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EVM: Allow userspace to signal an RSA key has been loaded

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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>
This commit is contained in:
Matthew Garrett 2017-10-11 12:10:14 -07:00 committed by Mimi Zohar
parent 096b854648
commit f00d797507
3 changed files with 54 additions and 27 deletions
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47
Documentation/ABI/testing/evm
View file

@ -7,17 +7,36 @@ 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.

3
security/integrity/evm/evm.h
View file

@ -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;

31
security/integrity/evm/evm_secfs.c
View file

@ -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)
ret = kstrtoint_from_user(buf, count, 0, &i);
if (ret)
return ret;
/* Reject invalid values */
if (!i || (i & ~EVM_INIT_MASK) != 0)
return -EINVAL;
if (copy_from_user(temp, buf, count) != 0)
return -EFAULT;
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;
}
temp[count] = '\0';
if ((sscanf(temp, "%d", &i) != 1) || (i != 1))
return -EINVAL;
evm_init_key();
evm_initialized |= i;
return count;
}