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

On Tue, 2020-05-12 at 16:31 +0000, Roberto Sassu wrote:

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From: Mimi Zohar [mailto:zohar@linux.ibm.com]

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Each time the EVM protected file metadata is updated, the EVM HMAC

is

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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,

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                                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