[PATCH v2 7/7] Smack: Handle labels consistently in untrusted mounts

[Seth Forshee]

On Wed, Oct 14, 2015 at 10:46:47PM -0700, Casey Schaufler wrote:
> On 10/13/2015 10:04 AM, Seth Forshee wrote:
> > The SMACK64, SMACK64EXEC, and SMACK64MMAP labels are all handled
> > differently in untrusted mounts. This is confusing and
> > potentically problematic. Change this to handle them all the same
> > way that SMACK64 is currently handled; that is, read the label
> > from disk and check it at use time. For SMACK64 and SMACK64MMAP
> > access is denied if the label does not match smk_root. To be
> > consistent with suid, a SMACK64EXEC label which does not match
> > smk_root will still allow execution of the file but will not run
> > with the label supplied in the xattr.
> >
> > Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
> 
> Aside from the one comment below (which I can be talked out of)
> this looks fine.
> 
> > ---
> >  security/smack/smack_lsm.c | 28 ++++++++++++++++++----------
> >  1 file changed, 18 insertions(+), 10 deletions(-)
> >
> > diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> > index 621200f86b56..bee0b2652bf4 100644
> > --- a/security/smack/smack_lsm.c
> > +++ b/security/smack/smack_lsm.c
> > @@ -891,6 +891,7 @@ static int smack_bprm_set_creds(struct linux_binprm *bprm)
> >  	struct inode *inode = file_inode(bprm->file);
> >  	struct task_smack *bsp = bprm->cred->security;
> >  	struct inode_smack *isp;
> > +	struct superblock_smack *sbsp;
> >  	int rc;
> >  
> >  	if (bprm->cred_prepared)
> > @@ -900,6 +901,10 @@ static int smack_bprm_set_creds(struct linux_binprm *bprm)
> >  	if (isp->smk_task == NULL || isp->smk_task == bsp->smk_task)
> >  		return 0;
> >  
> > +	sbsp = inode->i_sb->s_security;
> > +	if (sbsp->smk_flags & SMK_SB_UNTRUSTED && isp->smk_task != sbsp->smk_root)
> 
> Call me old fashioned, but how about
> 
> 	if ((sbsp->smk_flags & SMK_SB_UNTRUSTED) && isp->smk_task != sbsp->smk_root)
> 
> naked '&'s give me the willies. 

That's fine by me.

Seth