[patch 25/28] mm_for_maps: shift down_read(mmap_sem) to the caller

[Greg KH]

2.6.27-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Oleg Nesterov <oleg@xxxxxxxxxx>

commit 00f89d218523b9bf6b522349c039d5ac80aa536d upstream.

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@xxxxxxxxxx>
Acked-by: Serge Hallyn <serue@xxxxxxxxxx>
Signed-off-by: James Morris <jmorris@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 fs/proc/base.c       |    8 +++-----
 fs/proc/task_mmu.c   |    1 +
 fs/proc/task_nommu.c |    1 +
 3 files changed, 5 insertions(+), 5 deletions(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -240,9 +240,8 @@ static int check_mem_permission(struct t
 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.
@@ -250,10 +249,9 @@ struct mm_struct *mm_for_maps(struct tas
 		if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
 		    mm != task->mm) {
 			mmput(mm);
-			return NULL;
+			mm = NULL;
 		}
 	}
-	down_read(&mm->mmap_sem);
 	return mm;
 }
 
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -119,6 +119,7 @@ static void *m_start(struct seq_file *m,
 	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;
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -137,6 +137,7 @@ static void *m_start(struct seq_file *m,
 		priv->task = NULL;
 		return NULL;
 	}
+	down_read(&mm->mmap_sem);
 
 	/* start from the Nth VMA */
 	for (vml = mm->context.vmlist; vml; vml = vml->next)


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