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

Gregory Price
2023-11-22T21:12:39Z

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