mm, memcg: fix potential undefined behaviour in page stat accounting

Since commit d7365e783edb ("mm: memcontrol: fix missed end-writeback
page accounting") mem_cgroup_end_page_stat consumes locked and flags
variables directly rather than via pointers which might trigger C
undefined behavior as those variables are initialized only in the slow
path of mem_cgroup_begin_page_stat.

Although mem_cgroup_end_page_stat handles parameters correctly and
touches them only when they hold a sensible value it is caller which
loads a potentially uninitialized value which then might allow compiler
to do crazy things.

I haven't seen any warning from gcc and it seems that the current
version (4.9) doesn't exploit this type undefined behavior but Sasha has
reported the following:

  UBSan: Undefined behaviour in mm/rmap.c:1084:2
  load of value 255 is not a valid value for type '_Bool'
  CPU: 4 PID: 8304 Comm: rngd Not tainted 3.18.0-rc2-next-20141029-sasha-00039-g77ed13d-dirty #1427
  Call Trace:
    dump_stack (lib/dump_stack.c:52)
    ubsan_epilogue (lib/ubsan.c:159)
    __ubsan_handle_load_invalid_value (lib/ubsan.c:482)
    page_remove_rmap (mm/rmap.c:1084 mm/rmap.c:1096)
    unmap_page_range (./arch/x86/include/asm/atomic.h:27 include/linux/mm.h:463 mm/memory.c:1146 mm/memory.c:1258 mm/memory.c:1279 mm/memory.c:1303)
    unmap_single_vma (mm/memory.c:1348)
    unmap_vmas (mm/memory.c:1377 (discriminator 3))
    exit_mmap (mm/mmap.c:2837)
    mmput (kernel/fork.c:659)
    do_exit (./arch/x86/include/asm/thread_info.h:168 kernel/exit.c:462 kernel/exit.c:747)
    do_group_exit (include/linux/sched.h:775 kernel/exit.c:873)
    SyS_exit_group (kernel/exit.c:901)
    tracesys_phase2 (arch/x86/kernel/entry_64.S:529)

Fix this by using pointer parameters for both locked and flags and be
more robust for future compiler changes even though the current code is
implemented correctly.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -141,8 +141,8 @@ static inline bool mem_cgroup_disabled(void)
 
 struct mem_cgroup *mem_cgroup_begin_page_stat(struct page *page, bool *locked,
 					      unsigned long *flags);
-void mem_cgroup_end_page_stat(struct mem_cgroup *memcg, bool locked,
-			      unsigned long flags);
+void mem_cgroup_end_page_stat(struct mem_cgroup *memcg, bool *locked,
+			      unsigned long *flags);
 void mem_cgroup_update_page_stat(struct mem_cgroup *memcg,
 				 enum mem_cgroup_stat_index idx, int val);
 
@@ -297,7 +297,7 @@ static inline struct mem_cgroup *mem_cgroup_begin_page_stat(struct page *page,
 }
 
 static inline void mem_cgroup_end_page_stat(struct mem_cgroup *memcg,
-					bool locked, unsigned long flags)
+					bool *locked, unsigned long *flags)
 {
 }
 

mm/memcontrol.c

@@ -2053,11 +2053,11 @@ again:
  * @locked: value received from mem_cgroup_begin_page_stat()
  * @flags: value received from mem_cgroup_begin_page_stat()
  */
-void mem_cgroup_end_page_stat(struct mem_cgroup *memcg, bool locked,
-			      unsigned long flags)
+void mem_cgroup_end_page_stat(struct mem_cgroup *memcg, bool *locked,
+			      unsigned long *flags)
 {
-	if (memcg && locked)
-		spin_unlock_irqrestore(&memcg->move_lock, flags);
+	if (memcg && *locked)
+		spin_unlock_irqrestore(&memcg->move_lock, *flags);
 
 	rcu_read_unlock();
 }

mm/page-writeback.c

@@ -2357,7 +2357,7 @@ int test_clear_page_writeback(struct page *page)
 		dec_zone_page_state(page, NR_WRITEBACK);
 		inc_zone_page_state(page, NR_WRITTEN);
 	}
-	mem_cgroup_end_page_stat(memcg, locked, memcg_flags);
+	mem_cgroup_end_page_stat(memcg, &locked, &memcg_flags);
 	return ret;
 }
 
@@ -2399,7 +2399,7 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
 		mem_cgroup_inc_page_stat(memcg, MEM_CGROUP_STAT_WRITEBACK);
 		inc_zone_page_state(page, NR_WRITEBACK);
 	}
-	mem_cgroup_end_page_stat(memcg, locked, memcg_flags);
+	mem_cgroup_end_page_stat(memcg, &locked, &memcg_flags);
 	return ret;
 
 }

mm/rmap.c

@@ -1053,7 +1053,7 @@ void page_add_file_rmap(struct page *page)
 		__inc_zone_page_state(page, NR_FILE_MAPPED);
 		mem_cgroup_inc_page_stat(memcg, MEM_CGROUP_STAT_FILE_MAPPED);
 	}
-	mem_cgroup_end_page_stat(memcg, locked, flags);
+	mem_cgroup_end_page_stat(memcg, &locked, &flags);
 }
 
 static void page_remove_file_rmap(struct page *page)
@@ -1083,7 +1083,7 @@ static void page_remove_file_rmap(struct page *page)
 	if (unlikely(PageMlocked(page)))
 		clear_page_mlock(page);
 out:
-	mem_cgroup_end_page_stat(memcg, locked, flags);
+	mem_cgroup_end_page_stat(memcg, &locked, &flags);
 }
 
 /**