp, strlen(temp)); return rc; @@ -61,24 +61,29 @@ static ssize_t evm_read_key(struct file *filp, char __user *buf, static ssize_t evm_write_key(struct file *file, const char __user *buf, size_t count, loff_t *ppos) { - char temp[80]; - int i; + int i, ret; - if (!capable(CAP_SYS_ADMIN) || (evm_initialized & EVM_INIT_HMAC)) + if (!capable(CAP_SYS_ADMIN) || (evm_initialized & EVM_SETUP)) return -EPERM; - if (count >= sizeof(temp) || count == 0) - return -EINVAL; - - if (copy_from_user(temp, buf, count) != 0) - return -EFAULT; + ret = kstrtoint_from_user(buf, count, 0, &i); - temp[count] = '\0'; + if (ret) + return ret; - if ((sscanf(temp, "%d", &i) != 1) || (i != 1)) + /* Reject invalid values */ + if (!i || (i & ~EVM_INIT_MASK) != 0) return -EINVAL; - evm_init_key(); + if (i & EVM_INIT_HMAC) { + ret = evm_init_key(); + if (ret != 0) + return ret; + /* Forbid further writes after the symmetric key is loaded */ + i |= EVM_SETUP; + } + + evm_initialized |= i; return count; } -- 2.15.0.rc0.271.g36b669edcc-goog
