[PATCH] perf/x86/amd/uncore: fixup use after free due to hotplug conversion

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

The conversion change the behaviour of how the hotplug hooks were
invoked: Before one CPU at a time invoked them all. Now on registration
each hook is invoked by all CPUs (one after the other) before the next
state is invoked.
In amd_uncore_cpu_up_prepare() we allocate an uncore struct for each CPU
with id 0. In amd_uncore_cpu_starting() we set the ID and search of
others to replace it if available. CPU0 finds another struct with id 0
and puts its on the kill list. CPU1 find the struct of CPU0 and puts its
on the kill list.
The online event will free them.
To fix this I use an id of -1 assuming that no uncore with such an id
will show up. Step two is use a list to free the not needed elements.

Reported-by: Eric Sandeen <sandeen@sandeen.net>
Tested-by: Eric Sandeen <sandeen@sandeen.net>
Fixes: 96b2bd3866a0 ("perf/x86/amd/uncore: Convert to hotplug state machine")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 arch/x86/events/amd/uncore.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/x86/events/amd/uncore.c b/arch/x86/events/amd/uncore.c
index e6131d4454e6..65577f081d07 100644
--- a/arch/x86/events/amd/uncore.c
+++ b/arch/x86/events/amd/uncore.c
@@ -29,6 +29,8 @@
 
 #define COUNTER_SHIFT		16
 
+static HLIST_HEAD(uncore_unused_list);
+
 struct amd_uncore {
 	int id;
 	int refcnt;
@@ -39,7 +41,7 @@ struct amd_uncore {
 	cpumask_t *active_mask;
 	struct pmu *pmu;
 	struct perf_event *events[MAX_COUNTERS];
-	struct amd_uncore *free_when_cpu_online;
+	struct hlist_node node;
 };
 
 static struct amd_uncore * __percpu *amd_uncore_nb;
@@ -306,6 +308,7 @@ static int amd_uncore_cpu_up_prepare(unsigned int cpu)
 		uncore_nb->msr_base = MSR_F15H_NB_PERF_CTL;
 		uncore_nb->active_mask = &amd_nb_active_mask;
 		uncore_nb->pmu = &amd_nb_pmu;
+		uncore_nb->id = -1;
 		*per_cpu_ptr(amd_uncore_nb, cpu) = uncore_nb;
 	}
 
@@ -319,6 +322,7 @@ static int amd_uncore_cpu_up_prepare(unsigned int cpu)
 		uncore_l2->msr_base = MSR_F16H_L2I_PERF_CTL;
 		uncore_l2->active_mask = &amd_l2_active_mask;
 		uncore_l2->pmu = &amd_l2_pmu;
+		uncore_l2->id = -1;
 		*per_cpu_ptr(amd_uncore_l2, cpu) = uncore_l2;
 	}
 
@@ -348,7 +352,7 @@ amd_uncore_find_online_sibling(struct amd_uncore *this,
 			continue;
 
 		if (this->id == that->id) {
-			that->free_when_cpu_online = this;
+			hlist_add_head(&this->node, &uncore_unused_list);
 			this = that;
 			break;
 		}
@@ -388,13 +392,23 @@ static int amd_uncore_cpu_starting(unsigned int cpu)
 	return 0;
 }
 
+static void uncore_clean_online(void)
+{
+	struct amd_uncore *uncore;
+	struct hlist_node *n;
+
+	hlist_for_each_entry_safe(uncore, n, &uncore_unused_list, node) {
+		hlist_del(&uncore->node);
+		kfree(uncore);
+	}
+}
+
 static void uncore_online(unsigned int cpu,
 			  struct amd_uncore * __percpu *uncores)
 {
 	struct amd_uncore *uncore = *per_cpu_ptr(uncores, cpu);
 
-	kfree(uncore->free_when_cpu_online);
-	uncore->free_when_cpu_online = NULL;
+	uncore_clean_online();
 
 	if (cpu == uncore->cpu)
 		cpumask_set_cpu(cpu, uncore->active_mask);
-- 
2.9.3