On Wed, 2017-10-11 at 12:10 -0700, Matthew Garrett wrote: > 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. I assume you mean "block loading of a symmetric key". Other than this and a trailing blank line, the patch looks good. If you don't have objections, I'll fix these two things. Mimi > > 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; > }
