[RFC PATCH 05/11] mm/mempolicy: modify set_mempolicy_home_node to take a task argument

[Gregory Price <gourry.memverge@gmail.com>]

To make mempolicy fetchable by external tasks, we must first change
the callstack to take a task as an argument.

Modify the following functions to require a task argument:
    set_mempolicy_home_node

First we refactor set_mempolicy_home_node to __set_mempolicy_home_node
which accepts a task argument, and change the syscall definition to pass
in (current).

The only functional change in this patch is related to the way task->mm
is acquired. Originally, set_mempolicy_home_node would acquire task->mm
directly via (current->mm).  This is unsafe to do in a non-current
context.  However, utilizing get_task_mm would break the original
functionality of do_get_mempolicy due to the following check
in get_task_mm:

  if (mm) {
    if (task->flags & PF_KTHREAD)
      mm = NULL;
    else
      mmget(mm);
  }

To retain the original behavior, if (task == current) we access
the task->mm directly, but if (task != current) we will utilize
get_task_mm to safely access the mm.  We always take a reference
to the mm to keep the cleanup semantics simple.

Signed-off-by: Gregory Price <gregory.price@memverge.com>
---
 mm/mempolicy.c | 62 +++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 49 insertions(+), 13 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 4519f39b1a07..540163f5d349 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1521,39 +1521,67 @@ static long kernel_mbind(unsigned long start, unsigned long len,
 	return do_mbind(start, len, lmode, mode_flags, &nodes, flags);
 }
 
-SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, len,
-		unsigned long, home_node, unsigned long, flags)
+static long __set_mempolicy_home_node(struct task_struct *task,
+				      unsigned long start,
+				      unsigned long len,
+				      unsigned long home_node,
+				      unsigned long flags)
 {
-	struct mm_struct *mm = current->mm;
+	struct mm_struct *mm;
 	struct vm_area_struct *vma, *prev;
 	struct mempolicy *new, *old;
 	unsigned long end;
 	int err = -ENOENT;
+
+	/*
+	 * Behavior when task == current allows a task modifying itself
+	 * to bypass the check in get_task_mm and acquire the mm directly
+	 */
+	if (task == current) {
+		mm = task->mm;
+		mmget(mm);
+	} else
+		mm = get_task_mm(task);
+
+	if (!mm)
+		return -ENODEV;
+
 	VMA_ITERATOR(vmi, mm, start);
 
 	start = untagged_addr(start);
-	if (start & ~PAGE_MASK)
-		return -EINVAL;
+	if (start & ~PAGE_MASK) {
+		err = -EINVAL;
+		goto mm_out;
+	}
 	/*
 	 * flags is used for future extension if any.
 	 */
-	if (flags != 0)
-		return -EINVAL;
+	if (flags != 0) {
+		err = -EINVAL;
+		goto mm_out;
+	}
 
 	/*
 	 * Check home_node is online to avoid accessing uninitialized
 	 * NODE_DATA.
 	 */
-	if (home_node >= MAX_NUMNODES || !node_online(home_node))
-		return -EINVAL;
+	if (home_node >= MAX_NUMNODES || !node_online(home_node)) {
+		err = -EINVAL;
+		goto mm_out;
+	}
 
 	len = PAGE_ALIGN(len);
 	end = start + len;
 
-	if (end < start)
-		return -EINVAL;
-	if (end == start)
-		return 0;
+	if (end < start) {
+		err = -EINVAL;
+		goto mm_out;
+	}
+	if (end == start) {
+		err = 0;
+		goto mm_out;
+	}
+
 	mmap_write_lock(mm);
 	prev = vma_prev(&vmi);
 	for_each_vma_range(vmi, vma, end) {
@@ -1585,9 +1613,17 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
 			break;
 	}
 	mmap_write_unlock(mm);
+mm_out:
+	mmput(mm);
 	return err;
 }
 
+SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, len,
+		unsigned long, home_node, unsigned long, flags)
+{
+	return __set_mempolicy_home_node(current, start, len, home_node, flags);
+}
+
 SYSCALL_DEFINE6(mbind, unsigned long, start, unsigned long, len,
 		unsigned long, mode, const unsigned long __user *, nmask,
 		unsigned long, maxnode, unsigned int, flags)
-- 
2.39.1