powerpc/papr_scm: Fix buffer overflow issue with CONFIG_FORTIFY_SOURCE

With CONFIG_FORTIFY_SOURCE enabled, string functions will also perform
dynamic checks for string size which can panic the kernel, like incase
of overflow detection.

In papr_scm, papr_scm_pmu_check_events function uses stat->stat_id with
string operations, to populate the nvdimm_events_map array. Since
stat_id variable is not NULL terminated, the kernel panics with
CONFIG_FORTIFY_SOURCE enabled at boot time.

Below are the logs of kernel panic:

  detected buffer overflow in __fortify_strlen
  ------------[ cut here ]------------
  kernel BUG at lib/string_helpers.c:980!
  Oops: Exception in kernel mode, sig: 5 [#1]
  NIP [c00000000077dad0] fortify_panic+0x28/0x38
  LR [c00000000077dacc] fortify_panic+0x24/0x38
  Call Trace:
  [c0000022d77836e0] [c00000000077dacc] fortify_panic+0x24/0x38 (unreliable)
  [c00800000deb2660] papr_scm_pmu_check_events.constprop.0+0x118/0x220 [papr_scm]
  [c00800000deb2cb0] papr_scm_probe+0x288/0x62c [papr_scm]
  [c0000000009b46a8] platform_probe+0x98/0x150

Fix this issue by using kmemdup_nul() to copy the content of
stat->stat_id directly to the nvdimm_events_map array.

mpe: stat->stat_id comes from the hypervisor, not userspace, so there is
no security exposure.

Fixes: 4c08d4bb ("powerpc/papr_scm: Add perf interface support")
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220505153451.35503-1-kjain@linux.ibm.com

arch/powerpc/platforms/pseries/papr_scm.c

@@ -462,7 +462,6 @@ static int papr_scm_pmu_check_events(struct papr_scm_priv *p, struct nvdimm_pmu
 {
 	struct papr_scm_perf_stat *stat;
 	struct papr_scm_perf_stats *stats;
-	char *statid;
 	int index, rc, count;
 	u32 available_events;
 
@@ -493,14 +492,12 @@ static int papr_scm_pmu_check_events(struct papr_scm_priv *p, struct nvdimm_pmu
 
 	for (index = 0, stat = stats->scm_statistic, count = 0;
 		     index < available_events; index++, ++stat) {
-		statid = kzalloc(strlen(stat->stat_id) + 1, GFP_KERNEL);
-		if (!statid) {
+		p->nvdimm_events_map[count] = kmemdup_nul(stat->stat_id, 8, GFP_KERNEL);
+		if (!p->nvdimm_events_map[count]) {
 			rc = -ENOMEM;
 			goto out_nvdimm_events_map;
 		}
 
-		strcpy(statid, stat->stat_id);
-		p->nvdimm_events_map[count] = statid;
 		count++;
 	}
 	p->nvdimm_events_map[count] = NULL;