sched/fair: Use RCU accessors consistently for ->numa_group

The old code used RCU annotations and accessors inconsistently for
->numa_group, which can lead to use-after-frees and NULL dereferences.

Let all accesses to ->numa_group use proper RCU helpers to prevent such
issues.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Fixes: 8c8a743c5087 ("sched/numa: Use {cpu, pid} to create task groups for shared faults")
Link: https://lkml.kernel.org/r/20190716152047.14424-3-jannh@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 90 insertions, 40 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 8dc1811487f5..9f51932bd543 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1092,7 +1092,15 @@ struct task_struct {
         u64                             last_sum_exec_runtime;
         struct callback_head            numa_work;
 
-        struct numa_group               *numa_group;
+        /*
+         * This pointer is only modified for current in syscall and
+         * pagefault context (and for tasks being destroyed), so it can be read
+         * from any of the following contexts:
+         *  - RCU read-side critical section
+         *  - current->numa_group from everywhere
+         *  - task's runqueue locked, task not running
+         */
+        struct numa_group __rcu         *numa_group;
 
         /*
          * numa_faults is an array split into four regions:
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6adb0e0f5feb..bc9cfeaac8bd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1086,6 +1086,21 @@ struct numa_group {
         unsigned long faults[0];
 };
 
+/*
+ * For functions that can be called in multiple contexts that permit reading
+ * ->numa_group (see struct task_struct for locking rules).
+ */
+static struct numa_group *deref_task_numa_group(struct task_struct *p)
+{
+        return rcu_dereference_check(p->numa_group, p == current ||
+                (lockdep_is_held(&task_rq(p)->lock) && !READ_ONCE(p->on_cpu)));
+}
+
+static struct numa_group *deref_curr_numa_group(struct task_struct *p)
+{
+        return rcu_dereference_protected(p->numa_group, p == current);
+}
+
 static inline unsigned long group_faults_priv(struct numa_group *ng);
 static inline unsigned long group_faults_shared(struct numa_group *ng);
 
@@ -1129,10 +1144,12 @@ static unsigned int task_scan_start(struct task_struct *p)
 {
         unsigned long smin = task_scan_min(p);
         unsigned long period = smin;
+        struct numa_group *ng;
 
         /* Scale the maximum scan period with the amount of shared memory. */
-        if (p->numa_group) {
-                struct numa_group *ng = p->numa_group;
+        rcu_read_lock();
+        ng = rcu_dereference(p->numa_group);
+        if (ng) {
                 unsigned long shared = group_faults_shared(ng);
                 unsigned long private = group_faults_priv(ng);
 
@@ -1140,6 +1157,7 @@ static unsigned int task_scan_start(struct task_struct *p)
                 period *= shared + 1;
                 period /= private + shared + 1;
         }
+        rcu_read_unlock();
 
         return max(smin, period);
 }
@@ -1148,13 +1166,14 @@ static unsigned int task_scan_max(struct task_struct *p)
 {
         unsigned long smin = task_scan_min(p);
         unsigned long smax;
+        struct numa_group *ng;
 
         /* Watch for min being lower than max due to floor calculations */
         smax = sysctl_numa_balancing_scan_period_max / task_nr_scan_windows(p);
 
         /* Scale the maximum scan period with the amount of shared memory. */
-        if (p->numa_group) {
-                struct numa_group *ng = p->numa_group;
+        ng = deref_curr_numa_group(p);
+        if (ng) {
                 unsigned long shared = group_faults_shared(ng);
                 unsigned long private = group_faults_priv(ng);
                 unsigned long period = smax;
@@ -1186,7 +1205,7 @@ void init_numa_balancing(unsigned long clone_flags, struct task_struct *p)
         p->numa_scan_period             = sysctl_numa_balancing_scan_delay;
         p->numa_work.next               = &p->numa_work;
         p->numa_faults                  = NULL;
-        p->numa_group                   = NULL;
+        RCU_INIT_POINTER(p->numa_group, NULL);
         p->last_task_numa_placement     = 0;
         p->last_sum_exec_runtime        = 0;
 
@@ -1233,7 +1252,16 @@ static void account_numa_dequeue(struct rq *rq, struct task_struct *p)
 
 pid_t task_numa_group_id(struct task_struct *p)
 {
-        return p->numa_group ? p->numa_group->gid : 0;
+        struct numa_group *ng;
+        pid_t gid = 0;
+
+        rcu_read_lock();
+        ng = rcu_dereference(p->numa_group);
+        if (ng)
+                gid = ng->gid;
+        rcu_read_unlock();
+
+        return gid;
 }
 
 /*
@@ -1258,11 +1286,13 @@ static inline unsigned long task_faults(struct task_struct *p, int nid)
 
 static inline unsigned long group_faults(struct task_struct *p, int nid)
 {
-        if (!p->numa_group)
+        struct numa_group *ng = deref_task_numa_group(p);
+
+        if (!ng)
                 return 0;
 
