[Devel] Re: [PATCH] c/r: [signal 2/3] checkpoint/restart of rlimit

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at librato.com):
> 
> 
> Serge E. Hallyn wrote:
> > Quoting Oren Laadan (orenl at librato.com):
> >> This patch adds checkpoint and restart of rlimit information
> >> that is part of shared signal_struct.
> > 
> > ...
> > 
> >>  static int restore_signal(struct ckpt_ctx *ctx)
> >>  {
> >>  	struct ckpt_hdr_signal *h;
> >> +	struct rlimit rlim;
> >> +	int i, ret;
> >>
> >>  	h = ckpt_read_obj_type(ctx, sizeof(*h), CKPT_HDR_SIGNAL);
> >>  	if (IS_ERR(h))
> >>  		return PTR_ERR(h);
> >>
> >> -	/* fill in later */
> >> -
> >> +	/* rlimit */
> >> +	for (i = 0; i < RLIM_NLIMITS; i++) {
> >> +		rlim.rlim_cur = h->rlim[i].rlim_cur;
> >> +		rlim.rlim_max = h->rlim[i].rlim_max;
> >> +		ret = do_setrlimit(i, &rlim);
> > 
> > ...
> >> +int do_setrlimit(unsigned int resource, struct rlimit *new_rlim)
> >>  {
> >> -	struct rlimit new_rlim, *old_rlim;
> >> +	struct rlimit *old_rlim;
> >>  	int retval;
> >>
> >> -	if (resource >= RLIM_NLIMITS)
> >> -		return -EINVAL;
> >> -	if (copy_from_user(&new_rlim, rlim, sizeof(*rlim)))
> >> -		return -EFAULT;
> >> -	if (new_rlim.rlim_cur > new_rlim.rlim_max)
> >> -		return -EINVAL;
> >>  	old_rlim = current->signal->rlim + resource;
> >> -	if ((new_rlim.rlim_max > old_rlim->rlim_max) &&
> >> +	if ((new_rlim->rlim_max > old_rlim->rlim_max) &&
> >>  	    !capable(CAP_SYS_RESOURCE))
> >>  		return -EPERM;
> >> -	if (resource == RLIMIT_NOFILE && new_rlim.rlim_max > sysctl_nr_open)
> >> +	if (resource == RLIMIT_NOFILE && new_rlim->rlim_max > sysctl_nr_open)
> >>  		return -EPERM;
> >>
> >> -	retval = security_task_setrlimit(resource, &new_rlim);
> >> +	retval = security_task_setrlimit(resource, new_rlim);
> >>  	if (retval)
> >>  		return retval;
> >>
> >> -	if (resource == RLIMIT_CPU && new_rlim.rlim_cur == 0) {
> >> +	if (resource == RLIMIT_CPU && new_rlim->rlim_cur == 0) {
> >>  		/*
> >>  		 * The caller is asking for an immediate RLIMIT_CPU
> >>  		 * expiry.  But we use the zero value to mean "it was
> >>  		 * never set".  So let's cheat and make it one second
> >>  		 * instead
> >>  		 */
> >> -		new_rlim.rlim_cur = 1;
> >> +		new_rlim->rlim_cur = 1;
> >>  	}
> >>
> >>  	task_lock(current->group_leader);
> >> -	*old_rlim = new_rlim;
> >> +	*old_rlim = *new_rlim;
> >>  	task_unlock(current->group_leader);
> >>
> >>  	if (resource != RLIMIT_CPU)
> >> @@ -1189,14 +1183,27 @@ SYSCALL_DEFINE2(setrlimit, unsigned int, resource, struct rlimit __user *, rlim)
> >>  	 * very long-standing error, and fixing it now risks breakage of
> >>  	 * applications, so we live with it
> >>  	 */
> >> -	if (new_rlim.rlim_cur == RLIM_INFINITY)
> >> +	if (new_rlim->rlim_cur == RLIM_INFINITY)
> >>  		goto out;
> >>
> >> -	update_rlimit_cpu(new_rlim.rlim_cur);
> >> +	update_rlimit_cpu(new_rlim->rlim_cur);
> >>  out:
> >>  	return 0;
> >>  }
> >>
> >> +SYSCALL_DEFINE2(setrlimit, unsigned int, resource, struct rlimit __user *, rlim)
> >> +{
> >> +	struct rlimit new_rlim;
> >> +
> >> +	if (resource >= RLIM_NLIMITS)
> >> +		return -EINVAL;
> >> +	if (copy_from_user(&new_rlim, rlim, sizeof(*rlim)))
> >> +		return -EFAULT;
> >> +	if (new_rlim.rlim_cur > new_rlim.rlim_max)
> >> +		return -EINVAL;
> > 
> > Should the above check go into do_setrlimit()?  No sense trusting
> > the data sent to sys_checkpoint() any more than the data sent to
> > sys_setrlimit().
> 
> You are very correct.
> 
> I wonder though: moving the first check will change the order of
> input sanitizing, which will change the syscall behavior on bad
> input. E.g, setrlimit(4096, NULL) used to return EINVAL but now
> will return EFAULT.
> 
> Not that I really care that much, but I've seen a similar case
> that confused LTP scripts into seeing the "wrong" error from a
> syscall and failing a test.

Heh, I could be wrong, but when you mess up 2 ways, I don't think the kernel
needs to guarantee which one you'll be warned about :)  Of course there are
cases where that is well-defined (i.e. DAC-before-MAC).  Maybe we should ask at
linux-api?

Putting the same check before both callers of do_setrlimit() isn't *that*
bad, and I suppose we can put a comment above do_setrlimit() saying that
that any new callers need to do that check themselves...

-serge
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