mm owner: fix race between swapoff and exit

There's a race between mm->owner assignment and swapoff, more easily
seen when task slab poisoning is turned on.  The condition occurs when
try_to_unuse() runs in parallel with an exiting task.  A similar race
can occur with callers of get_task_mm(), such as /proc/<pid>/<mmstats>
or ptrace or page migration.

CPU0                                    CPU1
                                        try_to_unuse
                                        looks at mm = task0->mm
                                        increments mm->mm_users
task 0 exits
mm->owner needs to be updated, but no
new owner is found (mm_users > 1, but
no other task has task->mm = task0->mm)
mm_update_next_owner() leaves
                                        mmput(mm) decrements mm->mm_users
task0 freed
                                        dereferencing mm->owner fails

The fix is to notify the subsystem via mm_owner_changed callback(),
if no new owner is found, by specifying the new task as NULL.

Jiri Slaby:
mm->owner was set to NULL prior to calling cgroup_mm_owner_callbacks(), but
must be set after that, so as not to pass NULL as old owner causing oops.

Daisuke Nishimura:
mm_update_next_owner() may set mm->owner to NULL, but mem_cgroup_from_task()
and its callers need to take account of this situation to avoid oops.

Hugh Dickins:
Lockdep warning and hang below exec_mmap() when testing these patches.
exit_mm() up_reads mmap_sem before calling mm_update_next_owner(),
so exec_mmap() now needs to do the same.  And with that repositioning,
there's now no point in mm_need_new_owner() allowing for NULL mm.

Reported-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 31 insertions, 5 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 32993beecbe9..cecee501ce78 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -752,11 +752,11 @@ static int exec_mmap(struct mm_struct *mm)
         tsk->active_mm = mm;
         activate_mm(active_mm, mm);
         task_unlock(tsk);
-        mm_update_next_owner(old_mm);
         arch_pick_mmap_layout(mm);
         if (old_mm) {
                 up_read(&old_mm->mmap_sem);
                 BUG_ON(active_mm != old_mm);
+                mm_update_next_owner(old_mm);
                 mmput(old_mm);
                 return 0;
         }
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 13932abde159..a0123d75ec9a 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2738,14 +2738,15 @@ void cgroup_fork_callbacks(struct task_struct *child)
  */
 void cgroup_mm_owner_callbacks(struct task_struct *old, struct task_struct *new)
 {
-        struct cgroup *oldcgrp, *newcgrp;
+        struct cgroup *oldcgrp, *newcgrp = NULL;
 
         if (need_mm_owner_callback) {
                 int i;
                 for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                         struct cgroup_subsys *ss = subsys[i];
                         oldcgrp = task_cgroup(old, ss->subsys_id);
-                        newcgrp = task_cgroup(new, ss->subsys_id);
+                        if (new)
+                                newcgrp = task_cgroup(new, ss->subsys_id);
                         if (oldcgrp == newcgrp)
                                 continue;
                         if (ss->mm_owner_changed)
diff --git a/kernel/exit.c b/kernel/exit.c
index 16395644a98f..85a83c831856 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -583,8 +583,6 @@ mm_need_new_owner(struct mm_struct *mm, struct task_struct *p)
          * If there are other users of the mm and the owner (us) is exiting
          * we need to find a new owner to take on the responsibility.
          */
-        if (!mm)
-                return 0;
         if (atomic_read(&mm->mm_users) <= 1)
                 return 0;
         if (mm->owner != p)
@@ -627,6 +625,16 @@ retry:
         } while_each_thread(g, c);
 
         read_unlock(&tasklist_lock);
+        /*
+         * We found no owner yet mm_users > 1: this implies that we are
+         * most likely racing with swapoff (try_to_unuse()) or /proc or
+         * ptrace or page migration (get_task_mm()).  Mark owner as NULL,
+         * so that subsystems can understand the callback and take action.
+         */
+        down_write(&mm->mmap_sem);
+        cgroup_mm_owner_callbacks(mm->owner, NULL);
+        mm->owner = NULL;
+        up_write(&mm->mmap_sem);
         return;
 
 assign_new_owner:
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c0500e4d3a2f..36896f3eb7f5 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -250,6 +250,14 @@ static struct mem_cgroup *mem_cgroup_from_cont(struct cgroup *cont)
 
 struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
 {
+        /*
+         * mm_update_next_owner() may clear mm->owner to NULL
+         * if it races with swapoff, page migration, etc.
+         * So this can be called with p == NULL.
+         */
+        if (unlikely(!p))
+                return NULL;
+
         return container_of(task_subsys_state(p, mem_cgroup_subsys_id),
                                 struct mem_cgroup, css);
 }
@@ -549,6 +557,11 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
         if (likely(!memcg)) {
                 rcu_read_lock();
                 mem = mem_cgroup_from_task(rcu_dereference(mm->owner));
+                if (unlikely(!mem)) {
+                        rcu_read_unlock();
+                        kmem_cache_free(page_cgroup_cache, pc);
+                        return 0;
+                }
                 /*
                  * For every charge from the cgroup, increment reference count
                  */
@@ -801,6 +814,10 @@ int mem_cgroup_shrink_usage(struct mm_struct *mm, gfp_t gfp_mask)
 
         rcu_read_lock();
         mem = mem_cgroup_from_task(rcu_dereference(mm->owner));
+        if (unlikely(!mem)) {
+                rcu_read_unlock();
+                return 0;
+        }
         css_get(&mem->css);
         rcu_read_unlock();