Fix race in process_vm_rw_core

This fixes the race in process_vm_core found by Oleg (see

  http://article.gmane.org/gmane.linux.kernel/1235667/

for details).

This has been updated since I last sent it as the creation of the new
mm_access() function did almost exactly the same thing as parts of the
previous version of this patch did.

In order to use mm_access() even when /proc isn't enabled, we move it to
kernel/fork.c where other related process mm access functions already
are.
Signed-off-by: Chris Yeoh <yeohc@au1.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -198,26 +198,6 @@ static int proc_root_link(struct dentry *dentry, struct path *path)
 	return result;
 }
 
-static struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
-{
-	struct mm_struct *mm;
-	int err;
-
-	err =  mutex_lock_killable(&task->signal->cred_guard_mutex);
-	if (err)
-		return ERR_PTR(err);
-
-	mm = get_task_mm(task);
-	if (mm && mm != current->mm &&
-			!ptrace_may_access(task, mode)) {
-		mmput(mm);
-		mm = ERR_PTR(-EACCES);
-	}
-	mutex_unlock(&task->signal->cred_guard_mutex);
-
-	return mm;
-}
-
 struct mm_struct *mm_for_maps(struct task_struct *task)
 {
 	return mm_access(task, PTRACE_MODE_READ);

include/linux/sched.h

@@ -2259,6 +2259,12 @@ static inline void mmdrop(struct mm_struct * mm)
 extern void mmput(struct mm_struct *);
 /* Grab a reference to a task's mm, if it is not already going away */
 extern struct mm_struct *get_task_mm(struct task_struct *task);
+/*
+ * Grab a reference to a task's mm, if it is not already going away
+ * and ptrace_may_access with the mode parameter passed to it
+ * succeeds.
+ */
+extern struct mm_struct *mm_access(struct task_struct *task, unsigned int mode);
 /* Remove the current tasks stale references to the old mm_struct */
 extern void mm_release(struct task_struct *, struct mm_struct *);
 /* Allocate a new mm structure and copy contents from tsk->mm */

kernel/fork.c

@@ -647,6 +647,26 @@ struct mm_struct *get_task_mm(struct task_struct *task)
 }
 EXPORT_SYMBOL_GPL(get_task_mm);
 
+struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
+{
+	struct mm_struct *mm;
+	int err;
+
+	err =  mutex_lock_killable(&task->signal->cred_guard_mutex);
+	if (err)
+		return ERR_PTR(err);
+
+	mm = get_task_mm(task);
+	if (mm && mm != current->mm &&
+			!ptrace_may_access(task, mode)) {
+		mmput(mm);
+		mm = ERR_PTR(-EACCES);
+	}
+	mutex_unlock(&task->signal->cred_guard_mutex);
+
+	return mm;
+}
+
 /* Please note the differences between mmput and mm_release.
  * mmput is called whenever we stop holding onto a mm_struct,
  * error success whatever.

mm/process_vm_access.c

@@ -298,22 +298,17 @@ static ssize_t process_vm_rw_core(pid_t pid, const struct iovec *lvec,
 		goto free_proc_pages;
 	}
 
-	task_lock(task);
-	if (__ptrace_may_access(task, PTRACE_MODE_ATTACH)) {
-		task_unlock(task);
+	mm = mm_access(task, PTRACE_MODE_ATTACH);
+	if (!mm || IS_ERR(mm)) {
+		rc = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
+		/*
+		 * Explicitly map EACCES to EPERM as EPERM is a more a
+		 * appropriate error code for process_vw_readv/writev
+		 */
+		if (rc == -EACCES)
 			rc = -EPERM;
 		goto put_task_struct;
 	}
-	mm = task->mm;
-
-	if (!mm || (task->flags & PF_KTHREAD)) {
-		task_unlock(task);
-		rc = -EINVAL;
-		goto put_task_struct;
-	}
-
-	atomic_inc(&mm->mm_users);
-	task_unlock(task);
 
 	for (i = 0; i < riovcnt && iov_l_curr_idx < liovcnt; i++) {
 		rc = process_vm_rw_single_vec(