powerpc/hw_breakpoint: Avoid relying on caller synchronization

Internal data structures (cpu_bps, task_bps) of powerpc's hw_breakpoint
implementation have relied on nr_bp_mutex serializing access to them.

Before overhauling synchronization of kernel/events/hw_breakpoint.c,
introduce 2 spinlocks to synchronize cpu_bps and task_bps respectively,
thus avoiding reliance on callers synchronizing powerpc's hw_breakpoint.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20220829124719.675715-10-elver@google.com

diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
index 2669f80b3a4973fafb3b9104dfc47c42366f0f82..8db1a15d7acbe17815f2a843242b5ea5339fbcf1 100644 (file)
--- a/arch/powerpc/kernel/hw_breakpoint.c
+++ b/arch/powerpc/kernel/hw_breakpoint.c
@@ -15,6 +15,7 @@
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/smp.h>
+#include <linux/spinlock.h>
 #include <linux/debugfs.h>
 #include <linux/init.h>
 
@@ -129,7 +130,14 @@ struct breakpoint {
        bool ptrace_bp;
 };
 
+/*
+ * While kernel/events/hw_breakpoint.c does its own synchronization, we cannot
+ * rely on it safely synchronizing internals here; however, we can rely on it
+ * not requesting more breakpoints than available.
+ */
+static DEFINE_SPINLOCK(cpu_bps_lock);
 static DEFINE_PER_CPU(struct breakpoint *, cpu_bps[HBP_NUM_MAX]);
+static DEFINE_SPINLOCK(task_bps_lock);
 static LIST_HEAD(task_bps);
 
 static struct breakpoint *alloc_breakpoint(struct perf_event *bp)
@@ -174,7 +182,9 @@ static int task_bps_add(struct perf_event *bp)
        if (IS_ERR(tmp))
                return PTR_ERR(tmp);
 
+       spin_lock(&task_bps_lock);
        list_add(&tmp->list, &task_bps);
+       spin_unlock(&task_bps_lock);
        return 0;
 }
 
@@ -182,6 +192,7 @@ static void task_bps_remove(struct perf_event *bp)
 {
        struct list_head *pos, *q;
 
+       spin_lock(&task_bps_lock);
        list_for_each_safe(pos, q, &task_bps) {
                struct breakpoint *tmp = list_entry(pos, struct breakpoint, list);
 
@@ -191,6 +202,7 @@ static void task_bps_remove(struct perf_event *bp)
                        break;
                }
        }
+       spin_unlock(&task_bps_lock);
 }
 
 /*
@@ -200,12 +212,17 @@ static void task_bps_remove(struct perf_event *bp)
 static bool all_task_bps_check(struct perf_event *bp)
 {
        struct breakpoint *tmp;
+       bool ret = false;
 
+       spin_lock(&task_bps_lock);
        list_for_each_entry(tmp, &task_bps, list) {
-               if (!can_co_exist(tmp, bp))
-                       return true;
+               if (!can_co_exist(tmp, bp)) {
+                       ret = true;
+                       break;
+               }
        }
-       return false;
+       spin_unlock(&task_bps_lock);
+       return ret;
 }
 
 /*
@@ -215,13 +232,18 @@ static bool all_task_bps_check(struct perf_event *bp)
 static bool same_task_bps_check(struct perf_event *bp)
 {
        struct breakpoint *tmp;
+       bool ret = false;
 
+       spin_lock(&task_bps_lock);
        list_for_each_entry(tmp, &task_bps, list) {
                if (tmp->bp->hw.target == bp->hw.target &&
-                   !can_co_exist(tmp, bp))
-                       return true;
+                   !can_co_exist(tmp, bp)) {
+                       ret = true;
+                       break;
+               }
        }
-       return false;
+       spin_unlock(&task_bps_lock);
+       return ret;
 }
 
 static int cpu_bps_add(struct perf_event *bp)
@@ -234,6 +256,7 @@ static int cpu_bps_add(struct perf_event *bp)
        if (IS_ERR(tmp))
                return PTR_ERR(tmp);
 
+       spin_lock(&cpu_bps_lock);
        cpu_bp = per_cpu_ptr(cpu_bps, bp->cpu);
        for (i = 0; i < nr_wp_slots(); i++) {
                if (!cpu_bp[i]) {
@@ -241,6 +264,7 @@ static int cpu_bps_add(struct perf_event *bp)
                        break;
                }
        }
+       spin_unlock(&cpu_bps_lock);
        return 0;
 }
 
@@ -249,6 +273,7 @@ static void cpu_bps_remove(struct perf_event *bp)
        struct breakpoint **cpu_bp;
        int i = 0;
 
+       spin_lock(&cpu_bps_lock);
        cpu_bp = per_cpu_ptr(cpu_bps, bp->cpu);
        for (i = 0; i < nr_wp_slots(); i++) {
                if (!cpu_bp[i])
@@ -260,19 +285,25 @@ static void cpu_bps_remove(struct perf_event *bp)
                        break;
                }
        }
+       spin_unlock(&cpu_bps_lock);
 }
 
 static bool cpu_bps_check(int cpu, struct perf_event *bp)
 {
        struct breakpoint **cpu_bp;
+       bool ret = false;
        int i;
 
+       spin_lock(&cpu_bps_lock);
        cpu_bp = per_cpu_ptr(cpu_bps, cpu);
        for (i = 0; i < nr_wp_slots(); i++) {
-               if (cpu_bp[i] && !can_co_exist(cpu_bp[i], bp))
-                       return true;
+               if (cpu_bp[i] && !can_co_exist(cpu_bp[i], bp)) {
+                       ret = true;
+                       break;
+               }
        }
-       return false;
+       spin_unlock(&cpu_bps_lock);
+       return ret;
 }
 
 static bool all_cpu_bps_check(struct perf_event *bp)
@@ -286,10 +317,6 @@ static bool all_cpu_bps_check(struct perf_event *bp)
        return false;
 }
 
-/*
- * We don't use any locks to serialize accesses to cpu_bps or task_bps
- * because are already inside nr_bp_mutex.
- */
 int arch_reserve_bp_slot(struct perf_event *bp)
 {
        int ret;