x86/resctrl: Fix rdt_find_domain() return value and checks

rdt_find_domain() returns an ERR_PTR() that is generated from a provided
domain id when the value is negative.

Care needs to be taken when creating an ERR_PTR() from this value
because a subsequent check using IS_ERR() expects the error to
be within the MAX_ERRNO range. Using an invalid domain id as an
ERR_PTR() does work at this time since this is currently always -1.
Using this undocumented assumption is fragile since future users of
rdt_find_domain() may not be aware of thus assumption.

Two related issues are addressed:

- Ensure that rdt_find_domain() always returns a valid error value by
forcing the error to be -ENODEV when a negative domain id is provided.

- In a few instances the return value of rdt_find_domain() is just
checked for NULL - fix these to include a check of ERR_PTR.

Fixes: d89b737901 ("x86/intel_rdt/cqm: Add mon_data")
Fixes: 521348b011 ("x86/intel_rdt: Introduce utility to obtain CDP peer")
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: fenghua.yu@intel.com
Cc: gavin.hindman@intel.com
Cc: jithu.joseph@intel.com
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/b88cd4ff6a75995bf8db9b0ea546908fe50f69f3.1544479852.git.reinette.chatre@intel.com

arch/x86/kernel/cpu/resctrl/core.c

@@ -454,7 +454,7 @@ struct rdt_domain *rdt_find_domain(struct rdt_resource *r, int id,
 	struct list_head *l;
 
 	if (id < 0)
-		return ERR_PTR(id);
+		return ERR_PTR(-ENODEV);
 
 	list_for_each(l, &r->domains) {
 		d = list_entry(l, struct rdt_domain, list);

arch/x86/kernel/cpu/resctrl/ctrlmondata.c

@@ -534,7 +534,7 @@ int rdtgroup_mondata_show(struct seq_file *m, void *arg)
 
 	r = &rdt_resources_all[resid];
 	d = rdt_find_domain(r, domid, NULL);
-	if (!d) {
+	if (IS_ERR_OR_NULL(d)) {
 		ret = -ENOENT;
 		goto out;
 	}

arch/x86/kernel/cpu/resctrl/rdtgroup.c

@@ -1029,7 +1029,7 @@ static int rdt_cdp_peer_get(struct rdt_resource *r, struct rdt_domain *d,
 	 * peer RDT CDP resource. Hence the WARN.
 	 */
 	_d_cdp = rdt_find_domain(_r_cdp, d->id, NULL);
-	if (WARN_ON(!_d_cdp)) {
+	if (WARN_ON(IS_ERR_OR_NULL(_d_cdp))) {
 		_r_cdp = NULL;
 		ret = -EINVAL;
 	}