Re: [PATCH] mm/migrate: Avoid possible unnecessary ptrace_may_access() call in kernel_move_pages()

[Matthew Wilcox <willy@infradead.org>]

On Mon, Aug 17, 2020 at 07:59:33AM -0400, Miaohe Lin wrote:
> There is no need to check if this process has the right to modify the
> specified process when they are same.

We should probably also skip the security hook call if a process is
modifying its own pages.

How about this instead?

diff --git a/mm/migrate.c b/mm/migrate.c
index f37729673558..84dc1f42161d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1797,33 +1797,22 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
 	return nr_pages ? -EFAULT : 0;
 }
 
-/*
- * Move a list of pages in the address space of the currently executing
- * process.
- */
-static int kernel_move_pages(pid_t pid, unsigned long nr_pages,
-			     const void __user * __user *pages,
-			     const int __user *nodes,
-			     int __user *status, int flags)
+static struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes)
 {
 	struct task_struct *task;
 	struct mm_struct *mm;
-	int err;
-	nodemask_t task_nodes;
-
-	/* Check flags */
-	if (flags & ~(MPOL_MF_MOVE|MPOL_MF_MOVE_ALL))
-		return -EINVAL;
 
-	if ((flags & MPOL_MF_MOVE_ALL) && !capable(CAP_SYS_NICE))
-		return -EPERM;
+	if (!pid) {
+		mmget(current->mm);
+		*mem_nodes = cpuset_mems_allowed(current);
+		return current->mm;
+	}
 
-	/* Find the mm_struct */
 	rcu_read_lock();
-	task = pid ? find_task_by_vpid(pid) : current;
+	task = find_task_by_vpid(pid);
 	if (!task) {
 		rcu_read_unlock();
-		return -ESRCH;
+		return ERR_PTR(-ESRCH);
 	}
 	get_task_struct(task);
 
@@ -1833,22 +1822,49 @@ static int kernel_move_pages(pid_t pid, unsigned long nr_pages,
 	 */
 	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
 		rcu_read_unlock();
-		err = -EPERM;
+		mm = ERR_PTR(-EPERM);
 		goto out;
 	}
 	rcu_read_unlock();
 
- 	err = security_task_movememory(task);
- 	if (err)
+	mm = ERR_PTR(security_task_movememory(task));
+	if (IS_ERR(mm))
 		goto out;
 
-	task_nodes = cpuset_mems_allowed(task);
+	*mem_nodes = cpuset_mems_allowed(task);
 	mm = get_task_mm(task);
+out:
 	put_task_struct(task);
 
 	if (!mm)
+		mm = ERR_PTR(-EINVAL);
+	return mm;
+}
+
+/*
+ * Move a list of pages in the address space of the specified
+ * process (pid of 0 means current).
+ */
+static int kernel_move_pages(pid_t pid, unsigned long nr_pages,
+			     const void __user * __user *pages,
+			     const int __user *nodes,
+			     int __user *status, int flags)
+{
+	struct mm_struct *mm;
+	int err;
+	nodemask_t task_nodes;
+
+	/* Check flags */
+	if (flags & ~(MPOL_MF_MOVE|MPOL_MF_MOVE_ALL))
 		return -EINVAL;
 
+	if ((flags & MPOL_MF_MOVE_ALL) && !capable(CAP_SYS_NICE))
+		return -EPERM;
+
+	mm = find_mm_struct(pid, &task_nodes);
+	if (IS_ERR(mm))
+		return PTR_ERR(mm);
+
 	if (nodes)
 		err = do_pages_move(mm, task_nodes, nr_pages, pages,
 				    nodes, status, flags);
@@ -1857,10 +1873,6 @@ static int kernel_move_pages(pid_t pid, unsigned long nr_pages,
 
 	mmput(mm);
 	return err;
-
-out:
-	put_task_struct(task);
-	return err;
 }
 
 SYSCALL_DEFINE6(move_pages, pid_t, pid, unsigned long, nr_pages,