linux-kernel - Re: [RFC][PATCH 4/7] smack: fix label check in smack_kernel_act

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Date:	Thu, 28 Apr 2011 11:22:31 +0200
From:	Roberto Sassu <roberto.sassu@...ito.it>
To:	Casey Schaufler <casey@...aufler-ca.com>
Cc:	linux-security-module@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	dhowells@...hat.com, jmorris@...ei.org, zohar@...ux.vnet.ibm.com,
	safford@...son.ibm.com, tyhicks@...ux.vnet.ibm.com,
	kirkland@...onical.com, ecryptfs-devel@...ts.launchpad.net,
	eparis@...hat.com, sds@...ho.nsa.gov, selinux@...ho.nsa.gov,
	viro@...iv.linux.org.uk
Subject: Re: [RFC][PATCH 4/7] smack: fix label check in smack_kernel_act_as()

On Thursday, April 28, 2011 01:22:34 AM Casey Schaufler wrote:
> On 4/27/2011 5:34 AM, Roberto Sassu wrote:
> > The function smack_kernel_act_as() must return -EINVAL if the label
> > returned by smack_from_secid() is equal to 'smack_known_invalid.smk_known',
> > which means that no entries in the 'smack_known_list' list matching the
> > security identifier given are found.
> 
> I'll admit that the code here is wrong, but I disagree with the fix.
> 
> smack_from_secid() will never return NULL, so the check for NULL is
> pointless. Checking for known_invalid is not right either, as the
> Smack philosophy is to return a label in all cases, as is evident
> by the behavior of smack_from_secid(). Thus, the correct change
> would be to remove the error check completely and set the new task
> value to the value obtained from smack_from_secid in all cases.
> 
> Besides, where did the caller of this function get a secid that isn't
> going to map to a Smack label?
> 

I discovered the issue when calling the function set_security_override_from_ctx(),
located in kernel/cred.c, which will be used in eCryptfs to override the set
of initial credentials (i'm about to post the patches).
This function accepts an arbitrary string and uses it as a security label to replace
the one in the credentials provided. If the string passed is not yet imported
to SMACK, the 'smack_known_invalid.smk_known' label is applied without this fact
is notified to the caller.
I tried to solve the problem by importing the string during the execution of the
security_secctx_to_secid() hook, so that set_security_override() will never fail.
So, probably the string must be already been imported if no new memory can be
allocated during smack_secctx_to_secid().


> > Signed-off-by: Roberto Sassu <roberto.sassu@...ito.it>
> > ---
> >  security/smack/smack_lsm.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> > index e3c9e54..0e7ed31 100644
> > --- a/security/smack/smack_lsm.c
> > +++ b/security/smack/smack_lsm.c
> > @@ -1415,7 +1415,7 @@ static int smack_kernel_act_as(struct cred *new, u32 secid)
> >  	struct task_smack *new_tsp = new->security;
> >  	char *smack = smack_from_secid(secid);
> >  
> > -	if (smack == NULL)
> > +	if (smack == smack_known_invalid.smk_known)
> >  		return -EINVAL;
> >  
> >  	new_tsp->smk_task = smack;
> 
> 
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