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SubjectRe: [PATCH v3 06/11] rlimits: do security check under task_lock
On 06/23, Jiri Slaby wrote:
>
> On 06/07/2010 08:08 PM, Oleg Nesterov wrote:
> > First of all, my apologies for the huge delay. And I still didn't
> > read the whole series, sorry.
>
> Hi, never mind, my RTT of 2 weeks doesn't look like very short too :).
>
> > On 06/06, Jiri Slaby wrote:
> >> @@ -1339,13 +1364,19 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource,
> >>
> >> rlim = tsk->signal->rlim + resource;
> >> task_lock(tsk->group_leader);
> >> +again:
> >> + retval = 0;
> >> if (new_rlim) {
> >> if ((new_rlim->rlim_max > rlim->rlim_max) &&
> >> !capable(CAP_SYS_RESOURCE))
>
> BTW this capable() has the exactly same problem with being called with
> task_lock held. Is it OK to move it completely out of critical section?
> I'm asking because it sets a current->flags SU bit used for accounting.
> If I move it out of the section, it will set the bit always.

Well, with all these delays I do not know what "exactly same problem"
means ;) Please explain?

> >> retval = -EPERM;
> >> - if (!retval)
> >> - retval = security_task_setrlimit(tsk, resource,
> >> - new_rlim);
> >> + if (!retval) {
> >> + retval = check_security_task_setrlimit_unlocked(tsk,
> >> + resource, new_rlim, rlim);
> >> + if (retval == -EAGAIN) {
> >> + goto again;
> >> + }
> >> + }
> >
> > Oh. Can't we just ignore this (imho minor) race ? Or just verify/document that
> > current_has_perm() can be called under task_lock. Actually, I do not think
> > we have a race, selinux_task_setrlimit() only checks that the caller has
> > rights to change the rlimits.
>
> But does so only if current limits are different to the new ones. My
> opinion is that we can ignore it anyway.

Or call it under task_lock(), see below

> > And. Given that avc_has_perm() can be called from irq context (say,
> > selinux_file_send_sigiotask or selinux_task_kill), we can assume it is safe
> > to call it under task_lock() which is not irq-safe.
> >
> > But. OTOH, if we are really worried about security_ ops, then we have another
> > reason to call this hook under task_lock(), and we probably want to modify
> > selinux_bprm_committing_creds() to take this lock too:
> >
> > --- security/selinux/hooks.c
> > +++ security/selinux/hooks.c
> > @@ -2333,11 +2333,14 @@ static void selinux_bprm_committing_cred
> > rc = avc_has_perm(new_tsec->osid, new_tsec->sid, SECCLASS_PROCESS,
> > PROCESS__RLIMITINH, NULL);
> > if (rc) {
> > + /* protects against do_prlimit() */
> > + task_lock(current);
> > for (i = 0; i < RLIM_NLIMITS; i++) {
> > rlim = current->signal->rlim + i;
> > initrlim = init_task.signal->rlim + i;
> > rlim->rlim_cur = min(rlim->rlim_max, initrlim->rlim_cur);
> > }
> > + task_unlock(current);
> > update_rlimit_cpu(current->signal->rlim[RLIMIT_CPU].rlim_cur);
> > }
> > }
>
> Makes sense to me.

see above ;)

> > Finally. selinux_task_setrlimit(p) uses __task_cred(p) for the check.
> > This looks a bit strange, different threads can have different creds
> > but obviously rlimits are per-process.
>
> Sorry I can't see it. Could you point out in which function this is done?

selinux_task_setrlimit()->current_has_perm()->current_sid()->current_cred()

Oleg.



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