Re: [RFC][PATCH 1/3] evm: Move hooks outside LSM infrastructure

[Mimi Zohar]

On Tue, 2020-05-12 at 15:31 +0000, Roberto Sassu wrote:
> > From: owner-linux-security-module@xxxxxxxxxxxxxxx [mailto:owner-linux-
> > security-module@xxxxxxxxxxxxxxx] On Behalf Of Mimi Zohar
> > Sent: Tuesday, May 12, 2020 4:17 PM
> > On Tue, 2020-05-12 at 07:54 +0000, Roberto Sassu wrote:
> > > > > > Roberto, EVM is only triggered by IMA, unless you've modified the
> > > > > > kernel to do otherwise.
> > > > >
> > > > > EVM would deny xattr/attr operations even if IMA is disabled in the
> > > > > kernel configuration. For example, evm_setxattr() returns the value
> > > > > from evm_protect_xattr(). IMA is not involved there.
> > > >
> > > > CommitÂae1ba1676b88 ("EVM: Allow userland to permit modification of
> > > > EVM-protected metadata")
> > introducedÂEVM_ALLOW_METADATA_WRITES
> > > > to allow writing the EVM portable and immutable file signatures.
> > >
> > > According to Documentation/ABI/testing/evm:
> > >
> > > Note that once a key has been loaded, it will no longer be
> > > possible to enable metadata modification.
> > 
> > Not any key, but the HMAC key.
> > 
> > 2ÂÂÂÂÂÂÂÂÂPermit modification of EVM-protected metadata at
> > Â Â Â Â Â runtime. Not supported if HMAC validation and
> > Â Â Â Â Â creation is enabled.
> 
> #ifdef CONFIG_EVM_LOAD_X509
> void __init evm_load_x509(void)
> {
> [...]
>         rc = integrity_load_x509(INTEGRITY_KEYRING_EVM, CONFIG_EVM_X509_PATH);
>         if (!rc)
>                 evm_initialized |= EVM_INIT_X509;
> 
> 
> static ssize_t evm_write_key(struct file *file, const char __user *buf,
>                              size_t count, loff_t *ppos)
> {
> [...]
>         /* Don't allow a request to freshly enable metadata writes if
>          * keys are loaded.
>          */
>         if ((i & EVM_ALLOW_METADATA_WRITES) &&
>             ((evm_initialized & EVM_KEY_MASK) != 0) &&
>             !(evm_initialized & EVM_ALLOW_METADATA_WRITES))
>                 return -EPERM;
> 
> Should have been:
> 
>         if ((i & EVM_ALLOW_METADATA_WRITES) &&
>             ((evm_initialized & EVM_INIT_HMAC) != 0) &&
>             !(evm_initialized & EVM_ALLOW_METADATA_WRITES))
>                 return -EPERM;

Ok

> 
> > Each time the EVM protected file metadata is updated, the EVM HMAC is
> > updated, assuming the existing EVM HMAC is valid. ÂUserspace should
> > not have access to the HMAC key, so we only allow writing EVM
> > signatures.
> > 
> > The only difference between writing the original EVM signature and the
> > new portable and immutable signature is the security.ima xattr
> > requirement. ÂSince the new EVM signature does not include the
> > filesystem specific data, something else needs to bind the file
> > metadata to the file data. ÂThus the IMA xattr requirement.
> > 
> > Assuming that the new EVM signature is written last, as long as there
> > is an IMA xattr, there shouldn't be a problem writing the new EVM
> > signature.
> 
>         /* first need to know the sig type */
>         rc = vfs_getxattr_alloc(dentry, XATTR_NAME_EVM, (char **)&xattr_data, 0,
>                                 GFP_NOFS);
>         if (rc <= 0) {
>                 evm_status = INTEGRITY_FAIL;
>                 if (rc == -ENODATA) {
>                         rc = evm_find_protected_xattrs(dentry);
>                         if (rc > 0)
>                                 evm_status = INTEGRITY_NOLABEL;
>                         else if (rc == 0)
>                                 evm_status = INTEGRITY_NOXATTRS; /* new file */
> 
> If EVM_ALLOW_METADATA_WRITES is cleared, only the first xattr
> can be written (status INTEGRITY_NOXATTRS is ok). After,
> evm_find_protected_xattrs() returns rc > 0, so the status is
> INTEGRITY_NOLABEL, which is not ignored by evm_protect_xattr().

With EVM HMAC enabled, as a result of writing the first protected
xattr, an EVM HMAC should be calculated and written in
evm_inode_post_setxattr().

Mimi