Re: [PATCH 3/4] ima: Improvements in ima_appraise_measurement()

[Thiago Jung Bauermann]

Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> writes:
> On Wed, 2018-03-14 at 21:03 -0300, Thiago Jung Bauermann wrote:
>> Hello Serge,
>> 
>> Thanks for quickly reviewing these patches!
>> 
>> Serge E. Hallyn <serge@xxxxxxxxxx> writes:
>> 
>> > Quoting Thiago Jung Bauermann (bauerman@xxxxxxxxxxxxxxxxxx):
>> >> From: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
>> >> @@ -241,16 +241,20 @@ int ima_appraise_measurement(enum ima_hooks func,
>> >>  	}
>> >>  
>> >>  	status = evm_verifyxattr(dentry, XATTR_NAME_IMA, xattr_value, rc, iint);
>> >> -	if ((status != INTEGRITY_PASS) &&
>> >> -	    (status != INTEGRITY_PASS_IMMUTABLE) &&
>> >> -	    (status != INTEGRITY_UNKNOWN)) {
>> >> -		if ((status == INTEGRITY_NOLABEL)
>> >> -		    || (status == INTEGRITY_NOXATTRS))
>> >> -			cause = "missing-HMAC";
>> >> -		else if (status == INTEGRITY_FAIL)
>> >> -			cause = "invalid-HMAC";
>> >> +	switch (status) {
>> >> +	case INTEGRITY_PASS:
>> >> +	case INTEGRITY_PASS_IMMUTABLE:
>> >> +	case INTEGRITY_UNKNOWN:
>> >
>> > Wouldn't it be more future-proof to replace this with a 'default', or
>> > to at least add a "default: BUG()" to catch new status values?
>> 
>> I agree. I like the "default: BUG()" option.
>
> Agreed. I would put it at the end after INTEGRITY_FAIL.

Ok, what about the version below?

>> 
>> >> +		break;
>> >> +	case INTEGRITY_NOXATTRS:	/* No EVM protected xattrs. */
>> >> +	case INTEGRITY_NOLABEL:		/* No security.evm xattr. */
>> >> +		cause = "missing-HMAC";
>> >> +		goto out;
>> >> +	case INTEGRITY_FAIL:		/* Invalid HMAC/signature. */
>> >> +		cause = "invalid-HMAC";
>> >>  		goto out;
>> >>  	}
>> >> +
>> >>  	switch (xattr_value->type) {
>> >>  	case IMA_XATTR_DIGEST_NG:
>> >>  		/* first byte contains algorithm id */
>> >> @@ -316,17 +320,20 @@ int ima_appraise_measurement(enum ima_hooks func,
>> >>  		integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
>> >>  				    op, cause, rc, 0);
>> >>  	} else if (status != INTEGRITY_PASS) {
>> >> +		/* Fix mode, but don't replace file signatures. */
>> >>  		if ((ima_appraise & IMA_APPRAISE_FIX) &&
>> >>  		    (!xattr_value ||
>> >>  		     xattr_value->type != EVM_IMA_XATTR_DIGSIG)) {
>> >>  			if (!ima_fix_xattr(dentry, iint))
>> >>  				status = INTEGRITY_PASS;
>> >> -		} else if ((inode->i_size == 0) &&
>> >> -			   (iint->flags & IMA_NEW_FILE) &&
>> >> -			   (xattr_value &&
>> >> -			    xattr_value->type == EVM_IMA_XATTR_DIGSIG)) {
>> >> +		}
>> >> +
>> >> +		/* Permit new files with file signatures, but without data. */
>> >> +		if (inode->i_size == 0 && iint->flags & IMA_NEW_FILE &&
>> >
>> > This may be correct, but it's not identical to what you're replacing.
>> > Since in either case you're setting status to INTEGRITY_PASS the final
>> > result is the same, but with a few extra possible steps.  Not sure
>> > whether that matters.
>> 
>> Good point. I'll have to defer this one to Mimi though.
>
> The end result is the same, but add some needed comments.

The patch is unchanged here, then.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center