Mimi Zohar <zo...@linux.vnet.ibm.com> writes:
> On Wed, 2017-08-02 at 18:52 -0400, Mimi Zohar wrote:
>> On Wed, 2017-08-02 at 14:42 -0300, Thiago Jung Bauermann wrote:
>> > Mimi Zohar <zo...@linux.vnet.ibm.com> writes:
>
>> > >> @@ -229,8 +251,24 @@ int ima_appraise_measurement(enum ima_hooks func,
>> > >> goto out;
>> > >> }
>> > >>
>> > >> - status = evm_verifyxattr(dentry, XATTR_NAME_IMA, xattr_value,
>> > >> rc, iint);
>> > >> - if ((status != INTEGRITY_PASS) && (status !=
>> > >> INTEGRITY_UNKNOWN)) {
>> > >> + /*
>> > >> + * Appended signatures aren't protected by EVM but we still call
>> > >> + * evm_verifyxattr to check other security xattrs, if they
>> > >> exist.
>> > >> + */
>> > >> + if (appraising_modsig) {
>> > >> + xattr_value_evm = NULL;
>> > >> + xattr_len_evm = 0;
>> > >> + } else {
>> > >> + xattr_value_evm = xattr_value;
>> > >> + xattr_len_evm = xattr_len;
>> > >> + }
>> > >> +
>> > >> + status = evm_verifyxattr(dentry, XATTR_NAME_IMA,
>> > >> xattr_value_evm,
>> > >> + xattr_len_evm, iint);
>> > >> + if (appraising_modsig && status == INTEGRITY_FAIL) {
>> > >> + cause = "invalid-HMAC";
>> > >> + goto out;
>> > >
>> > > "modsig" is special, because having any security xattrs is not
>> > > required. This test doesn't prevent status from being set to
>> > > "missing-HMAC". This test is redundant with the original tests below.
>> >
>> > Indeed, that is wrong. I'm still a bit fuzzy about how EVM works and how
>> > it interacts with IMA. The only way I can think of singling out modsig
>> > without reintroduced the complex expression you didn't like in v2 is as
>> > below. What do you think?
>>
>> The original code, without any extra tests, should be fine.
>
> There is one major difference.
>
> EVM verifies a file's metadata has not been modified based on either
> an HMAC or signature stored as security.evm. Prior to the appended
> signatures patch set, all files in policy required a security.evm
> xattr. With IMA enabled we could guarantee that at least one security
> xattr existed. The only exception were new files, which hadn't yet
> been labeled.
>
> With appended signatures, there is now no guarantee that at least one
> security xattr exists.
>
> Perhaps the code snippet below will help clarify the meaning of the
> integrity_status results.
>
> switch (status) {
> case INTEGRITY_PASS:
> case INTEGRITY_UNKNOWN:
> break;
> case INTEGRITY_NOXATTRS:/* no EVM protected xattrs */
> if (appraising_modsig)
> break;
> case INTEGRITY_NOLABEL:/* no security.evm xattr */
> cause = "missing-HMAC";
> fail = 1;
> break;
> case INTEGRITY_FAIL:/* invalid HMAC/signature */
> default:
> cause = "invalid-HMAC";
> fail = 1;
> break;
> }
Thanks! I'll use the switch above in the next version of the patch.
--
Thiago Jung Bauermann
IBM Linux Technology Center
