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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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