mm_for_maps: shift down_read(mmap_sem) to the caller

mm_for_maps() takes ->mmap_sem after security checks, this looks
strange and obfuscates the locking rules. Move this lock to its
single caller, m_start().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

3 files changed, 5 insertions, 5 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 917f338a6739..f3c2e4085fed 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -235,9 +235,8 @@ static int check_mem_permission(struct task_struct *task)
 struct mm_struct *mm_for_maps(struct task_struct *task)
 {
         struct mm_struct *mm = get_task_mm(task);
-        if (!mm)
-                return NULL;
-        if (mm != current->mm) {
+
+        if (mm && mm != current->mm) {
                 /*
                  * task->mm can be changed before security check,
                  * in that case we must notice the change after.
@@ -245,10 +244,9 @@ struct mm_struct *mm_for_maps(struct task_struct *task)
                 if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
                     mm != task->mm) {
                         mmput(mm);
-                        return NULL;
+                        mm = NULL;
                 }
         }
-        down_read(&mm->mmap_sem);
         return mm;
 }
 
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 6f61b7cc32e0..9bd8be1d235c 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -119,6 +119,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
         mm = mm_for_maps(priv->task);
         if (!mm)
                 return NULL;
+        down_read(&mm->mmap_sem);
 
         tail_vma = get_gate_vma(priv->task);
         priv->tail_vma = tail_vma;
diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index 64a72e2e7650..8f5c05d3dbd3 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -189,6 +189,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
                 priv->task = NULL;
                 return NULL;
         }
+        down_read(&mm->mmap_sem);
 
         /* start from the Nth VMA */
         for (p = rb_first(&mm->mm_rb); p; p = rb_next(p))