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

[Mimi Zohar <zohar@linux.ibm.com>]

On Tue, 2020-05-12 at 16:31 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@linux.ibm.com]

> > > > Each time the EVM protected file metadata is updated, the EVM HMAC
> > is
> > > > updated, assuming the existing EVM HMAC is valid.  Userspace 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.  Since the new EVM signature does not include the
> > > > filesystem specific data, something else needs to bind the file
> > > > metadata to the file data.  Thus 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().
> 
> To solve the ordering issue, wouldn't allowing setxattr() on a file
> with portable signature that does not yet pass verification be safe?
> evm_update_evmxattr() checks if the signature is portable and
> if yes, does not calculate the HMAC.

Before agreeing to allowing the protected xattrs to be written on a
file with a portable signature that does not yet pass verification are
safe, would we be introducing any new types of attacks?

For example, would we differentiate between portable signatures that
don't pass verification and ones that do?  If we don't differentiate,
could it be used for DoS?  Should it be limited to new files?

Mimi