RE: [PATCH] ima: Allow imasig requirement to be satisfied by EVM portable signatures

[Roberto Sassu]

> -----Original Message-----
> From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx]
> Sent: Thursday, April 23, 2020 10:52 PM
> To: Roberto Sassu <roberto.sassu@xxxxxxxxxx>; mjg59@xxxxxxxxxx
> Cc: linux-integrity@xxxxxxxxxxxxxxx; linux-security-module@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; Silviu Vlasceanu
> <Silviu.Vlasceanu@xxxxxxxxxx>
> Subject: Re: [PATCH] ima: Allow imasig requirement to be satisfied by EVM
> portable signatures
> 
> On Tue, 2020-04-21 at 11:24 +0200, Roberto Sassu wrote:
> > System administrators can require that all accessed files have a signature
> > by specifying appraise_type=imasig in a policy rule.
> >
> > Currently, only IMA signatures satisfy this requirement. However, also
> EVM
> > portable signatures can satisfy it. Metadata, including security.ima, are
> > signed and cannot change.
> 
> Please expand this paragraph with a short comparison of the security
> guarantees provided by EVM immutable, portable signatures versus ima-
> sig.
> 
> >
> > This patch helps in the scenarios where system administrators want to
> > enforce this restriction but only EVM portable signatures are available.
> 
> Yes, I agree it "helps", but we still need to address the ability of
> setting/removing security.ima, which isn't possible with an IMA
> signature. ÂThis sounds like we need to define an immutable file hash.

I didn't understand. Can you explain better?

Thanks

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli


> ÂWhat do you think?
> 
> > The patch makes the following changes:
> >
> > file xattr types:
> > security.ima: IMA_XATTR_DIGEST/IMA_XATTR_DIGEST_NG
> > security.evm: EVM_XATTR_PORTABLE_DIGSIG
> >
> > execve(), mmap(), open() behavior (with appraise_type=imasig):
> > before: denied (file without IMA signature, imasig requirement not met)
> > after: allowed (file with EVM portable signature, imasig requirement met)
> >
> > open(O_WRONLY) behavior (without appraise_type=imasig):
> > before: allowed (file without IMA signature, not immutable)
> > after: denied (file with EVM portable signature, immutable)
> >
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > ---
> >  security/integrity/ima/ima_appraise.c | 14 +++++++++-----
> >  1 file changed, 9 insertions(+), 5 deletions(-)
> >
> > diff --git a/security/integrity/ima/ima_appraise.c
> b/security/integrity/ima/ima_appraise.c
> > index a9649b04b9f1..69a6a958f811 100644
> > --- a/security/integrity/ima/ima_appraise.c
> > +++ b/security/integrity/ima/ima_appraise.c
> > @@ -219,12 +219,16 @@ static int xattr_verify(enum ima_hooks func,
> struct integrity_iint_cache *iint,
> >  		hash_start = 1;
> >  		/* fall through */
> >  	case IMA_XATTR_DIGEST:
> > -		if (iint->flags & IMA_DIGSIG_REQUIRED) {
> > -			*cause = "IMA-signature-required";
> > -			*status = INTEGRITY_FAIL;
> > -			break;
> > +		if (*status != INTEGRITY_PASS_IMMUTABLE) {
> > +			if (iint->flags & IMA_DIGSIG_REQUIRED) {
> > +				*cause = "IMA-signature-required";
> > +				*status = INTEGRITY_FAIL;
> > +				break;
> > +			}
> > +			clear_bit(IMA_DIGSIG, &iint->atomic_flags);
> > +		} else {
> > +			set_bit(IMA_DIGSIG, &iint->atomic_flags);
> >  		}
> > -		clear_bit(IMA_DIGSIG, &iint->atomic_flags);
> >  		if (xattr_len - sizeof(xattr_value->type) - hash_start >=
> >  				iint->ima_hash->length)
> >  			/*
> 
> Nice!
> 
> Mimi