-        return p->numa_group->faults[task_faults_idx(NUMA_MEM, nid, 0)] +
-                p->numa_group->faults[task_faults_idx(NUMA_MEM, nid, 1)];
+        return ng->faults[task_faults_idx(NUMA_MEM, nid, 0)] +
+                ng->faults[task_faults_idx(NUMA_MEM, nid, 1)];
 }
 
 static inline unsigned long group_faults_cpu(struct numa_group *group, int nid)
@@ -1400,12 +1430,13 @@ static inline unsigned long task_weight(struct task_struct *p, int nid,
 static inline unsigned long group_weight(struct task_struct *p, int nid,
                                          int dist)
 {
+        struct numa_group *ng = deref_task_numa_group(p);
         unsigned long faults, total_faults;
 
-        if (!p->numa_group)
+        if (!ng)
                 return 0;
 
-        total_faults = p->numa_group->total_faults;
+        total_faults = ng->total_faults;
 
         if (!total_faults)
                 return 0;
@@ -1419,7 +1450,7 @@ static inline unsigned long group_weight(struct task_struct *p, int nid,
 bool should_numa_migrate_memory(struct task_struct *p, struct page * page,
                                 int src_nid, int dst_cpu)
 {
-        struct numa_group *ng = p->numa_group;
+        struct numa_group *ng = deref_curr_numa_group(p);
         int dst_nid = cpu_to_node(dst_cpu);
         int last_cpupid, this_cpupid;
 
@@ -1600,13 +1631,14 @@ static bool load_too_imbalanced(long src_load, long dst_load,
 static void task_numa_compare(struct task_numa_env *env,
                               long taskimp, long groupimp, bool maymove)
 {
+        struct numa_group *cur_ng, *p_ng = deref_curr_numa_group(env->p);
         struct rq *dst_rq = cpu_rq(env->dst_cpu);
+        long imp = p_ng ? groupimp : taskimp;
         struct task_struct *cur;
         long src_load, dst_load;
-        long load;
-        long imp = env->p->numa_group ? groupimp : taskimp;
-        long moveimp = imp;
         int dist = env->dist;
+        long moveimp = imp;
+        long load;
 
         if (READ_ONCE(dst_rq->numa_migrate_on))
                 return;
@@ -1645,21 +1677,22 @@ static void task_numa_compare(struct task_numa_env *env,
          * If dst and source tasks are in the same NUMA group, or not
          * in any group then look only at task weights.
          */
-        if (cur->numa_group == env->p->numa_group) {
+        cur_ng = rcu_dereference(cur->numa_group);
+        if (cur_ng == p_ng) {
                 imp = taskimp + task_weight(cur, env->src_nid, dist) -
                       task_weight(cur, env->dst_nid, dist);
                 /*
                  * Add some hysteresis to prevent swapping the
                  * tasks within a group over tiny differences.
                  */
-                if (cur->numa_group)
+                if (cur_ng)
                         imp -= imp / 16;
         } else {
                 /*
                  * Compare the group weights. If a task is all by itself
                  * (not part of a group), use the task weight instead.
                  */
-                if (cur->numa_group && env->p->numa_group)
+                if (cur_ng && p_ng)
                         imp += group_weight(cur, env->src_nid, dist) -
                                group_weight(cur, env->dst_nid, dist);
                 else
@@ -1757,11 +1790,12 @@ static int task_numa_migrate(struct task_struct *p)
                 .best_imp = 0,
                 .best_cpu = -1,
         };
+        unsigned long taskweight, groupweight;
         struct sched_domain *sd;
+        long taskimp, groupimp;
+        struct numa_group *ng;
         struct rq *best_rq;
-        unsigned long taskweight, groupweight;
         int nid, ret, dist;
-        long taskimp, groupimp;
 
         /*
          * Pick the lowest SD_NUMA domain, as that would have the smallest
@@ -1807,7 +1841,8 @@ static int task_numa_migrate(struct task_struct *p)
          *   multiple NUMA nodes; in order to better consolidate the group,
          *   we need to check other locations.
          */
-        if (env.best_cpu == -1 || (p->numa_group && p->numa_group->active_nodes > 1)) {
+        ng = deref_curr_numa_group(p);
+        if (env.best_cpu == -1 || (ng && ng->active_nodes > 1)) {
                 for_each_online_node(nid) {
                         if (nid == env.src_nid || nid == p->numa_preferred_nid)
                                 continue;
@@ -1840,7 +1875,7 @@ static int task_numa_migrate(struct task_struct *p)
          * A task that migrated to a second choice node will be better off
          * trying for a better one later. Do not set the preferred node here.
          */
-        if (p->numa_group) {
+        if (ng) {
                 if (env.best_cpu == -1)
                         nid = env.src_nid;
                 else
@@ -2135,6 +2170,7 @@ static void task_numa_placement(struct task_struct *p)
         unsigned long total_faults;
         u64 runtime, period;
         spinlock_t *group_lock = NULL;
+        struct numa_group *ng;
 
