Re: [PATCH] Split bprm_apply_creds into two functions

[Chris Wright]

* Serge E. Hallyn (serue@xxxxxxxxxx) wrote:
> Quoting Chris Wright (chrisw@xxxxxxxx):
> > * Serge E. Hallyn (serue@xxxxxxxxxx) wrote:
> > > The security_bprm_apply_creds() function is called from
> > > fs/exec.c:compute_creds() under task_lock(current).  SELinux must
> > > perform some work which is unsafe in that context, and therefore
> > > explicitly drops the task_lock, does the work, and re-acquires the
> > > task_lock.  This is unsafe if other security modules are stacked after
> > > SELinux, as their bprm_apply_creds assumes that the 'unsafe' variable is
> > > still meaningful, that is, that the task_lock has not been dropped.
> > 
> > I don't like this approach.  The whole point is to ensure safety, and
> > avoid races that have been found in the past.  This gives a new interface
> > that could be easily used under the wrong conditions, and breaking
> > the interface into two pieces looks kinda hackish.  Is there no other
> > solution?  I looked at this once before and wondered why task_unlock()
> > is needed to call avc_audit?  audit should be as lock friendly as printk
> > IMO, and I don't recall seeing any deadlock after short review of it.
> > But I didn't get much beyond that.  Is it all the flushing that can't
> > hold task_lock?
> 
> As Stephen points out, it was more a concern about lock nesting.  The
> attached patch simply removes the task_unlock from selinux_bprm_apply_creds,
> and runs just fine on my machine.  Stephen, do you have a preference
> either way, or was the task_unlock to relieve the concerns of others?

Unfortunately, running fine isn't indication that it's not
deadlock-able.  Issues are making sure nesting is appropriate, as
task_lock is normally inner lock, and making sure normal things like not
taking sem when holding spinlock.  It requires more inspection than just
running.

thanks,
-chris
-- 
Linux Security Modules     http://lsm.immunix.org     http://lsm.bkbits.net
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