linux-kernel - Re: [PATCH 2/6] ima: Return INTEGRITY_FAIL if digital signature can't be verified

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Message-ID: <1362490253.4392.159.camel@falcor1>
Date:	Tue, 05 Mar 2013 08:30:53 -0500
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, dmitry.kasatkin@...el.com
Subject: Re: [PATCH 2/6] ima: Return INTEGRITY_FAIL if digital signature
 can't be verified

On Mon, 2013-03-04 at 11:20 -0500, Vivek Goyal wrote:
> On Mon, Mar 04, 2013 at 08:48:36AM -0500, Mimi Zohar wrote:
> > On Thu, 2013-02-14 at 14:55 -0500, Vivek Goyal wrote:
> > > Digital signature verification happens using integrity_digsig_verify().
> > > Curently we set integrity to FAIL for all error codes except -EOPNOTSUPP.
> > > This sounds out of line.
> > > 
> > > - If appropriate kernel code is not compiled in to verify signature of
> > >   a file, then prractically it is a failed signature.
> > > 
> > > - For so many other possible errors we are setting the status to fail.
> > >   For example, -EINVAL, -ENOKEY, -ENOMEM, -EINVAL, -ENOTSUPP etc, it
> > >   beats me that why -EOPNOTSUPP is special.
> > > 
> > > This patch should make the semantics more consistent. That is, if digital
> > > signature is present in security.ima, then any error happened during
> > > signature processing leads to status INTEGRITY_FAIL.
> > > 
> > > AFAICS, it should not have any user visible effect on existing
> > > application. In some cases we will start returning INTEGRITY_FAIL
> > > instead of INTEGRITY_UNKNOWN. And process_measurement() will deny access
> > > to file both in case of INTEGRITY_UNKNOWN and INTEGRITY_FAIL.
> > > 
> > > Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> > 
> > A number of patches in this patchset more finely differentiate return
> > codes, which is good.  I agree with you totally that there is no good
> > reason for -EOPNOTSUPP to be handled differently.  Unfortunately, the
> > initramfs is CPIO, which doesn't support xattrs.  With the proposed
> > change and 'ima_appraise_tcb' flag enabled, we wouldn't be able to boot.
> > I really dislike hard coding policy in the kernel.
> 
> Hi Mimi,
> 
> If there are no xattr, then we will not even hit this code. We will
> bail out early in vfs_getxattr_alloc().
> 
> I thought that one of the DON_APPRAISE rules will kick in for initramfs
> and files in initramfs will not be appraised and boot will continue.
> 
> {.action = DONT_APPRAISE,.fsmagic = TMPFS_MAGIC,.flags = IMA_FSMAGIC},
> {.action = DONT_APPRAISE,.fsmagic = RAMFS_MAGIC,.flags = IMA_FSMAGIC},
> 
> Is that not the case here?

Right, thanks for the clarification.  Perhaps we could abbreviate the
patch description like:
 
Digital signature verification happens using integrity_digsig_verify().
If a digital signature is present in security.ima, then any error, which
happens during signature verification, should lead to status
INTEGRITY_FAIL.  In the future we might want to differentiate between
persistent (eg. -ENOMEM) vs. non-persistent errors, in order to cache
failures.  This patch removes the unnecessary -EOPNOTSUPP test.
 
thanks,

Mimi

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