         /*
          * The p->mm->numa_scan_seq field gets updated without
@@ -2152,8 +2188,9 @@ static void task_numa_placement(struct task_struct *p)
         runtime = numa_get_avg_runtime(p, &period);
 
         /* If the task is part of a group prevent parallel updates to group stats */
-        if (p->numa_group) {
-                group_lock = &p->numa_group->lock;
+        ng = deref_curr_numa_group(p);
+        if (ng) {
+                group_lock = &ng->lock;
                 spin_lock_irq(group_lock);
         }
 
@@ -2194,7 +2231,7 @@ static void task_numa_placement(struct task_struct *p)
                         p->numa_faults[cpu_idx] += f_diff;
                         faults += p->numa_faults[mem_idx];
                         p->total_numa_faults += diff;
-                        if (p->numa_group) {
+                        if (ng) {
                                 /*
                                  * safe because we can only change our own group
                                  *
@@ -2202,14 +2239,14 @@ static void task_numa_placement(struct task_struct *p)
                                  * nid and priv in a specific region because it
                                  * is at the beginning of the numa_faults array.
                                  */
-                                p->numa_group->faults[mem_idx] += diff;
-                                p->numa_group->faults_cpu[mem_idx] += f_diff;
-                                p->numa_group->total_faults += diff;
-                                group_faults += p->numa_group->faults[mem_idx];
+                                ng->faults[mem_idx] += diff;
+                                ng->faults_cpu[mem_idx] += f_diff;
+                                ng->total_faults += diff;
+                                group_faults += ng->faults[mem_idx];
                         }
                 }
 
-                if (!p->numa_group) {
+                if (!ng) {
                         if (faults > max_faults) {
                                 max_faults = faults;
                                 max_nid = nid;
@@ -2220,8 +2257,8 @@ static void task_numa_placement(struct task_struct *p)
                 }
         }
 
-        if (p->numa_group) {
-                numa_group_count_active_nodes(p->numa_group);
+        if (ng) {
+                numa_group_count_active_nodes(ng);
                 spin_unlock_irq(group_lock);
                 max_nid = preferred_group_nid(p, max_nid);
         }
@@ -2255,7 +2292,7 @@ static void task_numa_group(struct task_struct *p, int cpupid, int flags,
         int cpu = cpupid_to_cpu(cpupid);
         int i;
 
-        if (unlikely(!p->numa_group)) {
+        if (unlikely(!deref_curr_numa_group(p))) {
                 unsigned int size = sizeof(struct numa_group) +
                                     4*nr_node_ids*sizeof(unsigned long);
 
@@ -2291,7 +2328,7 @@ static void task_numa_group(struct task_struct *p, int cpupid, int flags,
         if (!grp)
                 goto no_join;
 
-        my_grp = p->numa_group;
+        my_grp = deref_curr_numa_group(p);
         if (grp == my_grp)
                 goto no_join;
 
@@ -2362,7 +2399,8 @@ no_join:
  */
 void task_numa_free(struct task_struct *p, bool final)
 {
-        struct numa_group *grp = p->numa_group;
+        /* safe: p either is current or is being freed by current */
+        struct numa_group *grp = rcu_dereference_raw(p->numa_group);
         unsigned long *numa_faults = p->numa_faults;
         unsigned long flags;
         int i;
@@ -2442,7 +2480,7 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)
          * actively using should be counted as local. This allows the
          * scan rate to slow down when a workload has settled down.
          */
-        ng = p->numa_group;
+        ng = deref_curr_numa_group(p);
         if (!priv && !local && ng && ng->active_nodes > 1 &&
                                 numa_is_active_node(cpu_node, ng) &&
                                 numa_is_active_node(mem_node, ng))
@@ -10460,18 +10498,22 @@ void show_numa_stats(struct task_struct *p, struct seq_file *m)
 {
         int node;
         unsigned long tsf = 0, tpf = 0, gsf = 0, gpf = 0;
+        struct numa_group *ng;
 
+        rcu_read_lock();
+        ng = rcu_dereference(p->numa_group);
         for_each_online_node(node) {
                 if (p->numa_faults) {
                         tsf = p->numa_faults[task_faults_idx(NUMA_MEM, node, 0)];
                         tpf = p->numa_faults[task_faults_idx(NUMA_MEM, node, 1)];
                 }
-                if (p->numa_group) {
-                        gsf = p->numa_group->faults[task_faults_idx(NUMA_MEM, node, 0)],
-                        gpf = p->numa_group->faults[task_faults_idx(NUMA_MEM, node, 1)];
+                if (ng) {
+                        gsf = ng->faults[task_faults_idx(NUMA_MEM, node, 0)],
+                        gpf = ng->faults[task_faults_idx(NUMA_MEM, node, 1)];
                 }
                 print_numa_stats(m, node, tsf, tpf, gsf, gpf);
         }
+        rcu_read_unlock();
 }
 #endif /* CONFIG_NUMA_BALANCING */
 #endif /* CONFIG_SCHED_DEBUG */