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authorMatthew Garrett <mjg59@google.com>2017-10-11 12:10:14 -0700
committerMimi Zohar <zohar@linux.vnet.ibm.com>2017-11-08 15:16:36 -0500
commitf00d79750712511d0a83c108eea0d44b680a915f (patch)
treef0bdb9499c9e6b7bb7c37ace6b70d25bc5035ac5
parent096b85464832d2a7bd7bd6d4db2fafed2ab77244 (diff)
downloadblackbird-op-linux-f00d79750712511d0a83c108eea0d44b680a915f.tar.gz
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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>
-rw-r--r--Documentation/ABI/testing/evm47
-rw-r--r--security/integrity/evm/evm.h3
-rw-r--r--security/integrity/evm/evm_secfs.c29
3 files changed, 53 insertions, 26 deletions
diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm
index 8374d4557e5d..a0bbccb00736 100644
--- a/Documentation/ABI/testing/evm
+++ b/Documentation/ABI/testing/evm
@@ -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.
diff --git a/security/integrity/evm/evm.h b/security/integrity/evm/evm.h
index f5f12727771a..241aca315b0c 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;
}
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