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>

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;
 }